Methylation, of nucleotides

A DNA modification which changes gene transcription is methylation of nucleotide bases. Heterochromatin contains heavily methylated DNA which cannot be transcribed. Genes in euchromatin are less methylated and are transcribable. Methylation at C5 of cytosine in CpG sequences by a methyltransferase, with 5-adenosyl methionine (SAM) as methyl donor, silences genes in heterochromatin. Methylation may affect the higher-order structure of DNA and may impede access of the transcriptional machinery to DNA. Members of the histone Hj family have been associated with transcriptional repression, because these histones bind preferentially to methylated CpG sequences (Fig. 9.10).4i... 

Reflect and t ply Can methylation of nucleotides play a role in DNA replication If so, what sort of role ... 

Enzymes that catalyze the methylation of nucleotides in DNA are found in a wide variety of prokaryotes and eukaryotes. In prokaryotes, the DNA MTases are most often identified as elements of restriction-modification systems. In eukaryotes, DNA MTases have multiple functions, but their involvement in R-M is not clearly known. All DNA MTases use AdoMet as the methyl group donor (Scheme 4.3). 

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