Cytosine was discovered and named by Albrecht Kossel and Albert Neumann in 1894 when it was hydrolyzed from calf thymus tissues.[4][5] A structure was proposed in 1903, and was synthesized (and thus confirmed) in the laboratory in the same year.
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.[7]
Cytosine and guanine with the direction of hydrogen bonding indicated (arrow points positive to negative charge). Methylation of cytosine occurs on carbon number 5.
In DNA and RNA, cytosine is paired with guanine. 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 repairenzymes such as uracil glycosylase, which cleaves a uracil in DNA.
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.[9] 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.[10]
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.[11]
References
^Dawson, R.M.C.; et al. (1959). Data for Biochemical Research. Oxford: Clarendon Press.
^A. Kossel and Albert Neumann (1894) "Darstellung und Spaltungsprodukte der Nucleïnsäure (Adenylsäure)" (Preparation and cleavage products of nucleic acids (adenic acid)), Berichte der Deutschen Chemischen Gesellschaft zu Berlin, 27 : 2215–2222. The name "cytosine" is coined on page 2219: " … ein Produkt von basischen Eigenschaften, für welches wir den Namen "Cytosin" vorschlagen." ( … a product with basic properties, for which we suggest the name "cytosine".)
^Chahwan R.; Wontakal S.N.; Roa S. (2010). "Crosstalk between genetic and epigenetic information through cytosine deamination". Trends in Genetics. 26 (10): 443–448. doi:10.1016/j.tig.2010.07.005. PMID20800313.
