Cytosine

Cytosine (C) is one of the four bases in DNA. It is also one of the two pyrimidine bases in DNA or RNA.

It forms complementary base pair with Guanine (G) with three hydrogen bonds. This means that fragments of DNA and RNA rich in Guanine-Cytosine (GC) pairing have higher melting points than those richer in Adenine-Thymine (AT) pairing, as Adenine (A) and Thymine (T) only bond with 2 hydrogen bonds.

In deamination mutations, Cytosine (C) loses an NH2/amine group and becomes Uracil (U), an RNA base which base pairs with Adenine (A). The complimentary strand at this gene loci will now have an Adenine (A) where it originally had a Guanine (G) so the base sequence has changed. These kinds of mutations cause snp's (single nucleotide polymorphisms); single base changes in the sequence of a gene that may change the amino acid coded for and, therefore, the structure of the protein expressed. This simple structural change to Cytosine (C) can have drastic effects as the protein synthesised may be non-functional.

Hartl, DL. and Jones, EW., 2005, Genetics: Analysis of Genes and Genomes, Sixth Ed., Jones and Bartlett Publishers.