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Genetic recombination (also known as genetic reshuffling) is the exchange of
genetic material
genetic material
between different
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 interactions, Physiology, physiological me ...

organism
s which leads to production of offspring with combinations of traits that differ from those found in either parent. In
eukaryote Eukaryotes () are 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 (biology), tax ...

eukaryote
s, genetic recombination during
meiosis Meiosis (; , because it is a reductional division) is a special type of of in organisms used to produce the , such as or . It involves two rounds of division that ultimately result in four cells with only one copy of each (). Additionall ...

meiosis
can lead to a novel set of
genetic
genetic
information that can be passed on from the
parent A parent is a caregiver of the offspring in their own species. In humans, a parent is the caretaker of a child (where "child" refers to offspring, not necessarily age). A ''biological parent'' is a person whose gamete resulted in a child, a male ...

parent
s to the offspring. Most recombination is naturally occurring. During
meiosis Meiosis (; , because it is a reductional division) is a special type of of in organisms used to produce the , such as or . It involves two rounds of division that ultimately result in four cells with only one copy of each (). Additionall ...

meiosis
in eukaryotes, genetic recombination involves the pairing of
homologous chromosome A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome A chromosome is a long DNA The structure of part of a DNA double helix Deoxyribonucleic acid (; DNA) is a molecule File:Pen ...
s. This may be followed by information transfer between the chromosomes. The information transfer may occur without physical exchange (a section of genetic material is copied from one chromosome to another, without the donating chromosome being changed) (see SDSA pathway in Figure); or by the breaking and rejoining 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
strands, which forms new molecules of DNA (see DHJ pathway in Figure). Recombination may also occur during
mitosis 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 proce ...

mitosis
in eukaryotes where it ordinarily involves the two sister chromosomes formed after chromosomal replication. In this case, new combinations of
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 are not produced since the sister chromosomes are usually identical. In meiosis and mitosis, recombination occurs between similar molecules of DNA (
homologous sequence Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life The history of life on Earth Earth is the third planet from the Sun and the ...
s). In meiosis, non-sister homologous chromosomes pair with each other so that recombination characteristically occurs between non-sister homologues. In both meiotic and mitotic cells, recombination between homologous chromosomes is a common mechanism used in
DNA repair DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome In the fields of molecular biology and genetics Genetics is a branch of biology concerned with th ...

DNA repair
.
Gene conversionGene conversion is the process by which one DNA sequence replaces a homologous sequence such that the sequences become identical after the conversion event. Gene conversion can be either allelic, meaning that one allele of the same gene replaces anot ...
- the process during which homologous sequences are made identical also falls under genetic recombination. Genetic recombination and recombinational
DNA repair DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome In the fields of molecular biology and genetics Genetics is a branch of biology concerned with th ...

DNA repair
also occurs in
bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the sm ...

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
, which use
asexual reproduction Asexual reproduction is a type of that does not involve the fusion of s or change in the number of . The offspring that arise by asexual reproduction from either unicellular or s inherit the full set of genes of their single parent. Asexual rep ...
. Recombination can be artificially induced in laboratory (''in vitro'') settings, producing
recombinant DNA Recombinant DNA (rDNA) molecules are 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 ...

recombinant DNA
for purposes including
vaccine A vaccine is a biological preparation that provides active acquired immunity to a particular infectious disease. A vaccine typically contains an agent that resembles a disease-causing microorganism and is often made from weakened or killed for ...

vaccine
development.
V(D)J recombination V(D)J recombination is the mechanism of somatic recombination that occurs only in developing lymphocyte A lymphocyte is a type of white blood cell in the immune system The immune system is a network of biological processes that protects an ...
in organisms with an
adaptive immune system The adaptive immune system, also referred as the acquired immune system, is a subsystem of the immune system The immune system is a network of biological processes that protects an organism In biology, an organism (from Ancient Greek, G ...
is a type of site-specific genetic recombination that helps immune cells rapidly diversify to recognize and adapt to new
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory ...
s.


Synapsis

During meiosis, synapsis (the pairing of homologous chromosomes) ordinarily precedes genetic recombination.


Mechanism

Genetic recombination is
catalyzed that utilizes a low-temperature oxidation catalyst to convert carbon monoxide to less toxic carbon dioxide at room temperature. It can also remove formaldehyde from the air. Catalysis () is the process of increasing the reaction rate, rate of a ...

catalyzed
by many different
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), substrates, and the enzyme converts the substrates in ...

enzyme
s.
Recombinase Recombinases are genetic recombination Genetic recombination (also known as genetic reshuffling) is the exchange of DNA, genetic material between different organisms which leads to production of offspring with combinations of traits that differ fro ...
s are key enzymes that catalyse the strand transfer step during recombination.
RecA RecA is a 38 kilodalton The dalton or unified atomic mass unit (symbols: Da or u) is a unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of ac ...

RecA
, the chief recombinase found in ''
Escherichia coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative bacteria, Gram-negative, Facultative anaerobic organism, facultative anaer ...

Escherichia coli
'', is responsible for the repair of DNA double strand breaks (DSBs). In yeast and other eukaryotic organisms there are two recombinases required for repairing DSBs. The
RAD51 DNA repair protein RAD51 homolog 1 is a protein encoded by the 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 ...
protein is required for
mitotic In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the total number of chromosomes is maintained. In gene ...

mitotic
and
meiotic Meiosis (; , because it is a reductional division) is a special type of cell division of germ cells in Sexual reproduction, sexually-reproducing organisms used to produce the gametes, such as sperm or egg cells. It involves two rounds of divi ...

meiotic
recombination, whereas the DNA repair protein,
DMC1 Meiotic recombination protein DMC1/LIM15 homolog is a protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one ...
, is specific to meiotic recombination. In the archaea, the
ortholog Sequence homology is the biological homology between 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 f ...
of the bacterial RecA protein is RadA. ;Bacterial recombination In
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
there are: *regular
bacterial recombinationBacterial recombination is a type of genetic recombination Genetic recombination (also known as genetic reshuffling) is the exchange of DNA, genetic material between different organisms which leads to production of offspring with combinations of tra ...
, as well as noneffective transfer of
genetic material Nucleic acids are biopolymer Biopolymers are natural polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese busin ...
, expressed as * unsuccessful transfer or abortive transfer which is any
bacterial Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell. They constitute a large domain of prokaryotic microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology, an ...
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
transfer of the donor cell to recipients who have set the incoming DNA as part of the genetic material of the recipient. Abortive transfer was registered in the following transduction and conjugation. In all cases, the transmitted fragment is diluted by the culture growth.


Chromosomal crossover

In
eukaryotes Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym for "Outline ...
, recombination during
meiosis Meiosis (; , because it is a reductional division) is a special type of of in organisms used to produce the , such as or . It involves two rounds of division that ultimately result in four cells with only one copy of each (). Additionall ...

meiosis
is facilitated by
chromosomal crossover Chromosomal crossover, or crossing over, is the exchange of genetic material during sexual reproduction between two homologous chromosomes' sister chromatids, non-sister chromatids that results in recombinant chromosomes. It is one of the final ...
. The crossover process leads to offspring having different combinations of genes from those of their parents, and can occasionally produce new chimeric
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. The shuffling of genes brought about by genetic recombination produces increased
genetic variation thumb File:Genetic Variation and Inheritance.svg, Parents have similar gene coding in this specific situation where they reproduce and variation in the offspring is seen. Offspring containing the variation also reproduce and passes down traits t ...

genetic variation
. It also allows sexually reproducing organisms to avoid
Muller's ratchet In evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes that are passed on from pa ...
, in which 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
s of an asexual
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
tend to accumulate more deleterious mutations over time than other types of beneficial or reversing mutations. Chromosomal crossover involves recombination between the paired
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 inherited from each of one's parents, generally occurring during
meiosis Meiosis (; , because it is a reductional division) is a special type of of in organisms used to produce the , such as or . It involves two rounds of division that ultimately result in four cells with only one copy of each (). Additionall ...

meiosis
. During
prophase I spermatocyte, played back at 120× the recorded speed Meiosis (; from Greek language, Greek μείωσις, ''meiosis'', meaning "lessening") is a special type of cell division Cell division is the process by which a parent Cell (biology), ...
(pachytene stage) the four available
chromatid In the diagram, (1) refers to a chromatid: 1-half of two identical threadlike strands of a replicated sister chromatid pair") are joined at the region called the centromere">ister_chromatids.html" ;"title="chromosome. During cell division, the ide ...
s are in tight formation with one another. While in this formation, homologous sites on two chromatids can closely pair with one another, and may exchange genetic information. Because recombination can occur with small probability at any location along chromosome, the frequency of recombination between two locations depends on the distance separating them. Therefore, for genes sufficiently distant on the same chromosome, the amount of crossover is high enough to destroy the correlation between
alleles 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 ...

alleles
. Tracking the movement of genes resulting from crossovers has proven quite useful to geneticists. Because two genes that are close together are less likely to become separated than genes that are farther apart, geneticists can deduce roughly how far apart two genes are on a chromosome if they know the frequency of the crossovers. Geneticists can also use this method to infer the presence of certain genes. Genes that typically stay together during recombination are said to be linked. One gene in a linked pair can sometimes be used as a marker to deduce the presence of another gene. This is typically used in order to detect the presence of a disease-causing gene. The recombination frequency between two loci observed is the '' crossing-over value''. It is the frequency of crossing over between two linked
gene 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 mecha ...

gene
loci ( markers), and depends on the mutual distance of the genetic
loci 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 ...
observed. For any fixed set of genetic and environmental conditions, recombination in a particular region of a linkage structure (
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
) tends to be constant, and the same is then true for the crossing-over value which is used in the production of
genetic map Genetic linkage is the tendency of DNA sequences that are close together on a chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins ...

genetic map
s.


Gene conversion

In gene conversion, a section of genetic material is copied from one chromosome to another, without the donating chromosome being changed. Gene conversion occurs at high frequency at the actual site of the recombination event during
meiosis Meiosis (; , because it is a reductional division) is a special type of of in organisms used to produce the , such as or . It involves two rounds of division that ultimately result in four cells with only one copy of each (). Additionall ...

meiosis
. It is a process by which a DNA sequence is copied from one DNA helix (which remains unchanged) to another DNA helix, whose sequence is altered. Gene conversion has often been studied in fungal crosses where the 4 products of individual meioses can be conveniently observed. Gene conversion events can be distinguished as deviations in an individual meiosis from the normal 2:2 segregation pattern (e.g. a 3:1 pattern).


Nonhomologous recombination

Recombination can occur between DNA sequences that contain no sequence homology. This can cause
chromosomal translocation 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 interactions ...
s, sometimes leading to cancer.


In B cells

B cells #REDIRECT B cell 3D rendering of a B cell B cells, also known as B lymphocytes, are a type of white blood cell White blood cells (WBCs), also called leukocytes or leucocytes, are the cells of the immune system The immune system is a ne ...
of the
immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as Tumor immunology, cancer cells and objects such ...
perform genetic recombination, called
immunoglobulin class switching Immunoglobulin class switching, also known as isotype switching, isotypic commutation or class-switch recombination (CSR), is a biological mechanism that changes a B cell's production of immunoglobulin An antibody (Ab), also known as an immun ...
. It is a biological mechanism that changes an
antibody An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and Viral disease, viruses. The antibody recognizes a unique mo ...

antibody
from one class to another, for example, from an isotype called
IgM Immunoglobulin M (IgM) is one of several isotypes of antibody An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacte ...

IgM
to an isotype called
IgG Immunoglobulin G (IgG) is a type Type may refer to: Science and technology Computing * Typing, producing text via a keyboard, typewriter, etc. * Data type, collection of values used for computations. * File type * TYPE (DOS command), a command to ...
.


Genetic engineering

In
genetic engineering Genetic engineering, also called genetic modification or genetic manipulation, is the direct manipulation of an organism's gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_in ...
, recombination can also refer to artificial and deliberate recombination of disparate pieces of DNA, often from different organisms, creating what is called
recombinant DNA Recombinant DNA (rDNA) molecules are 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 ...

recombinant DNA
. A prime example of such a use of genetic recombination is
gene targeting mouse gene targeted for the agouti coat color gene, with its offspring Gene targeting (also, replacement strategy based on homologous recombination) is a genetics, genetic technique that uses homologous recombination to modify an endogenous Endo ...
, which can be used to add, delete or otherwise change an organism's genes. This technique is important to
biomedical research Medical research (or biomedical research), also known as experimental medicine, encompasses a wide array of research, extending from "basic research Basic research, also called pure research or fundamental research, is a type of scientific r ...
ers as it allows them to study the effects of specific genes. Techniques based on genetic recombination are also applied in
protein engineeringProtein engineering is the process of developing useful or valuable protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast ar ...
to develop new proteins of biological interest.


Recombinational repair

DNA damages caused by a variety of exogenous agents (e.g.
UV light Ultraviolet (UV) is a form of with from 10 (with a corresponding frequency around 30 ) to 400 nm (750 ), shorter than that of , but longer than s. UV radiation is present in , and constitutes about 10% of the total electromagnetic radi ...

UV light
,
X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Moti ...

X-ray
s, chemical
cross-link In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * Chin ...
ing agents) can be repaired by homologous recombinational repair (HRR). These findings suggest that DNA damages arising from natural processes, such as exposure to reactive oxygen species that are byproducts of normal metabolism, are also repaired by HRR. In humans, deficiencies in the gene products necessary for HRR during meiosis likely cause infertility In humans, deficiencies in gene products necessary for HRR, such as
BRCA1 Breast cancer type 1 susceptibility protein is 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 Si ...

BRCA1
and
BRCA2 ''BRCA2'' and BRCA2 () are a human 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 ' ...

BRCA2
, increase the risk of cancer (see
DNA repair-deficiency disorder A DNA repair-deficiency disorder is a medical condition due to reduced functionality of DNA repair DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome In t ...
). In bacteria, transformation is a process of gene transfer that ordinarily occurs between individual cells of the same bacterial species. Transformation involves integration of donor DNA into the recipient chromosome by recombination. This process appears to be an adaptation for repairing DNA damages in the recipient chromosome by HRR. Transformation may provide a benefit to pathogenic bacteria by allowing repair of DNA damage, particularly damages that occur in the inflammatory, oxidizing environment associated with infection of a host. When two or more viruses, each containing lethal genomic damages, infect the same host cell, the virus genomes can often pair with each other and undergo HRR to produce viable progeny. This process, referred to as multiplicity reactivation, has been studied in lambda and
T4 bacteriophage T4 or T-4 may refer to: Airports and airlines * Heathrow Terminal 4 * Tiyas Military Airbase, also known as the T-4 Airbase Biology and medicine * T4 phage, a bacteriophage * Thyroxine (T4), a form of thyroid hormone * the T4 spinal nerve * th ...

T4 bacteriophage
s, as well as in several pathogenic viruses. In the case of pathogenic viruses, multiplicity reactivation may be an adaptive benefit to the virus since it allows the repair of DNA damages caused by exposure to the oxidizing environment produced during host infection. See also
reassortment 300px, thumb , The process of reassortment in biotechnology Reassortment is the mixing of the genetic material of a species into new combinations in different individuals. Several different processes contribute to reassortment, including assortme ...

reassortment
.


Meiotic recombination

Molecular models of meiotic recombination have evolved over the years as relevant evidence accumulated. A major incentive for developing a fundamental understanding of the mechanism of meiotic recombination is that such understanding is crucial for solving the problem of the adaptive function of sex, a major unresolved issue in biology. A recent model that reflects current understanding was presented by Anderson and Sekelsky, and is outlined in the first figure in this article. The figure shows that two of the four chromatids present early in meiosis (prophase I) are paired with each other and able to interact. Recombination, in this version of the model, is initiated by a double-strand break (or gap) shown in the DNA molecule (chromatid) at the top of the first figure in this article. However, other types of DNA damage may also initiate recombination. For instance, an inter-strand cross-link (caused by exposure to a cross-linking agent such as mitomycin C) can be repaired by HRR. As indicated in the first figure, above, two types of recombinant product are produced. Indicated on the right side is a “crossover” (CO) type, where the flanking regions of the chromosomes are exchanged, and on the left side, a “non-crossover” (NCO) type where the flanking regions are not exchanged. The CO type of recombination involves the intermediate formation of two “Holliday junctions” indicated in the lower right of the figure by two X shaped structures in each of which there is an exchange of single strands between the two participating chromatids. This pathway is labeled in the figure as the DHJ (double-Holliday junction) pathway. The NCO recombinants (illustrated on the left in the figure) are produced by a process referred to as “synthesis dependent strand annealing” (SDSA). Recombination events of the NCO/SDSA type appear to be more common than the CO/DHJ type. The NCO/SDSA pathway contributes little to genetic variation, since the arms of the chromosomes flanking the recombination event remain in the parental configuration. Thus, explanations for the adaptive function of meiosis that focus exclusively on crossing-over are inadequate to explain the majority of recombination events.


Achiasmy and heterochiasmy

Achiasmy is the phenomenon where autosomal recombination is completely absent in one sex of a species. Achiasmatic chromosomal segregation is well documented in male ''''. Heterochiasmy occurs when recombination rates differ between the sexes of a species. This sexual dimorphic pattern in recombination rate has been observed in many species. In mammals, females most often have higher rates of recombination. The "Haldane-Huxley rule" states that achiasmy usually occurs in the
heterogametic sex Heterogametic sex (digametic sex) refers to the sex of a species in which the sex chromosomes are not the same. For example, in humans, males, with an X and a Y sex chromosome, would be referred to as the heterogametic sex, and females having tw ...
.


RNA virus recombination

Numerous RNA viruses are capable of genetic recombination when at least two viral
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
s are present in the same host cell. Recombination is largely responsible for RNA virus diversity and immune evasion. RNA recombination appears to be a major driving force in determining genome architecture and the course of viral evolution among
picornaviridae Picornaviruses are a group of related Viral envelope, nonenveloped RNA viruses which infect vertebrates including fish, mammals, and birds. They are viruses that represent a large family of small, Positive-sense single-stranded RNA virus, positiv ...

picornaviridae
( (+)ssRNA) (e.g.
poliovirus A poliovirus, the causative agent of polio Poliomyelitis, commonly shortened to polio, is an infectious disease An infection is the invasion of an organism's body Tissue (biology), tissues by Pathogen, disease-causing agents, their multip ...
). In the
retroviridae A retrovirus is a type of virus A virus is a submicroscopic infectious agent that Viral replication, replicates only inside the living Cell (biology), cells of an organism. Viruses infect all types of life forms, from animals and plant ...

retroviridae
((+)ssRNA)(e.g.
HIV The human immunodeficiency viruses (HIV) are two species of ''Lentivirus ''Lentivirus'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, inc ...

HIV
), damage in the RNA genome appears to be avoided during
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 ...
by strand switching, a form of recombination. Recombination also occurs in the
reoviridae ''Reoviridae'' is a family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of families is to maintain the ...
(dsRNA)(e.g. reovirus),
orthomyxoviridae ''Orthomyxoviridae'' (ὀρθός, ''orthós'', Greek for "straight"; μύξα, ''mýxa'', Greek for "mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mu ...
((-)ssRNA)(e.g.
influenza virus ''Orthomyxoviridae'' (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 ...

influenza virus
) and
coronaviridae ''Coronaviridae'' is a family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain the well-being of its members and of soc ...
((+)ssRNA) (e.g.
SARS Severe acute respiratory syndrome (SARS) is a viral respiratory disease Respiratory diseases, or lung diseases, are pathology, pathological conditions affecting the organs and tissues that make gas exchange difficult in Breathing, air-breathing ...
). Recombination in RNA viruses appears to be an adaptation for coping with genome damage. Switching between template strands during genome replication, referred to as copy-choice recombination, was originally proposed to explain the positive correlation of recombination events over short distances in organisms with a DNA genome (see first Figure, SDSA pathway). Recombination can occur infrequently between animal viruses of the same species but of divergent lineages. The resulting recombinant viruses may sometimes cause an outbreak of infection in humans. Especially in coronaviruses, recombination may also occur even among distantly related evolutionary groups (subgenera), due to their characteristic transcription mechanism, that involves subgenomic mRNAs that are formed by template switching. When replicating its (+)ssRNA genome, the
poliovirus A poliovirus, the causative agent of polio Poliomyelitis, commonly shortened to polio, is an infectious disease An infection is the invasion of an organism's body Tissue (biology), tissues by Pathogen, disease-causing agents, their multip ...
RNA-dependent RNA polymerase RNA-dependent RNA polymerase (RdRp) or RNA replicase is an enzyme Enzymes () are s that act as s (biocatalysts). Catalysts accelerate . The molecules upon which enzymes may act are called , and the enzyme converts the substrates into diff ...
(RdRp) is able to carry out recombination. Recombination appears to occur by a copy choice mechanism in which the RdRp switches (+)ssRNA templates during negative strand synthesis. Recombination by RdRp strand switching also occurs in the (+)ssRNA plant carmoviruses and tombusviruses. Recombination appears to be a major driving force in determining genetic variability within coronaviruses, as well as the ability of coronavirus species to jump from one host to another and, infrequently, for the emergence of novel species, although the mechanism of recombination in is unclear. During the first months of the COVID-19 pandemic, such a recombination event was suggested to have been a critical step in the evolution of SARS-CoV-2's ability to infect humans. SARS-CoV-2's entire receptor binding motif appeared, based on preliminary observations, to have been introduced through recombination from coronaviruses of
pangolin Pangolins, sometimes known as scaly anteaters, are mammals Mammals (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in ...

pangolin
s. However, more comprehensive analyses later refuted this suggestion and showed that SARS-CoV-2 likely evolved solely within bats and with little or no recombination.


Role of recombination in the origin of life

Nowak and Ohtsuki noted that the origin of life (
abiogenesis In evolutionary biology Evolutionary biology is the subfield of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molec ...
) is also the origin of biological
evolution Evolution is change in the heritable 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, ...

evolution
. They pointed out that all known life on earth is based on
biopolymer Biopolymers are natural polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries ...
s and proposed that any theory for the origin of life must involve biological polymers that act as information carriers and catalysts. Lehman argued that recombination was an evolutionary development as ancient as the origins of life. Smail et al.Smail BA, Clifton BE, Mizuuchi R, Lehman N. Spontaneous advent of genetic diversity in RNA populations through multiple recombination mechanisms. RNA. 2019 Apr;25(4):453-464. doi: 10.1261/rna.068908.118. Epub 2019 Jan 22. PMID 30670484; PMCID: PMC6426292 proposed that in the primordial Earth, recombination played a key role in the expansion of the initially short informational polymers (presumed to be
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
) that were the precursors to life.


See also

*
Eukaryote hybrid genome Eukaryote hybrid genomes result from interspecific hybridization, where closely related species mate and produce offspring with Genetic admixture, admixed genomes. The advent of large-scale genomic sequencing has shown that Hybridisation (biology), ...
* Four-gamete test *Independent assortment *Recombination frequency *Recombination hotspot *Site-specific recombinase technology *Site-specific recombination *Reassortment *
V(D)J recombination V(D)J recombination is the mechanism of somatic recombination that occurs only in developing lymphocyte A lymphocyte is a type of white blood cell in the immune system The immune system is a network of biological processes that protects an ...


References


External links


Animations – homologous recombination
Animations showing several models of homologous recombination
The Holliday Model of Genetic Recombination
*

{{Portal bar, Biology Cellular processes Modification of genetic information Molecular genetics