why does histone methylation occur over large areas of chromatin
Histone methylation 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.
Nucleolus The nucleolus (/njuːˈkliːələs/ or /ˌnjuːkliˈoʊləs/, plural nucleoli /njuːˈkliːəˌlaɪ/ or /ˌnjuːkliˈoʊlaɪ/) is the largest structure in the nucleus of eukaryotic cells.[1] It is best known as the site of ribosome biogenesis. Nucleoli also participate in the formation of signal recognition particles and play a role in the cell's response to stress.[2] Nucleoli are made of proteins, DNA and RNA and form around specific chromosomal regions called nucleolar organizing regions. Malfunction of nucleoli can be the cause of several human conditions called "nucleolopathies"[3] and the nucleolus is being investigated as a target for cancer chemotherapy.[4][5]