The following illustrates methylation of cytidine (The nucleotide cytosine attached to a molecule of ribose, which is its normal free state) where H is replaced by CH3, producing 3-methylcytidine or 5-methylcytidine.
Consequently it can affect the availability of hydrogen bonds. Because it becomes part of the chromosome, it is carried through cell division.
Citation: Bland.J., (2003) About Gender: Methylation http://www.gender.org.uk/about/03gene/38acmnt.htm
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