DNA Base modifications

The expression of genes is influenced by the chromatin structure of a chromosome and regions of that have low or no gene expression usually contain high levels of methylation of cytosine bases. For example, cytosine methylation, producing 5-methylcytosine, is important for X-chromosome inactivation.^[38] The average level of methylation varies between organisms, with Caenorhabditis elegans lacking cytosine methylation, while vertebrates show higher levels, with up to 1% of their DNA containing 5-methylcytosine.^[39] Despite the biological role of 5-methylcytosine it can deaminate to leave a thymine base, methylated cytosines are therefore particularly prone to mutations.^[40] Other base modifications include adenine methylation in bacteria and the glycosylation of uracil to produce the "J-base" in kinetoplastids.

cytosine	5-methylcytosine	thymine

Structure of cytosine with and without the 5-methyl group. After deamination the 5-methylcytosine has the same structure as thymine

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