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Nucleobases, also known as ''nitrogenous bases'' or often simply ''bases'', are nitrogen-containing biological compounds that form
nucleosides Nucleosides are glycosylamines that can be thought of as nucleotide Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers deoxyribonucleic acid (DNA) and ribonuc ...
, which, in turn, are components 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
s, with all of these
monomer In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in th ...

monomer
s constituting the basic building blocks of
nucleic acids 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 ...
. The ability of nucleobases to form
base pair 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_ ...
s and to stack one upon another leads directly to long-chain helical structures such as
ribonucleic acid 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(111) STM.jpg, A scanning tunneling microscopy image of ...
(RNA) and
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). Five nucleobases—
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
(A),
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
(C),
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
(G),
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
(T), and
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
(U)—are called ''primary'' or ''canonical''. They function as the fundamental units of 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
, with the bases A, G, C, and T being found in DNA while A, G, C, and U are found in RNA. Thymine and uracil are distinguished by merely the presence or absence of a methyl group on the fifth carbon (C5) of these heterocyclic six-membered rings. In addition, some viruses have
aminoadenine
aminoadenine
(Z) instead of adenine. It differs in having an extra amine group, creating a more stable bond to thymine. Adenine and guanine have a fused-ring skeletal structure derived of
purine Purine is a heterocyclic 125px, Pyridine, a heterocyclic compound A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different chemical element, elements as members of its ring(s). Heterocyclic chemi ...

purine
, hence they are called purine bases. The purine nitrogenous bases are characterized by their single
amino group In organic chemistry, amines (, ) are organic compound, compounds and functional groups that contain a base (chemistry), basic nitrogen atom with a lone pair. Amines are formally derivative (chemistry), derivatives of ammonia, wherein one or ...
(NH2), at the C6 carbon in adenine and C2 in guanine. Similarly, the simple-ring structure of cytosine, uracil, and thymine is derived of
pyrimidine Pyrimidine is an aromatic In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts ...

pyrimidine
, so those three bases are called the pyrimidine bases. Each of the base pairs in a typical double-
helix A helix (), plural helixes or helices (), is a shape like a corkscrew or spiral staircase. It is a type of smooth Smooth may refer to: Mathematics * Smooth function is a smooth function with compact support. In mathematical analysis, the ...

helix
DNA comprises a purine and a pyrimidine: either an A paired with a T or a C paired with a G. These purine-pyrimidine pairs, which are called ''base complements'', connect the two strands of the helix and are often compared to the rungs of a ladder. The pairing of purines and pyrimidines may result, in part, from dimensional constraints, as this combination enables a geometry of constant width for the DNA spiral helix. The A–T and C–G pairings are based on double or triple
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 between the
amine In organic chemistry Organic chemistry is a branch of 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, ...

amine
and
carbonyl In organic chemistry Organic chemistry is a branch of 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, pro ...

carbonyl
groups on the complementary bases. Nucleobases such as adenine, guanine,
xanthine Xanthine ( or ; archaically xanthic acid; systematic name 3,7-dihydropurine-2,6-dione) is a purine Purine is a heterocyclic compound, heterocyclic aromatic organic compound that consists of two rings (pyrimidine and imidazole). It is water-solubl ...

xanthine
,
hypoxanthine Hypoxanthine is a naturally occurring purine Purine is a heterocyclic compound, heterocyclic aromatic organic compound that consists of two rings (pyrimidine and imidazole). It is water-soluble. Purine also gives its name to the wider class of mo ...

hypoxanthine
, purine,
2,6-diaminopurine
2,6-diaminopurine
, and 6,8-diaminopurine may have formed in outer space as well as on earth. The origin of the term ''
base Base or BASE may refer to: Brands and enterprises * Base (mobile telephony provider), a Belgian mobile telecommunications operator *Base CRM Base CRM (originally Future Simple or PipeJump) is an enterprise software company based in Mountain Vie ...
'' reflects these compounds' chemical properties in
acid–base reaction An acid–base reaction is a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In classical physics and general che ...
s, but those properties are not especially important for understanding most of the biological functions of nucleobases.


Structure

At the sides of nucleic acid structure, phosphate molecules successively connect the two sugar-rings of two adjacent nucleotide monomers, thereby creating a long chain
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 Nobel Prize in Chemistry, Nobel Prize i ...
. These chain-joins of phosphates with sugars (
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
or
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
) create the "backbone" strands for a single- or double helix biomolecule. In the double helix of DNA, the two strands are oriented chemically in opposite directions, which permits base pairing by providing complementarity between the two bases, and which is essential for
replication
replication
of or transcription of the encoded information found in DNA.


Modified nucleobases

DNA and RNA also contain other (non-primary) bases that have been modified after the nucleic acid chain has been formed. In DNA, the most common modified base is
5-methylcytosine
5-methylcytosine
(m5C). In RNA, there are many modified bases, including those contained in the nucleosides
pseudouridine Pseudouridine (abbreviated by the Greek letter psi- Ψ) is an isomer of the nucleoside uridine in which the uracil is attached via a carbon-carbon instead of a nitrogen-carbon glycosidic bond. (In this configuration, uracil is sometimes referred ...

pseudouridine
(Ψ),
dihydrouridine Dihydrouridine (abbreviated as D, DHU, or UH2) is a pyrimidine Pyrimidine is an aromatic heterocyclic compound, heterocyclic organic compound similar to pyridine. One of the three diazines (six-membered heterocyclics with two nitrogen atoms in ...

dihydrouridine
(D),
inosine Inosine is a nucleoside Nucleosides are s that can be thought of as s without a . A nucleoside consists simply of a (also termed a nitrogenous base) and a five-carbon sugar ( or 2'-deoxyribose) whereas a nucleotide is composed of a nucleobase, ...

inosine
(I), and
7-methylguanosine 7-Methylguanosine (m7G) is a modified purine Purine is a heterocyclic 125px, Pyridine, a heterocyclic compound A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different chemical element, element ...

7-methylguanosine
(m7G).
Hypoxanthine Hypoxanthine is a naturally occurring purine Purine is a heterocyclic 125px, Pyridine, a heterocyclic compound A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different chemical element, elements ...

Hypoxanthine
and
xanthine Xanthine ( or ; archaically xanthic acid; systematic name 3,7-dihydropurine-2,6-dione) is a purine Purine is a heterocyclic compound, heterocyclic aromatic organic compound that consists of two rings (pyrimidine and imidazole). It is water-solubl ...

xanthine
are two of the many bases created through
mutagen In genetics, a mutagen is a physical or chemical agent that permanently changes genetic material, usually DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pent ...
presence, both of them through
deamination Deamination is the removal of an amino group In organic chemistry, amines (, ) are organic compound, compounds and functional groups that contain a base (chemistry), basic nitrogen atom with a lone pair. Amines are formally derivative (chemi ...
(replacement of the amine-group with a carbonyl-group). Hypoxanthine is produced from adenine, xanthine from guanine, and uracil results from deamination of cytosine.


Modified purine nucleobases

These are examples of modified adenosine or guanosine.


Modified pyrimidine nucleobases

These are examples of modified cytosine, thymine or uridine.


Artificial nucleobases

A vast number of nucleobase analogues exist. The most common applications are used as fluorescent probes, either directly or indirectly, such as
aminoallyl nucleotide Aminoallyl nucleotide is a nucleotide with a modified base containing an allylamine. They are used in post-labeling of nucleic acids by fluorescence detection in microarray. They are reactive with N-Hydroxysuccinimide ester group which helps attach ...
, which are used to label cRNA or cDNA in
microarrays A microarray is a multiplex (assay), multiplex lab-on-a-chip. It is a two-dimensional array on a Substrate (materials science), solid substrate—usually a glass slide or silicon thin-film cell—that assays (tests) large amounts of biotic materi ...
. Several groups are working on alternative "extra" base pairs to extend the genetic code, such as
isoguanine
isoguanine
and
isocytosine
isocytosine
or the fluorescent
2-amino-6-(2-thienyl)purine
2-amino-6-(2-thienyl)purine
and pyrrole-2-carbaldehyde. In medicine, several
nucleoside analogue Nucleoside analogues are nucleoside Nucleosides are s that can be thought of as s without a . A nucleoside consists simply of a (also termed a nitrogenous base) and a five-carbon sugar ( or 2'-deoxyribose) whereas a nucleotide is compose ...
s are used as anticancer and antiviral agents. The viral polymerase incorporates these compounds with non-canonical bases. These compounds are activated in the cells by being converted into nucleotides; they are administered as
nucleoside Nucleosides are glycosylamine 120px, Cyclic hemiaminal ether bond derived from an aldehyde Glycosylamines are a class of biochemical compounds consisting of a Glycosyl, glycosyl group attached to an amine, amino group, -NR2. They are also known ...

nucleoside
s as charged nucleotides cannot easily cross cell membranes. At least one set of new base pairs has been announced as of May 2014.


Prebiotic condensation of nucleobases with ribose

In order to understand how
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
arose knowledge is required of chemical pathways that permit formation of the key building blocks of life under plausible prebiotic conditions. According to the
RNA world The RNA world is a hypothetical stage in the evolutionary history of life The history of life on Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surfa ...
hypothesis free-floating
ribonucleotide In biochemistry, a ribonucleotide is a nucleotide containing ribose as its pentose component. It is considered a molecular precursor of nucleic acids. Nucleotides are the basic building blocks of DNA and RNA. Ribonucleotides themselves are basic mon ...
s were present in the primitive soup. These were the fundamental molecules that combined in series to form
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
. Molecules as complex as RNA must have arisen from small molecules whose reactivity was governed by physico-chemical processes. RNA is composed of
purine Purine is a heterocyclic 125px, Pyridine, a heterocyclic compound A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different chemical element, elements as members of its ring(s). Heterocyclic chemi ...

purine
and
pyrimidine Pyrimidine is an aromatic In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts ...

pyrimidine
nucleotides, both of which are necessary for reliable information transfer, and thus Darwinian
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
. Nam et al. demonstrated the direct condensation of nucleobases with ribose to give ribonucleosides in aqueous microdroplets, a key step leading to RNA formation. Similar results were obtained by Becker et al.Becker S, Feldmann J, Wiedemann S, Okamura H, Schneider C, Iwan K, Crisp A, Rossa M, Amatov T, Carell T. Unified prebiotically plausible synthesis of pyrimidine and purine RNA ribonucleotides. Science. 2019 Oct 4;366(6461):76-82. doi: 10.1126/science.aax2747. PMID 31604305.


See also

* * * *


References


External links


Base pairing in DNA Double Helix (shows specific hydrogen bonds)
{{Authority control DNA RNA