Cytosine
Cytosine is one of the four nucleotide bases found in DNA and RNA, along with adenine, guanine, and thymine (uracil in RNA)[1][3]. 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)[3]. The nucleoside of cytosine is cytidine[3].
Definition and Structure
Cytosine is a nitrogenous base derived from pyrimidine that occurs in nucleic acids, the heredity-controlling components of all living cells, and in some coenzymes, substances that act in conjunction with enzymes in chemical reactions in the body[2]. Within a double-stranded DNA molecule, cytosine bases on one strand pair with guanine bases on the opposite strand[1].
Biological Function
When found third in a codon of RNA, cytosine is synonymous with uracil, as they are interchangeable as the third base[3]. When found as the second base in a codon, the third is always interchangeable[3]. 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[3].
Chemical Reactions and Modifications
Cytosine can be found as part of DNA, as part of RNA, or as a part of a nucleotide[3]. 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)[3]. Cytosine can also be methylated into 5-methylcytosine by an enzyme called DNA methyltransferase or be methylated and hydroxylated to make 5-hydroxymethylcytosine[3]. The difference in rates of deamination of cytosine and 5-methylcytosine (to uracil and thymine) forms the basis of bisulfite sequencing[3].
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[3]. Cytosine likely formed within some meteorite parent bodies, however did not persist within these bodies due to an effective deamination reaction into uracil[3]. 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[3].
Curious Chemical Biology
Cytosine typically serves as a stable reservoir of information, permitting gene expression and providing coding information[4]. Deamination, methylation and oxidation as modulators of genomic potential have evolved around cytosine within DNA, endowing the base with physiological significance beyond simple information storage[4]. This versatility arises from enzymes that chemically modify cytosine to expand the potential of the genome[4].
Citations:
[1] https://www.genome.gov/genetics-glossary/Cytosine
[2] https://www.britannica.com/science/cytosine
[3] https://en.wikipedia.org/wiki/Cytosine
[4] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3262930/
[5] https://www.sigmaaldrich.com/US/en/product/sigma/c3506