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In
biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, Development ...

biology
, a gene (from ''genos'' "...
Wilhelm Johannsen Wilhelm Johannsen (3 February 1857 – 11 November 1927) was a Danish pharmacist, botanist Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist i ...
coined the word gene to describe the ..."
(
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
) meaning ''generation'' or ''birth'' or
gender Gender is the range of characteristics pertaining to, and differentiating between femininity Femininity (also called womanliness or girlishness) is a set of attributes, behaviors, and roles generally associated with women A woman is a ...
) is a basic unit of
heredity Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cell (biology), cells or or ...

heredity
and a sequence of
nucleotides Nucleotides are organic molecules , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's ability to Catenation, ...

nucleotides
in
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
that the
synthesis Synthesis or synthesize may also refer to: Science Chemistry and biochemistry *Chemical synthesis, the execution of chemical reactions to form a more complex molecule from chemical precursors **Organic synthesis, the chemical synthesis of o ...

synthesis
of a
gene product A gene product is the biochemical material, either RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecule File:Pentacene on Ni( ...
, either RNA or
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
. During
gene expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. The ...

gene expression
, the DNA is first copied into RNA. The RNA can be directly functional or be the intermediate
template Template may refer to: Tools * Die (manufacturing) A die is a specialized machine tool A machine tool is a machine A machine is a man-made device that uses power to apply forces and control movement to perform an action. Machines can be ...
for a
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
that performs a function. The transmission of genes to an organism's
offspring In biology, offspring are the young creation of living organisms, produced either by a Asexual reproduction, single organism or, in the case of sexual reproduction, two organisms. Collective offspring may be known as a brood or progeny in a more ...

offspring
is the basis of the inheritance of
phenotypic trait A phenotypic trait, simply trait, or character state is a distinct variant of a phenotypic right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of petal color in pea plants. Th ...
s. These genes make up different DNA sequences called
genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a specific ...
s. Genotypes along with environmental and developmental factors determine what the phenotypes will be. Most biological traits are under the influence of
polygeneA polygene is a member of a group of non- epistatic gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, Mendelian units of heredity..." (Greek language, Greek) mean ...
s (many different genes) as well as
gene–environment interaction Gene–environment interaction (or genotype–environment interaction or GxE or G×E) is when two different genotypes The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles An allel ...
s. Some genetic traits are instantly visible, such as
eye color Eye color is a polygenicA polygene is a member of a group of non- epistatic gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, Mendelian units of heredity.. ...

eye color
or the number of limbs, and some are not, such as
blood type A blood type (also known as a blood group) is a classification of blood Blood is a body fluid Body fluids, bodily fluids, or biofluids are liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum ...
, the risk for specific diseases, or the thousands of basic
biochemical Biochemistry or biological chemistry, is the study of chemical process In a scientific Science (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. L ...

biochemical
processes that constitute
life Life is a characteristic that distinguishes physical entities A bubble of exhaled gas in water In common usage and classical mechanics, a physical object or physical body (or simply an object or body) is a collection of matter within a ...

life
. Genes can acquire
mutation In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechan ...
s in their sequence, leading to different variants, known as
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is one of two, or more, forms of a given gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance ...
s, in the
population Population typically refers the number of people in a single area whether it be a city or town, region, country, or the world. Governments typically quantify the size of the resident population within their jurisdiction by a process called a ...

population
. These alleles encode slightly different versions of a protein, which cause different
phenotypical right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of petal color in pea plants. The letters B and b represent gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen ...
traits. Usage of the term "having a gene" (e.g., "good genes," "hair color gene") typically refers to containing a different allele of the same, shared gene. Genes due to
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
/
survival of the fittest "Survival of the fittest" is a phrase that originated from Darwinian Darwinism is a theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemp ...
and
genetic drift Genetic drift (allelic drift or the Sewall Wright effect) is the change in the frequency of an existing gene In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical stru ...

genetic drift
of the alleles. The concept of gene continues to be refined as new phenomena are discovered. For example,
regulatory regions A regulatory sequence is a segment of a nucleic acid molecule which is capable of increasing or decreasing the Gene expression, expression of specific genes within an organism. Regulation of gene expression is an essential feature of all living orga ...
of a gene can be far removed from its
coding region The coding region of a gene, also known as the CDS (from ''coding sequence''), is the portion of a gene's DNA or RNA that codes for protein. Studying the length, composition, regulation, splicing, structures, and functions of coding regions compare ...
s, and coding regions can be split into several
exon An exon is any part of a gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, Mendelian units of heredity..." (Greek language, Greek) meaning ''generation'' or ...
s. Some
viruses A virus is a wikt:submicroscopic, submicroscopic infectious agent that Viral replication, replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, incl ...
store their
genome In the fields of molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...

genome
in
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Re ...

RNA
instead of DNA and some gene products are functional
non-coding RNA A non-coding RNA (ncRNA) is an RNA molecule that is not Translation (genetics), translated into a protein. The DNA sequence from which a functional non-coding RNA is transcribed is often called an RNA gene. Abundant and functionally important li ...
s. Therefore, a broad, modern working definition of a gene is any discrete
locus Locus (plural loci) is Latin for "place". It may refer to: Entertainment * Locus (comics), a Marvel Comics mutant villainess, a member of the Mutant Liberation Front * Locus (magazine), ''Locus'' (magazine), science fiction and fantasy magazine ...
of heritable, genomic sequence which affect an organism's traits by being as a functional product or by
regulation of gene expression Regulation of gene expression, or gene regulation, includes a wide range of mechanisms that are used by cells to increase or decrease the production of specific gene products (protein Proteins are large biomolecules or macromolecules that a ...

regulation of gene expression
. The term ''gene'' was introduced by Danish
botanist Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Ancient Greek wo ...

botanist
, plant physiologist and
geneticist A geneticist is a biologist who studies genetics, the science of genes, heredity, and genetic variation, variation of organisms. A geneticist can be employed as a scientist or a lecturer. Geneticists may perform general research on genetic proce ...
Wilhelm Johannsen Wilhelm Johannsen (3 February 1857 – 11 November 1927) was a Danish pharmacist, botanist Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist i ...
in 1909. From p. 124: ''"Dieses "etwas" in den Gameten bezw. in der Zygote, … — kurz, was wir eben Gene nennen wollen — bedingt sind."'' (This "something" in the gametes or in the zygote, which has crucial importance for the character of the organism, is usually called by the quite ambiguous term ''Anlagen'' rimordium, from the German word ''Anlage'' for "plan, arrangement ; rough sketch" Many other terms have been suggested, mostly unfortunately in closer connection with certain hypothetical opinions. The word "pangene", which was introduced by Darwin, is perhaps used most frequently in place of ''Anlagen''. However, the word "pangene" was not well chosen, as it is a compound word containing the roots ''pan'' (the neuter form of Πας all, every) and ''gen'' (from γί-γ(ε)ν-ομαι, to become). Only the meaning of this latter .e., ''gen''comes into consideration here ; just the basic idea — amely,that a trait in the developing organism can be determined or is influenced by "something" in the gametes — should find expression. No hypothesis about the nature of this "something" should be postulated or supported by it. For that reason it seems simplest to use in isolation the last syllable ''gen'' from Darwin's well-known word, which alone is of interest to us, in order to replace, with it, the poor, ambiguous word ''Anlage''. Thus we will say simply "gene" and "genes" for "pangene" and "pangenes". The word gene is completely free of any hypothesis ; it expresses only the established fact that in any case many traits of the organism are determined by specific, separable, and thus independent "conditions", "foundations", "plans" — in short, precisely what we want to call genes.) It is inspired by the
ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ), refers collectively to the diale ...
: γόνος, ''gonos'', that means offspring and procreation.


History


Discovery of discrete inherited units

The existence of discrete inheritable units was first suggested by
Gregor Mendel Gregor Johann Mendel (; cs, Řehoř Jan Mendel; 20 July 1822 – 6 January 1884) was a meteorologist, mathematician, biologist, AugustinianAugustinian may refer to: *Augustinians Augustinians are members of Christian religious orders th ...

Gregor Mendel
(1822–1884). From 1857 to 1864, in
Brno Brno ( , ; german: Brünn, ) is a city A city is a large human settlement.Goodall, B. (1987) ''The Penguin Dictionary of Human Geography''. London: Penguin.Kuper, A. and Kuper, J., eds (1996) ''The Social Science Encyclopedia''. 2nd edition. ...

Brno
,
Austrian Empire The Austrian Empire (german: Kaiserthum Oesterreich, modern spelling ') was a Central Europe Central Europe is an area of Europe Europe is a which is also recognised as part of , located entirely in the and mostly in the . It compr ...
(today's Czech Republic), he studied inheritance patterns in 8000 common edible pea plants, tracking distinct traits from parent to offspring. He described these mathematically as 2n combinations where n is the number of differing characteristics in the original peas. Although he did not use the term ''gene'', he explained his results in terms of discrete inherited units that give rise to observable physical characteristics. This description prefigured
Wilhelm Johannsen Wilhelm Johannsen (3 February 1857 – 11 November 1927) was a Danish pharmacist, botanist Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist i ...
's distinction between
genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a specific ...
(the genetic material of an organism) and
phenotype In genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular inter ...

phenotype
(the observable traits of that organism). Mendel was also the first to demonstrate
independent assortment Mendelian inheritance is a type of biology, biological inheritance (biology), inheritance that follows the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 and popularized by William Bateson. These princi ...

independent assortment
, the distinction between
dominant Domination or dominant may refer to: Society * World domination, which is mainly a conspiracy theory * Colonialism in which one group (usually a nation) invades another region for material gain or to eliminate competition * Chauvinism in which a p ...
and
recessive In genetics Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) Though heredity had been observed for millennia, Gregor Mendel, Moravia, Moravian scientist ...
traits, the distinction between a
heterozygote Zygosity (the noun, zygote A zygote (from Greek ζυγωτός ''zygōtos'' "joined" or "yoked", from ζυγοῦν ''zygoun'' "to join" or "to yoke") is a eukaryotic cell formed by a fertilization event between two gamete A gamete ( /ˈ ...
and
homozygote Zygosity (the noun, zygote A zygote (from Greek ζυγωτός ''zygōtos'' "joined" or "yoked", from ζυγοῦν ''zygoun'' "to join" or "to yoke") is a eukaryotic cell formed by a fertilization event between two gamete A gamete ( /ˈ ...
, and the phenomenon of discontinuous inheritance. Prior to Mendel's work, the dominant theory of heredity was one of
blending inheritance Blending inheritance is an Superseded scientific theories, obsolete theory in biology from the 19th century. The theory is that the progeny inheritance (biology), inherits any characteristic as the average of the parents' values of that characteris ...

blending inheritance
, which suggested that each parent contributed fluids to the fertilisation process and that the traits of the parents blended and mixed to produce the offspring.
Charles Darwin Charles Robert Darwin (; ; 12 February 1809 – 19 April 1882) was an English naturalist Natural history is a domain of inquiry involving organism In biology, an organism () is any organic, life, living system that fu ...

Charles Darwin
developed a theory of inheritance he termed
pangenesis Pangenesis was Charles Darwin's hypothetical mechanism for heredity, in which he proposed that each part of the body continually emitted its own type of small organic particles called gemmules that aggregated in the gonads, contributing heritable ...

pangenesis
, from
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
pan ("all, whole") and genesis ("birth") / genos ("origin"). Darwin used the term ''
gemmule Gemmules are internal buds found in sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal Metazoa (animal) clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group ...
'' to describe hypothetical particles that would mix during reproduction. Mendel's work went largely unnoticed after its first publication in 1866, but was rediscovered in the late 19th century by
Hugo de Vries Hugo Marie de Vries () (16 February 1848 – 21 May 1935) was a Dutch botanist Botany, also called , plant biology or phytology, is the science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterpris ...

Hugo de Vries
,
Carl Correns Carl Erich Correns (19 September 1864 – 14 February 1933) was a German botanist Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a sci ...

Carl Correns
, and
Erich von TschermakErich Tschermak, Edler von Seysenegg (15 November 1871 – 11 October 1962) was an Austria Austria, officially the Republic of Austria, is a landlocked country in the southern part of Central Europe, located on the Eastern Alps. It ...
, who (claimed to have) reached similar conclusions in their own research. Specifically, in 1889, Hugo de Vries published his book ''Intracellular Pangenesis'', Translated in 1908 from German to English by Open Court Publishing Co., Chicago, 1910 in which he postulated that different characters have individual hereditary carriers and that inheritance of specific traits in organisms comes in particles. De Vries called these units "pangenes" (''Pangens'' in German), after Darwin's 1868 pangenesis theory. Twenty years later, in 1909,
Wilhelm Johannsen Wilhelm Johannsen (3 February 1857 – 11 November 1927) was a Danish pharmacist, botanist Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist i ...
introduced the term 'gene' and in 1906,
William Bateson William Bateson (8 August 1861 – 8 February 1926) was an English biology, biologist who was the first person to use the term genetics to describe the study of heredity, and the chief populariser of the ideas of Gregor Mendel following their red ...

William Bateson
, that of '
genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, ...

genetics
' while
Eduard Strasburger Eduard Adolf Strasburger (1 February 1844 – 18 May 1912) was a Polish- German professor and one of the most famous botanist Botany, also called , plant biology or phytology, is the science Science (from the Latin word ''scientia'', ...
, amongst others, still used the term 'pangene' for the fundamental physical and functional unit of heredity.


Discovery of DNA

Advances in understanding genes and inheritance continued throughout the 20th century.
Deoxyribonucleic acid Deoxyribonucleic acid (; DNA) is a molecule composed of two polynucleotide chains that coil around each other to form a Nucleic acid double helix, double helix carrying genetics, genetic instructions for the development, functioning, growth ...
(DNA) was shown to be the molecular repository of genetic information by experiments in the 1940s to 1950s. Reprint: The structure of DNA was studied by
Rosalind Franklin Rosalind Elsie Franklin (25 July 192016 April 1958) was an English chemist and X-ray crystallographer whose work was central to the understanding of the molecular structures of DNA (deoxyribonucleic acid), RNA (ribonucleic acid), viruses, co ...

Rosalind Franklin
and
Maurice Wilkins Maurice Hugh Frederick Wilkins (15 December 1916 – 5 October 2004) was a New Zealand-born and whose research spanned multiple areas of physics and biophysics, contributing to the scientific understanding of , , and , and to the developme ...
using
X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract into many specific directions. By measurin ...

X-ray crystallography
, which led James D. Watson and
Francis Crick Francis Harry Compton Crick (8 June 1916 – 28 July 2004) was a British molecular biologist, biophysicist, and neuroscientist A neuroscientist (or neurobiologist) is a scientist A scientist is a person who conducts Scientific method, scie ...

Francis Crick
to publish a model of the double-stranded DNA molecule whose paired nucleotide bases indicated a compelling hypothesis for the mechanism of genetic replication. In the early 1950s the prevailing view was that the genes in a chromosome acted like discrete entities, indivisible by recombination and arranged like beads on a string. The experiments of using
mutant In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechan ...

mutant
s defective in the rII region of bacteriophage T4 (1955–1959) showed that individual genes have a simple linear structure and are likely to be equivalent to a linear section of DNA. Collectively, this body of research established the
central dogma of molecular biology The central dogma of molecular biology is an explanation of the flow of genetic information within a biological system. It is often stated as "DNA makes RNA, and RNA makes protein", although this is not its original meaning. It was first stated by ...

central dogma of molecular biology
, which states that
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
s are translated from
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Re ...

RNA
, which is transcribed from
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
. This dogma has since been shown to have exceptions, such as
reverse transcription A reverse transcriptase (RT) is an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrate ...

reverse transcription
in
retrovirus A retrovirus is a type of virus A virus is a submicroscopic infectious agent In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical ...

retrovirus
es. The modern study of
genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, ...

genetics
at the level of DNA is known as
molecular genetics Molecular genetics is a sub-field of biology that addresses how differences in the structures or expression of DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image ...
. In 1972,
Walter Fiers Walter Fiers (31 January 1931 in Ypres, West Flanders – 28 July 2019 in Destelbergen) was a Belgian molecular biologist. He obtained a degree of Engineer for Chemistry and Agricultural Industries at the University of Ghent in 1954, and star ...
and his team were the first to determine the sequence of a gene: that of
Bacteriophage MS2 ''Escherichia virus MS2'' is an icosahedral, positive-sense single-stranded RNA virus that infects the bacterium ''Escherichia coli'' and other members of the Enterobacteriaceae. MS2 is a member of a family of closely related bacterial viruses t ...
coat protein. The subsequent development of chain-termination
DNA sequencing DNA sequencing is the process of determining the nucleic acid sequence A nucleic acid sequence is a succession of bases signified by a series of a set of five different letters that indicate the order of nucleotides Nucleotides are organic ...

DNA sequencing
in 1977 by
Frederick Sanger Frederick Sanger (; 13 August 1918 – 19 November 2013) was a British biochemist Biochemists are scientists who are trained in biochemistry Biochemistry or biological chemistry, is the study of chemical processes within and relatin ...

Frederick Sanger
improved the efficiency of sequencing and turned it into a routine laboratory tool. An automated version of the Sanger method was used in early phases of the
Human Genome Project The Human Genome Project (''HGP'') was an international scientific research The scientific method is an Empirical evidence, empirical method of acquiring knowledge that has characterized the development of science since at least the 17th cen ...
.


Modern synthesis and its successors

The theories developed in the early 20th century to integrate
Mendelian genetics Mendelian inheritance is a type of biological inheritance Inheritance is the practice of passing on private property, titles A title is one or more words used before or after a person's name, in certain contexts. It may signify either ...
with
Darwinian evolution Darwinism is a theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as obser ...
are called the
modern synthesis Modern synthesis or modern evolutionary synthesis refers to several perspectives on evolutionary biology, namely: * Modern synthesis (20th century), the term coined by Julian Huxley in 1942 to denote the synthesis between Mendelian genetics and s ...
, a term introduced by
Julian Huxley Sir Julian Sorell Huxley (22 June 1887 – 14 February 1975) was an English evolutionary biologist Evolutionary biology is the subfield of biology that studies the evolutionary processes ( natural selection, common descent, speciation) t ...
. Evolutionary biologists have subsequently modified this concept, such as George C. Williams' gene-centric view of evolution. He proposed an evolutionary concept of the gene as a
unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in a theatrical presentation Music * Unit (album), ...
of
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
with the definition: "that which segregates and recombines with appreciable frequency." In this view, the molecular gene ''transcribes'' as a unit, and the evolutionary gene ''inherits'' as a unit. Related ideas emphasizing the centrality of genes in evolution were popularized by
Richard Dawkins Richard Dawkins (born 26 March 1941) is a British evolutionary biology, evolutionary biologist and author. He is an Oxford fellow, emeritus fellow of New College, Oxford and was Simonyi Professor for the Public Understanding of Science, Prof ...

Richard Dawkins
.


Molecular basis


DNA

The vast majority of organisms encode their genes in long strands of
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
(deoxyribonucleic acid). DNA consists of a
chain A chain is a wikt:series#Noun, serial assembly of connected pieces, called links, typically made of metal, with an overall character similar to that of a rope in that it is flexible and curved in compression (physics), compression but line (g ...

chain
made from four types of
nucleotide Nucleotides are organic molecules , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's ability to Catenation, ...

nucleotide
subunits, each composed of: a five-carbon sugar (), a
phosphate In chemistry, a phosphate is an anion, salt (chemistry), salt, functional group or ester derived from a phosphoric acids and phosphates, phosphoric acid. It most commonly means orthophosphate, a derivative of phosphoric acid, orthophosphoric a ...

phosphate
group, and one of the four bases
adenine Adenine (A, Ade) is a nucleobase 230px, Pyrimidine nucleobases are simple ring molecules. Nucleobases, also known as ''nitrogenous bases'' or often simply ''bases'', are nitrogen-containing biological compounds that form nucleosides Nucleos ...

adenine
,
cytosine Cytosine () (symbol A symbol is a mark, sign, or word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical ...

cytosine
,
guanine Guanine () (symbol A symbol is a mark, sign, or word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical me ...

guanine
, and
thymine Thymine () (symbol A symbol is a mark, sign, or word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical m ...

thymine
. Two chains of DNA twist around each other to form a DNA
double helix In molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, ...
with the phosphate-sugar backbone spiraling around the outside, and the bases pointing inwards with adenine
base pairing A base pair (bp) is a fundamental unit of double-stranded nucleic acids Nucleic acids are biopolymer Biopolymers are natural polymers produced by the cells of Organism, living organisms. Biopolymers consist of monomeric units that are Covalent_ ...
to thymine and guanine to cytosine. The specificity of base pairing occurs because adenine and thymine align to form two
hydrogen bond A hydrogen bond (or H-bond) is a primarily electrostatic Electrostatics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...

hydrogen bond
s, whereas cytosine and guanine form three hydrogen bonds. The two strands in a double helix must, therefore, be
complementary A complement is often something that completes something else, or at least adds to it in some useful way. Thus it may be: * Complement (linguistics), a word or phrase having a particular syntactic role ** Subject complement, a word or phrase addi ...
, with their sequence of bases matching such that the adenines of one strand are paired with the thymines of the other strand, and so on. Due to the chemical composition of the
pentose In , a pentose is a (simple sugar) with five . The of all pentoses is , and their is 150.13 g/mol.-Ri ...

pentose
residues of the bases, DNA strands have directionality. One end of a DNA polymer contains an exposed
hydroxyl A hydroxy or hydroxyl group is a functional group with the chemical formula -OH and composed of one oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the ...

hydroxyl
group on the
deoxyribose Deoxyribose, or more precisely 2-deoxyribose, is a monosaccharide Monosaccharides (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Rep ...

deoxyribose
; this is known as the 3' end of the molecule. The other end contains an exposed
phosphate In chemistry, a phosphate is an anion, salt (chemistry), salt, functional group or ester derived from a phosphoric acids and phosphates, phosphoric acid. It most commonly means orthophosphate, a derivative of phosphoric acid, orthophosphoric a ...

phosphate
group; this is the 5' end. The two strands of a double-helix run in opposite directions. Nucleic acid synthesis, including
DNA replication In molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, mo ...

DNA replication
and transcription occurs in the 5'→3' direction, because new nucleotides are added via a
dehydration reaction In chemistry, a dehydration reaction, also known as Zimmer's hydrogenesis, is a chemical reaction that involves the loss of water from the reacting molecule or ion. It is the most common type of condensation reaction. Dehydration reactions are co ...
that uses the exposed 3' hydroxyl as a
nucleophile In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a ...

nucleophile
. The of genes encoded in DNA begins by transcribing the gene into
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Re ...

RNA
, a second type of nucleic acid that is very similar to DNA, but whose monomers contain the sugar
ribose Ribose is a simple sugar and carbohydrate with molecular formula C5H10O5 and the linear-form composition H−(C=O)−(CHOH)4−H. The naturally-occurring form, , is a component of the ribonucleotides from which RNA is built, and so this compoun ...

ribose
rather than
deoxyribose Deoxyribose, or more precisely 2-deoxyribose, is a monosaccharide Monosaccharides (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Rep ...

deoxyribose
. RNA also contains the base
uracil Uracil () ( U or Ura) is one of the four s in the that are represented by the letters A, G, C and U. The others are (A), (C), and (G). In RNA, uracil binds to via two . In , the uracil nucleobase is replaced by . Uracil is a form of . Ura ...

uracil
in place of
thymine Thymine () (symbol A symbol is a mark, sign, or word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical m ...

thymine
. RNA molecules are less stable than DNA and are typically single-stranded. Genes that encode proteins are composed of a series of three-
nucleotide Nucleotides are organic molecules , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's ability to Catenation, ...

nucleotide
sequences called
codon The genetic code is the set of rules used by living cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or reli ...

codon
s, which serve as the "words" in the genetic "language". The
genetic code The genetic code is the set of rules used by living cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or rel ...

genetic code
specifies the correspondence during
protein translation In molecular biology and genetics, translation is the process in which ribosomes in the cytoplasm or endoplasmic reticulum synthesize proteins after the process of transcription (biology), transcription of DNA to RNA in the cell's nucleus ( ...
between codons and
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

amino acid
s. The genetic code is nearly the same for all known organisms.


Chromosomes

The total complement of genes in an organism or cell is known as its
genome In the fields of molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...

genome
, which may be stored on one or more
chromosome A chromosome is a long DNA molecule with part or all of the genome, genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by Chaperone (protein), chaperone proteins, bind to and ...

chromosome
s. A chromosome consists of a single, very long DNA helix on which thousands of genes are encoded. The region of the chromosome at which a particular gene is located is called its
locus Locus (plural loci) is Latin for "place". It may refer to: Entertainment * Locus (comics), a Marvel Comics mutant villainess, a member of the Mutant Liberation Front * Locus (magazine), ''Locus'' (magazine), science fiction and fantasy magazine ...
. Each locus contains one
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is one of two, or more, forms of a given gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance ...
of a gene; however, members of a population may have different alleles at the locus, each with a slightly different gene sequence. The majority of
eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...
genes are stored on a set of large, linear chromosomes. The chromosomes are packed within the
nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...

nucleus
in complex with storage proteins called
histone In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
s to form a unit called a
nucleosome A nucleosome is the basic structural unit of DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon ...

nucleosome
. DNA packaged and condensed in this way is called
chromatin Chromatin is a complex of DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecu ...
. The manner in which DNA is stored on the histones, as well as chemical modifications of the histone itself, regulate whether a particular region of DNA is accessible for
gene expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. The ...

gene expression
. In addition to genes, eukaryotic chromosomes contain sequences involved in ensuring that the DNA is copied without degradation of end regions and sorted into daughter cells during cell division: replication origins,
telomere A telomere ( or , from and ) is a region of repetitive nucleotide Nucleotides are organic molecules , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-h ...

telomere
s and the
centromere The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis In cell b ...
. Replication origins are the sequence regions where
DNA replication In molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, mo ...

DNA replication
is initiated to make two copies of the chromosome. Telomeres are long stretches of repetitive sequences that cap the ends of the linear chromosomes and prevent degradation of coding and regulatory regions during
DNA replication In molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, mo ...

DNA replication
. The length of the telomeres decreases each time the genome is replicated and has been implicated in the
aging Ageing or aging (see spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from the Ancient Greek word , , "discourse", from , , "through" and , , "I speak") is used in two distinct ways ...

aging
process. The centromere is required for binding spindle fibres to separate sister chromatids into daughter cells during
cell division Cell division is the process by which a parent cell (biology), cell divides into two or more daughter cells. Cell division usually occurs as part of a larger cell cycle. In eukaryotes, there are two distinct types of cell division; a vegetative ...

cell division
.
Prokaryote A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual conti ...
s (
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
and
archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...

archaea
) typically store their genomes on a single large,
circular chromosome A circular chromosome is a chromosome in bacteria, archaea, Mitochondrial DNA#Genome structure and diversity, mitochondria, and Chloroplast DNA#Molecular structure, chloroplasts, in the form of a molecule of circular DNA, unlike the linear chromoso ...
. Similarly, some eukaryotic
organelles In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
contain a remnant circular chromosome with a small number of genes. Prokaryotes sometimes supplement their chromosome with additional small circles of DNA called
plasmid A plasmid is a small, extrachromosomal DNA Extrachromosomal DNA (abbreviated ecDNA) is any DNA that is found off the chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryo ...
s, which usually encode only a few genes and are transferable between individuals. For example, the genes for
antibiotic resistance Antimicrobial resistance (AMR or AR) occurs when microbe A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is some ...

antibiotic resistance
are usually encoded on bacterial plasmids and can be passed between individual cells, even those of different species, via
horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient G ...
. Whereas the chromosomes of prokaryotes are relatively gene-dense, those of eukaryotes often contain regions of DNA that serve no obvious function. Simple single-celled eukaryotes have relatively small amounts of such DNA, whereas the genomes of complex
multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biol ...
s, including humans, contain an absolute majority of DNA without an identified function. This DNA has often been referred to as "
junk DNA Non-coding DNA sequences are components of an organism's DNA that do not encode protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins per ...
". However, more recent analyses suggest that, although protein-coding DNA makes up barely 2% of the
human genome The human genome is a complete set of nucleic acid sequence A nucleic acid sequence is a succession of bases signified by a series of a set of five different letters that indicate the order of nucleotides Nucleotides are organic molecules c ...

human genome
, about 80% of the bases in the genome may be expressed, so the term "junk DNA" may be a misnomer.


Structure and function


Structure

The consists of many elements of which the actual protein coding sequence is often only a small part. These include DNA regions that are not transcribed as well as untranslated regions of the RNA. Flanking the open reading frame, genes contain a
regulatory sequence A regulatory sequence is a segment of a nucleic acid Nucleic acids are biopolymers, macromolecules, essential to all Organism, known forms of life. They are composed of nucleotides, which are the monomers made of three components: a pentose, 5-car ...
that is required for their expression. First, genes require a promoter sequence. The promoter is recognized and bound by
transcription factors In molecular biology Molecular biology is the branch of biology that seeks to understand the molecule, molecular basis of biological activity in and between Cell (biology), cells, including biomolecule, molecular synthesis, modification, m ...

transcription factors
that recruit and help
RNA polymerase In molecular biology Molecular biology is the branch of biology that seeks to understand the molecule, molecular basis of biological activity in and between Cell (biology), cells, including biomolecule, molecular synthesis, modification, m ...

RNA polymerase
bind to the region to initiate transcription. The recognition typically occurs as a
consensus sequence In molecular biology and bioinformatics, the consensus sequence (or canonical sequence) is the calculated order of most frequent residues, either nucleotide or amino acid, found at each position in a sequence alignment. It serves as a simplified r ...
like the
TATA box In molecular biology, the TATA box (also called the Goldberg–Hogness box) is a DNA sequence, sequence of DNA found in the Promoter (genetics), core promoter region of genes in archaea and eukaryotes. The bacterial homolog of the TATA box is c ...
. A gene can have more than one promoter, resulting in messenger RNAs (
mRNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein. mRNA i ...

mRNA
) that differ in how far they extend in the 5' end. Highly transcribed genes have "strong" promoter sequences that form strong associations with transcription factors, thereby initiating transcription at a high rate. Others genes have "weak" promoters that form weak associations with transcription factors and initiate transcription less frequently.
Eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

Eukaryotic
promoter regions are much more complex and difficult to identify than
prokaryotic A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual conti ...
promoters. Additionally, genes can have regulatory regions many kilobases upstream or downstream of the open reading frame that alter expression. These act by binding to transcription factors which then cause the DNA to loop so that the regulatory sequence (and bound transcription factor) become close to the RNA polymerase binding site. For example,
enhancers In genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interacti ...
increase transcription by binding an activator protein which then helps to recruit the RNA polymerase to the promoter; conversely silencers bind
repressor In molecular genetics, a repressor is a DNA-binding protein, DNA- or RNA-binding protein that inhibits the Gene expression, expression of one or more genes by binding to the Operator (biology), operator or associated Silencer (DNA), silencers. A ...
proteins and make the DNA less available for RNA polymerase. The transcribed
pre-mRNA A primary transcript is the single-stranded ribonucleic acid (RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material cons ...

pre-mRNA
contains untranslated regions at both ends which contain binding sites for ribosome binding site, ribosomes, RNA-binding protein, RNA-binding proteins, microRNA, miRNA, as well as Terminator (genetics), terminator, and start codon, start and stop codons. In addition, most eukaryotic open reading frames contain untranslated introns, which are removed and exons, which are connected together in a process known as RNA splicing. Finally, the ends of gene transcripts are defined by polyadenylation, cleavage and polyadenylation (CPA) sites, where newly produced pre-mRNA gets cleaved and a string of ~200 adenosine monophosphates is added at the 3′ end. The polyadenylation, poly(A) tail protects mature mRNA from degradation and has other functions, affecting translation, localization, and transport of the transcript from the nucleus. Splicing, followed by CPA, generate the final mature mRNA, which encodes the protein or RNA product. Although the general mechanisms defining locations of human genes are known, identification of the exact factors regulating these cellular processes is an area of active research. For example, known sequence features in the three prime untranslated region, 3′-UTR can only explain half of all human gene ends. Many prokaryotic genes are organized into operons, with multiple protein-coding sequences that are transcribed as a unit. The genes in an operon are transcribed as a continuous messenger RNA, referred to as a Messenger RNA#Monocistronic versus polycistronic mRNA, polycistronic mRNA. The term cistron in this context is equivalent to gene. The transcription of an operon's mRNA is often controlled by a
repressor In molecular genetics, a repressor is a DNA-binding protein, DNA- or RNA-binding protein that inhibits the Gene expression, expression of one or more genes by binding to the Operator (biology), operator or associated Silencer (DNA), silencers. A ...
that can occur in an active or inactive state depending on the presence of specific metabolites. When active, the repressor binds to a DNA sequence at the beginning of the operon, called the Operon#General structure of an operon, operator region, and represses Transcription (genetics), transcription of the operon; when the repressor is inactive transcription of the operon can occur (see e.g. Lac operon). The products of operon genes typically have related functions and are involved in the same gene regulatory network, regulatory network.


Functional definitions

Defining exactly what section of a DNA sequence comprises a gene is difficult.Enhancer (genetics), Regulatory regions of a gene such as Enhancer (genetics), enhancers do not necessarily have to be close to the coding sequence on the linear molecule because the intervening DNA can be looped out to bring the gene and its regulatory region into proximity. Similarly, a gene's introns can be much larger than its exons. Regulatory regions can even be on entirely different chromosomes and operate ''in trans'' to allow regulatory regions on one chromosome to come in contact with target genes on another chromosome. Early work in molecular genetics suggested the concept that one gene-one enzyme hypothesis, one gene makes one protein. This concept (originally called the one gene-one enzyme hypothesis) emerged from an influential 1941 paper by George Beadle and Edward Tatum on experiments with mutants of the fungus ''Neurospora crassa''. Norman Horowitz, an early colleague on the ''Neurospora'' research, reminisced in 2004 that “these experiments founded the science of what Beadle and Tatum called ''biochemical genetics''. In actuality they proved to be the opening gun in what became
molecular genetics Molecular genetics is a sub-field of biology that addresses how differences in the structures or expression of DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image ...
and all the developments that have followed from that.” The one gene-one protein concept has been refined since the discovery of genes that can encode multiple proteins by alternative splicing and coding sequences split in short section across the genome whose mRNAs are concatenated by trans-splicing. A broad operational definition is sometimes used to encompass the complexity of these diverse phenomena, where a gene is defined as a union of genomic sequences encoding a coherent set of potentially overlapping functional products. This definition categorizes genes by their functional products (proteins or RNA) rather than their specific DNA loci, with regulatory elements classified as ''gene-associated'' regions.


Overlap between genes

It is also possible for genes to overlap the same DNA sequence and be considered distinct but Overlapping gene, overlapping genes. The current definition of an overlapping gene is different across eukaryotes, prokaryotes, and viruses. In Eukaryotes they have recently been defined as "when at least one nucleotide is shared between the outermost boundaries of the Primary transcript, primary transcripts of two or more genes, such that a DNA base mutation at the point of overlap would affect transcripts of all genes involved in the overlap." In Prokaryotes and Viruses they have recently been defined as "when the Coding region, coding sequences of two genes share a nucleotide either on the same or opposite strands."


Gene expression

In all organisms, two steps are required to read the information encoded in a gene's DNA and produce the protein it specifies. First, the gene's DNA is ''transcription (genetics), transcribed'' to messenger RNA (
mRNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein. mRNA i ...

mRNA
). Second, that mRNA is ''translation (genetics), translated'' to protein. RNA-coding genes must still go through the first step, but are not translated into protein. The process of producing a biologically functional molecule of either RNA or protein is called
gene expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. The ...

gene expression
, and the resulting molecule is called a
gene product A gene product is the biochemical material, either RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecule File:Pentacene on Ni( ...
.


Genetic code

The nucleotide sequence of a gene's DNA specifies the amino acid sequence of a protein through the
genetic code The genetic code is the set of rules used by living cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or rel ...

genetic code
. Sets of three nucleotides, known as
codon The genetic code is the set of rules used by living cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or reli ...

codon
s, each correspond to a specific amino acid. The principle that three sequential bases of DNA code for each amino acid was demonstrated in 1961 using frameshift mutations in the rIIB gene of bacteriophage T4 (see Crick, Brenner et al. experiment). Additionally, a "start codon", and three "stop codons" indicate the beginning and end of the Coding region, protein coding region. There are 64 possible codons (four possible nucleotides at each of three positions, hence 43 possible codons) and only 20 standard amino acids; hence the code is redundant and multiple codons can specify the same amino acid. The correspondence between codons and amino acids is nearly universal among all known living organisms.


Transcription

transcription (genetics), Transcription produces a single-stranded
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Re ...

RNA
molecule known as messenger RNA, whose nucleotide sequence is complementary to the DNA from which it was transcribed. The mRNA acts as an intermediate between the DNA gene and its final protein product. The gene's DNA is used as a template to generate a Base pair, complementary mRNA. The mRNA matches the sequence of the gene's DNA coding strand because it is synthesised as the complement of the template strand. Transcription is performed by an enzyme called an
RNA polymerase In molecular biology Molecular biology is the branch of biology that seeks to understand the molecule, molecular basis of biological activity in and between Cell (biology), cells, including biomolecule, molecular synthesis, modification, m ...

RNA polymerase
, which reads the template strand in the 3' end, 3' to 5' end, 5' direction and synthesizes the RNA from 5' end, 5' to 3' end, 3'. To initiate transcription, the polymerase first recognizes and binds a promoter (biology), promoter region of the gene. Thus, a major mechanism of gene regulation is the blocking or sequestering the promoter region, either by tight binding by
repressor In molecular genetics, a repressor is a DNA-binding protein, DNA- or RNA-binding protein that inhibits the Gene expression, expression of one or more genes by binding to the Operator (biology), operator or associated Silencer (DNA), silencers. A ...
molecules that physically block the polymerase or by organizing the DNA so that the promoter region is not accessible. In prokaryotes, transcription occurs in the cytoplasm; for very long transcripts, translation may begin at the 5'  end of the RNA while the 3' end is still being transcribed. In eukaryotes, transcription occurs in the nucleus, where the cell's DNA is stored. The RNA molecule produced by the polymerase is known as the primary transcript and undergoes post-transcriptional modifications before being exported to the cytoplasm for translation. One of the modifications performed is the splicing (genetics), splicing of introns which are sequences in the transcribed region that do not encode a protein. Alternative splicing mechanisms can result in mature transcripts from the same gene having different sequences and thus coding for different proteins. This is a major form of regulation in eukaryotic cells and also occurs in some prokaryotes.


Translation

Translation (genetics), Translation is the process by which a Mature messenger RNA, mature mRNA molecule is used as a template for synthesizing a new
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
. Translation is carried out by ribosomes, large complexes of RNA and protein responsible for carrying out the chemical reactions to add new
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

amino acid
s to a growing polypeptide chain by the formation of peptide bonds. The genetic code is read three nucleotides at a time, in units called
codon The genetic code is the set of rules used by living cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or reli ...

codon
s, via interactions with specialized RNA molecules called transfer RNA (tRNA). Each tRNA has three unpaired bases known as the anticodon that are complementary to the codon it reads on the mRNA. The tRNA is also covalently attached to the
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

amino acid
specified by the complementary codon. When the tRNA binds to its complementary codon in an mRNA strand, the ribosome attaches its amino acid cargo to the new polypeptide chain, which is synthesized from N-terminus, amino terminus to C-terminus, carboxyl terminus. During and after synthesis, most new proteins must protein folding, fold to their active tertiary structure, three-dimensional structure before they can carry out their cellular functions.


Regulation

Regulation of gene expression, Genes are regulated so that they are gene expression, expressed only when the product is needed, since expression draws on limited resources. A cell regulates its gene expression depending on its Environment (biophysical), external environment (e.g. nutrient, available nutrients, Heat shock protein, temperature and other Cellular stress response, stresses), its internal environment (e.g. cell division cycle, metabolism, infection, infection status), and its Cellular differentiation, specific role if in a multicellular organism. Gene expression can be regulated at any step: from Transcriptional regulation, transcriptional initiation, to RNA processing, to post-translational modification of the protein. The regulation of lactose metabolism genes in ''E. coli'' (lac operon, ''lac'' operon) was the first such mechanism to be described in 1961.


RNA genes

A typical protein-coding gene is first copied into
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Re ...

RNA
as an intermediate in the manufacture of the final protein product. In other cases, the RNA molecules are the actual functional products, as in the synthesis of ribosomal RNA and transfer RNA. Some RNAs known as ribozymes are capable of enzyme, enzymatic function, and microRNA has a regulatory role. The
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
sequences from which such RNAs are transcribed are known as non-coding RNA, non-coding RNA genes. Some viruses store their entire genomes in the form of
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Re ...

RNA
, and contain no DNA at all. Because they use RNA to store genes, their cell (biology), cellular host (biology), hosts may synthesize their proteins as soon as they are infection, infected and without the delay in waiting for transcription. On the other hand, RNA
retrovirus A retrovirus is a type of virus A virus is a submicroscopic infectious agent In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical ...

retrovirus
es, such as HIV, require the
reverse transcription A reverse transcriptase (RT) is an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrate ...

reverse transcription
of their
genome In the fields of molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...

genome
from RNA into DNA before their proteins can be synthesized. RNA-mediated epigenetic inheritance has also been observed in plants and very rarely in animals.


Inheritance

Organisms inherit their genes from their parents. Asexual reproduction, Asexual organisms simply inherit a complete copy of their parent's genome. Sexual reproduction, Sexual organisms have two copies of each chromosome because they inherit one complete set from each parent.


Mendelian inheritance

According to Mendelian inheritance, variations in an organism's
phenotype In genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular inter ...

phenotype
(observable physical and behavioral characteristics) are due in part to variations in its
genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a specific ...
(particular set of genes). Each gene specifies a particular trait with a different sequence of a gene (alleles) giving rise to different phenotypes. Most eukaryotic organisms (such as the pea plants Mendel worked on) have two alleles for each trait, one inherited from each parent. Alleles at a locus may be
dominant Domination or dominant may refer to: Society * World domination, which is mainly a conspiracy theory * Colonialism in which one group (usually a nation) invades another region for material gain or to eliminate competition * Chauvinism in which a p ...
or recessive gene, recessive; dominant alleles give rise to their corresponding phenotypes when paired with any other allele for the same trait, whereas recessive alleles give rise to their corresponding phenotype only when paired with another copy of the same allele. If you know the genotypes of the organisms, you can determine which alleles are dominant and which are recessive. For example, if the allele specifying tall stems in pea plants is dominant over the allele specifying short stems, then pea plants that inherit one tall allele from one parent and one short allele from the other parent will also have tall stems. Mendel's work demonstrated that alleles assort independently in the production of gametes, or germ cells, ensuring variation in the next generation. Although Mendelian inheritance remains a good model for many traits determined by single genes (including a number of well-known Genetic disorder#Single-gene disorder, genetic disorders) it does not include the physical processes of DNA replication and cell division.


DNA replication and cell division

The growth, development, and reproduction of organisms relies on
cell division Cell division is the process by which a parent cell (biology), cell divides into two or more daughter cells. Cell division usually occurs as part of a larger cell cycle. In eukaryotes, there are two distinct types of cell division; a vegetative ...

cell division
; the process by which a single cell (biology), cell divides into two usually identical daughter cells. This requires first making a duplicate copy of every gene in the
genome In the fields of molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...

genome
in a process called
DNA replication In molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, mo ...

DNA replication
. The copies are made by specialized enzymes known as DNA polymerases, which "read" one strand of the double-helical DNA, known as the template strand, and synthesize a new complementary strand. Because the DNA double helix is held together by base pairing, the sequence of one strand completely specifies the sequence of its complement; hence only one strand needs to be read by the enzyme to produce a faithful copy. The process of DNA replication is semiconservative replication, semiconservative; that is, the copy of the genome inherited by each daughter cell contains one original and one newly synthesized strand of DNA. The rate of DNA replication in living cells was first measured as the rate of phage T4 DNA elongation in phage-infected ''E. coli'' and found to be impressively rapid. During the period of exponential DNA increase at 37 °C, the rate of elongation was 749 nucleotides per second. After DNA replication is complete, the cell must physically separate the two copies of the genome and divide into two distinct membrane-bound cells. In prokaryotes (
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
and
archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...

archaea
) this usually occurs via a relatively simple process called binary fission, in which each circular genome attaches to the cell membrane and is separated into the daughter cells as the membrane invagination, invaginates to split the cytoplasm into two membrane-bound portions. Binary fission is extremely fast compared to the rates of cell division in eukaryotes. Eukaryotic cell division is a more complex process known as the cell cycle; DNA replication occurs during a phase of this cycle known as S phase, whereas the process of segregating
chromosome A chromosome is a long DNA molecule with part or all of the genome, genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by Chaperone (protein), chaperone proteins, bind to and ...

chromosome
s and splitting the cytoplasm occurs during M phase.


Molecular inheritance

The duplication and transmission of genetic material from one generation of cells to the next is the basis for molecular inheritance and the link between the classical and molecular pictures of genes. Organisms inherit the characteristics of their parents because the cells of the offspring contain copies of the genes in their parents' cells. In asexual reproduction, asexually reproducing organisms, the offspring will be a genetic copy or Clone (genetics), clone of the parent organism. In sexual reproduction, sexually reproducing organisms, a specialized form of cell division called meiosis produces cells called gametes or germ cells that are haploid, or contain only one copy of each gene. The gametes produced by females are called egg (biology), eggs or ova, and those produced by males are called sperm. Two gametes fuse to form a diploid fertilized egg, a single cell that has two sets of genes, with one copy of each gene from the mother and one from the father. During the process of meiotic cell division, an event called genetic recombination or ''crossing-over'' can sometimes occur, in which a length of DNA on one chromatid is swapped with a length of DNA on the corresponding homologous non-sister chromatid. This can result in reassortment of otherwise linked alleles. The Mendelian principle of independent assortment asserts that each of a parent's two genes for each trait will sort independently into gametes; which allele an organism inherits for one trait is unrelated to which allele it inherits for another trait. This is in fact only true for genes that do not reside on the same chromosome or are located very far from one another on the same chromosome. The closer two genes lie on the same chromosome, the more closely they will be associated in gametes and the more often they will appear together (known as genetic linkage). Genes that are very close are essentially never separated because it is extremely unlikely that a crossover point will occur between them.


Molecular evolution


Mutation

DNA replication is for the most part extremely accurate, however errors (mutations) do occur. The error rate in eukaryote, eukaryotic cell (biology), cells can be as low as 10−8 per
nucleotide Nucleotides are organic molecules , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's ability to Catenation, ...

nucleotide
per replication, whereas for some RNA viruses it can be as high as 10−3. This means that each generation, each human genome accumulates 1–2 new mutations. Small mutations can be caused by
DNA replication In molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, mo ...

DNA replication
and the aftermath of DNA repair, DNA damage and include point mutations in which a single base is altered and frameshift mutations in which a single base is inserted or deleted. Either of these mutations can change the gene by Missense mutation, missense (change a
codon The genetic code is the set of rules used by living cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or reli ...

codon
to encode a different amino acid) or nonsense mutation, nonsense (a premature stop codon). Larger mutations can be caused by errors in recombination to cause chromosomal abnormality, chromosomal abnormalities including the gene duplication, duplication, deletion, rearrangement or inversion of large sections of a chromosome. Additionally, DNA repair mechanisms can introduce mutational errors when repairing physical damage to the molecule. The repair, even with mutation, is more important to survival than restoring an exact copy, for example when repairing double-strand breaks. When multiple different
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is one of two, or more, forms of a given gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance ...
s for a gene are present in a species's population it is called Polymorphism (biology), polymorphic. Most different alleles are functionally equivalent, however some alleles can give rise to different
phenotypic trait A phenotypic trait, simply trait, or character state is a distinct variant of a phenotypic right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of petal color in pea plants. Th ...
s. A gene's most common allele is called the wild type, and rare alleles are called
mutant In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechan ...

mutant
s. The genetic variation in relative frequencies of different alleles in a population is due to both
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
and
genetic drift Genetic drift (allelic drift or the Sewall Wright effect) is the change in the frequency of an existing gene In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical stru ...

genetic drift
. The wild-type allele is not necessarily the ancestor of less common alleles, nor is it necessarily fitness (biology), fitter. Most mutations within genes are neutral mutation, neutral, having no effect on the organism's phenotype (silent mutations). Some mutations do not change the amino acid sequence because multiple codons encode the same amino acid (synonymous mutations). Other mutations can be neutral if they lead to amino acid sequence changes, but the protein still functions similarly with the new amino acid (e.g. conservative mutations). Many mutations, however, are deleterious mutation, deleterious or even Lethal allele, lethal, and are removed from populations by natural selection. Genetic disorders are the result of deleterious mutations and can be due to spontaneous mutation in the affected individual, or can be inherited. Finally, a small fraction of mutations are Beneficial mutation, beneficial, improving the organism's fitness (biology), fitness and are extremely important for evolution, since their directional selection leads to adaptive evolution.


Sequence homology

Genes with a most recent common ancestor, and thus a shared evolutionary ancestry, are known as Sequence homology, homologs. These genes appear either from gene duplication within an organism's genome, where they are known as paralogous genes, or are the result of divergence of the genes after a speciation event, where they are known as orthologous genes, and often perform the same or similar functions in related organisms. It is often assumed that the functions of orthologous genes are more similar than those of paralogous genes, although the difference is minimal. The relationship between genes can be measured by comparing the sequence alignment of their DNA. The degree of sequence similarity between homologous genes is called conserved sequence. Most changes to a gene's sequence do not affect its function and so genes accumulate mutations over time by Neutral theory of molecular evolution, neutral molecular evolution. Additionally, any selection on a gene will cause its sequence to diverge at a different rate. Genes under stabilizing selection are Biological constraints, constrained and so change more slowly whereas genes under directional selection change sequence more rapidly. The sequence differences between genes can be used for phylogenetic analyses to study how those genes have evolved and how the organisms they come from are related.


Origins of new genes

The most common source of new genes in eukaryotic lineages is gene duplication, which creates copy number variation of an existing gene in the genome. The resulting genes (paralogs) may then diverge in sequence and in function. Sets of genes formed in this way compose a gene family. Gene duplications and losses within a family are common and represent a major source of evolutionary biodiversity. Sometimes, gene duplication may result in a nonfunctional copy of a gene, or a functional copy may be subject to mutations that result in loss of function; such nonfunctional genes are called pseudogenes. orphan gene, "Orphan" genes, whose sequence shows no similarity to existing genes, are less common than gene duplicates. The human genome contains an estimate 18 to 60 genes with no identifiable homologs outside humans. Orphan genes arise primarily from either De novo gene birth, ''de novo'' emergence from previously non-coding sequence, or gene duplication followed by such rapid sequence change that the original relationship becomes undetectable. ''De novo'' genes are typically shorter and simpler in structure than most eukaryotic genes, with few if any introns. Over long evolutionary time periods, ''de novo'' gene birth may be responsible for a significant fraction of taxonomically-restricted gene families. Horizontal gene transfer refers to the transfer of genetic material through a mechanism other than reproduction. This mechanism is a common source of new genes in prokaryotes, sometimes thought to contribute more to genetic variation than gene duplication. It is a common means of spreading
antibiotic resistance Antimicrobial resistance (AMR or AR) occurs when microbe A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is some ...

antibiotic resistance
, virulence, and adaptive metabolism, metabolic functions. Although horizontal gene transfer is rare in eukaryotes, likely examples have been identified of protist and alga genomes containing genes of bacterial origin.


Genome

The
genome In the fields of molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...

genome
is the total genetic material of an organism and includes both the genes and non-coding sequences. Eukaryotic genes can be annotated using FINDER.


Number of genes

The genome size, and the number of genes it encodes varies widely between organisms. The smallest genomes occur in viruses, and viroids (which act as a single non-coding RNA gene). Conversely, plants can have extremely large genomes, with Oryza sativa, rice containing >46,000 protein-coding genes. The total number of protein-coding genes (the Earth's proteome) is estimated to be 5 million sequences. Although the number of base-pairs of DNA in the human genome has been known since the 1960s, the estimated number of genes has changed over time as definitions of genes, and methods of detecting them have been refined. Initial theoretical predictions of the number of human genes were as high as 2,000,000. Early experimental measures indicated there to be 50,000–100,000 ''transcribed'' genes (expressed sequence tags). Subsequently, the sequencing in the
Human Genome Project The Human Genome Project (''HGP'') was an international scientific research The scientific method is an Empirical evidence, empirical method of acquiring knowledge that has characterized the development of science since at least the 17th cen ...
indicated that many of these transcripts were Alternative splicing, alternative variants of the same genes, and the total number of protein-coding genes was revised down to ~20,000 with 13 genes encoded on the mitochondrial genome. With the GENCODE annotation project, that estimate has continued to fall to 19,000. Of the human genome, only 1–2% consists of protein-coding sequences, with the remainder being Noncoding DNA, 'noncoding' DNA such as introns, retrotransposons, and noncoding RNAs. Every multicellular organism has all its genes in each cell of its body but not every gene functions in every cell .


Essential genes

Essential genes are the set of genes thought to be critical for an organism's survival. This definition assumes the abundant availability of all relevant nutrients and the absence of environmental stress. Only a small portion of an organism's genes are essential. In bacteria, an estimated 250–400 genes are essential for ''Escherichia coli'' and ''Bacillus subtilis'', which is less than 10% of their genes. Half of these genes are orthologs in both organisms and are largely involved in protein synthesis. In the budding yeast ''Saccharomyces cerevisiae'' the number of essential genes is slightly higher, at 1000 genes (~20% of their genes). Although the number is more difficult to measure in higher eukaryotes, mice and humans are estimated to have around 2000 essential genes (~10% of their genes). The synthetic organism, ''Syn 3'', has a minimal genome of 473 essential genes and quasi-essential genes (necessary for fast growth), although 149 have unknown function. Essential genes include housekeeping genes (critical for basic cell functions) as well as genes that are expressed at different times in the organisms Developmental biology, development or biological life cycle, life cycle. Housekeeping genes are used as experimental controls when Gene expression analysis, analysing gene expression, since they are Gene expression, constitutively expressed at a relatively constant level.


Genetic and genomic nomenclature

Gene nomenclature has been established by the HUGO Gene Nomenclature Committee (HGNC), a committee of the Human Genome Organisation, for each known human gene in the form of an approved gene name and symbol (short-form abbreviation), which can be accessed through a database maintained by HGNC. Symbols are chosen to be unique, and each gene has only one symbol (although approved symbols sometimes change). Symbols are preferably kept consistent with other members of a gene family and with homologs in other species, particularly the mouse due to its role as a common model organism.


Genetic engineering

Genetic engineering is the modification of an organism's
genome In the fields of molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...

genome
through biotechnology. Since the 1970s, a Genetic engineering techniques, variety of techniques have been developed to specifically add, remove and edit genes in an organism. Recently developed genome engineering techniques use engineered nuclease enzymes to create targeted DNA repair in a
chromosome A chromosome is a long DNA molecule with part or all of the genome, genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by Chaperone (protein), chaperone proteins, bind to and ...

chromosome
to either disrupt or edit a gene when the break is repaired. The related term synthetic biology is sometimes used to refer to extensive genetic engineering of an organism. Genetic engineering is now a routine research tool with model organisms. For example, genes are easily added to
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
and lineages of knockout mice with a specific gene's function disrupted are used to investigate that gene's function. Many organisms have been genetically modified for applications in agriculture, industrial biotechnology, and medicine. For multicellular organisms, typically the embryo is engineered which grows into the adult genetically modified organism. However, the genomes of cells in an adult organism can be edited using gene therapy techniques to treat genetic diseases.


See also


References


Citations


Sources

; Main textbook * – A molecular biology textbook available free online through NCBI Bookshelf.
Glossary

Ch 1: Cells and genomes

1.1: The Universal Features of Cells on Earth

Ch 2: Cell Chemistry and Biosynthesis

2.1: The Chemical Components of a Cell

Ch 3: Proteins

Ch 4: DNA and Chromosomes

4.1: The Structure and Function of DNA

4.2: Chromosomal DNA and Its Packaging in the Chromatin Fiber

Ch 5: DNA Replication, Repair, and Recombination

5.2: DNA Replication Mechanisms

5.4: DNA Repair

5.5: General Recombination

Ch 6: How Cells Read the Genome: From DNA to Protein

6.1: DNA to RNA

6.2: RNA to Protein

Ch 7: Control of Gene Expression

7.1: An Overview of Gene Control

7.2: DNA-Binding Motifs in Gene Regulatory Proteins

7.3: How Genetic Switches Work

7.5: Posttranscriptional Controls

7.6: How Genomes Evolve

Ch 14: Energy Conversion: Mitochondria and Chloroplasts

14.4: The Genetic Systems of Mitochondria and Plastids

Ch 18: The Mechanics of Cell Division

18.1: An Overview of M Phase

18.2: Mitosis

Ch 20: Germ Cells and Fertilization

20.2: Meiosis


Further reading

* *
Google Book Search
first published 1976. * *


External links


Comparative Toxicogenomics Database

DNA From The Beginning – a primer on genes and DNA

Entrez Gene – a searchable database of genes

IDconverter – converts gene IDs between public databases

iHOP – Information Hyperlinked over Proteins

TranscriptomeBrowser – Gene expression profile analysis

The Protein Naming Utility, a database to identify and correct deficient gene names

''Genes''
nbsp;– an Open Access journal
IMPC (International Mouse Phenotyping Consortium)
nbsp;– Encyclopedia of mammalian gene function
Global Genes Project
nbsp;– Leading non-profit organization supporting people living with genetic diseases
ENCODE threads Explorer
Characterization of intergenic regions and gene definition. ''Nature (journal), Nature'' {{Authority control Cloning Genes, Molecular biology Wikipedia articles with sections published in WikiJournal of Medicine