DNA methylation is the addition of a methyl group (CH3) to DNA. The methyl group is attached to the base cytosine, usually where cytosine is followed by guanine (a CpG site). Methylation is catalysed by enzymes called DNA methyltransferases. It does not change the base sequence.Increased methylation of the DNA inhibits transcription. It does this in two ways:
• The methyl groups prevent transcription factors from binding to the DNA, especially in the promoter region.
• The methyl groups attract proteins that bind to them and make the chromatin more condensed, so the gene cannot be reached by transcription factors or RNA polymerase.Either way, the gene is switched off. Decreased methylation has the opposite effect: the gene becomes accessible and can be transcribed.
Methylation patterns are copied when DNA replicates, so they pass to daughter cells in mitosis. This is one way specialised cells keep genes they do not need permanently switched off. In a liver cell, for example, genes for proteins made only by nerve cells are heavily methylated.Worked example: a promoter contains 40 CpG sites and 34 of them are methylated.percentage methylation = 34 ÷ 40 × 100 = 85% This high level of methylation suggests the gene is not being transcribed in these cells.Environmental factors, including diet, can change methylation. For example, folate in the diet is involved in supplying the methyl groups used in methylation.
• The methyl groups prevent transcription factors from binding to the DNA, especially in the promoter region.
• The methyl groups attract proteins that bind to them and make the chromatin more condensed, so the gene cannot be reached by transcription factors or RNA polymerase.Either way, the gene is switched off. Decreased methylation has the opposite effect: the gene becomes accessible and can be transcribed.
Methylation patterns are copied when DNA replicates, so they pass to daughter cells in mitosis. This is one way specialised cells keep genes they do not need permanently switched off. In a liver cell, for example, genes for proteins made only by nerve cells are heavily methylated.Worked example: a promoter contains 40 CpG sites and 34 of them are methylated.