N H

5-Methylcytosine

110.18 In eukaryotic DNA, cytosine bases are often methylated to form 5-methylcytosine.

commonly methylated base. Bacterial DNA is frequently methylated to distinguish it from foreign, unmethylated DNA that may be introduced by viruses; bacteria use proteins called restriction enzymes to cut up any unmethylated viral DNA (see Chapter 18).

In eukaryotic DNA, cytosine bases are often methylated to form 5-methylcytosine ( FIGURE 10.18). The extent of cytosine methylation varies; in most animal cells, about 5% of the cytosine bases are methylated, but more than 50% of the cytosine bases in some plants are methylated. On the other hand, no methylation of cytosine has been detected in yeast cells, and only very low levels of methylation (about 1 methylated cytosine base per 12,500 nucleotides) are found in Drosophila. Why eukaryotic organisms differ so widely in their degree of methylation is not clear.

Methylation is most frequent on cytosine nucleotides that sit next to guanine nucleotides on the same strand:

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