Cytosine () (
symbol C or Cyt) is one of the four
nucleobases
Nucleobases, also known as ''nitrogenous bases'' or often simply ''bases'', are nitrogen-containing biological compounds that form nucleosides, which, in turn, are components of nucleotides, with all of these monomers constituting the bas ...
found in
DNA and
RNA, along with
adenine,
guanine
Guanine () ( symbol G or Gua) is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine ( uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside is ...
, and
thymine
Thymine () ( symbol T or Thy) is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidin ...
(
uracil in RNA). It is a
pyrimidine derivative, with a
heterocyclic aromatic ring and two substituents attached (an
amine group at position 4 and a
keto group at position 2). The
nucleoside of cytosine is
cytidine. In Watson-Crick
base pairing, it forms three
hydrogen bonds with
guanine
Guanine () ( symbol G or Gua) is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine ( uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside is ...
.
History
Cytosine was discovered and named by
Albrecht Kossel and Albert Neumann in 1894 when it was hydrolyzed from calf
thymus
The thymus is a specialized primary lymphoid organ of the immune system. Within the thymus, thymus cell lymphocytes or '' T cells'' mature. T cells are critical to the adaptive immune system, where the body adapts to specific foreign invaders ...
tissues. A structure was proposed in 1903, and was synthesized (and thus confirmed) in the laboratory in the same year.
In 1998, cytosine was used in an early demonstration of
quantum information processing when Oxford University researchers implemented the
Deutsch-Jozsa algorithm on a two
qubit nuclear magnetic resonance quantum computer (NMRQC).
In March 2015, NASA scientists reported the formation of cytosine, along with uracil and thymine, from
pyrimidine under the space-like laboratory conditions, which is of interest because pyrimidine has been found in meteorites although its origin is unknown.
Chemical reactions
Cytosine can be found as part of DNA, as part of RNA, or as a part of a
nucleotide. As
cytidine triphosphate (CTP), it can act as a co-factor to enzymes, and can transfer a phosphate to convert
adenosine diphosphate (ADP) to
adenosine triphosphate (ATP).
In DNA and RNA, cytosine is paired with
guanine
Guanine () ( symbol G or Gua) is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine ( uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside is ...
. However, it is inherently unstable, and can change into
uracil (
spontaneous deamination). This can lead to a
point mutation if not repaired by the
DNA repair enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
s such as uracil glycosylase, which cleaves a uracil in DNA.
Cytosine can also be
methylated into
5-methylcytosine by an enzyme called
DNA methyltransferase or be methylated and
hydroxylated to make
5-hydroxymethylcytosine. The difference in rates of deamination of cytosine and 5-methylcytosine (to uracil and
thymine
Thymine () ( symbol T or Thy) is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidin ...
) forms the basis of
bisulfite sequencing
Bisulfite sequencing (also known as bisulphite sequencing) is the use of bisulfite treatment of DNA before routine sequencing to determine the pattern of methylation. DNA methylation was the first discovered epigenetic mark, and remains the mo ...
.
Biological function
When found third in a
codon of
RNA, cytosine is synonymous with
uracil, as they are interchangeable as the third base.
When found as the second base in a codon, the third is always interchangeable. For example, UCU, UCC, UCA and UCG are all
serine, regardless of the third base.
Active enzymatic deamination of cytosine or 5-methylcytosine by the
APOBEC family of cytosine deaminases could have both beneficial and detrimental implications on various cellular processes as well as on organismal evolution.
The implications of deamination on 5-hydroxymethylcytosine, on the other hand, remains less understood.
Theoretical aspects
Until October 2021, Cytosine had not been found in meteorites, which suggested the first strands of RNA and DNA had to look elsewhere to obtain this building block. Cytosine likely formed within some meteorite parent bodies, however did not persist within these bodies due to an effective
deamination reaction into
uracil.
In October 2021, Cytosine was announced as having been found in meteorites by researchers in a joint Japan/NASA project, that used novel methods of detection which avoided damaging nucleotides as they were extracted from meteorites.
References
External links and citations
Cytosine MS Spectrum*
*
{{Authority control
Nucleobases
Amines
Pyrimidones