<P> Lysine and arginine residues both contain amino groups, which confer basic and hydrophobic characteristics . Lysine is able to be mono -, di -, or trimethylated with a methyl group replacing each hydrogen of its NH3+ group . With a free NH2 and NH2+ group, arginine is able to be mono - or dimethylated . This dimethylation can occur symmetrically on the NH2 group or asymmetrically with one methylation on each group . Each addition of a methyl group on each residue requires a specific set of protein enzymes with various substrates and cofactors . Generally, methylation of an arginine residue requires a complex including protein arginine methyltransferase (PRMT) while lysine requires a specific histone methyltransferase (HMT), usually containing an evolutionarily conserved SET domain . </P> <P> Different degrees of residue methylation can confer different functions, as exemplified in the methylation of the commonly studied H4K20 residue . Monomethylated H4K20 (H4K20me1) is involved in the compaction of chromatin and therefore transcriptional repression . However, H4K20me2 is vital in the repair of damaged DNA . When dimethylated, the residue provides a platform for the binding of protein 53BP1 involved in the repair of double - stranded DNA breaks . H4K20me3 is observed to be concentrated in heterochromatin and reductions in this trimethylation are observed in cancer progression . Therefore, H4K20me3 serves an additional role in chromatin repression . </P> <P> The genome is tightly condensed into chromatin, which needs to be loosened for transcription to occur . In order to halt the transcription of a gene the DNA must be wound tighter . This can be done by modifying histones at certain sites by methylation . Histone methyltransferases are enzymes which transfer methyl groups from S - Adenosyl methionine onto the lysine or arginine residues of the H3 and H4 histones . There are instances of the core globular domains of histones being methylated as well . </P> <P> The histone methyltransferases are specific to either lysine or arginine . The lysine - specific transferases are further broken down into whether or not they have a SET domain or a non-SET domain . These domains specify exactly how the enzyme catalyzes the transfer of the methyl from SAM to the transfer protein and further to the histone residue . The methyltransferases can add 1 - 3 methyls on the target residues . </P>

Why does histone methylation occur over large areas of chromatin
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