Methylation In Plants Draft 3

In plants, DNA methylation also occurs at CpNpG trinucleotides, where N represents a nucleotide with any base. Some DNA regions have many CpG dinucleotides and are referred to as CpG islands. In mammalian cells, CpG islands are often located in or near the promoters of genes. These CpG islands are usually not methylated when genes are being actively transcribed. However, methylation of CpG islands near a gene leads to repression of transcription. Cells repress and activate genes by methylating and demethylating cytosine bases. Enzymes called DNA methyltransferases to methylate DNA by adding methyl groups to cytosine bases to create 5-methylcytosine. Other enzymes, called demethylases, remove methyl groups, converting 5-methylcytosine back to cytosine.