Th2L01 Modification of histones

Modification of histones Tails are vulnerable to covalent modifications esp. addition or removal of acetyl, phosphate or methyl groups by enzymes in the nucleus Little affect on individual nucleosomes but can affect the stability of the chromatin fibre or higher order structures Attract specific enzymes which can either condense or decondense the chromatin These proteins signal things e.g. new replications has occurred; these genes need to be expressed Histone enzymes Tightly regulated Brought to a particular region with specific cues esp. interactions with proteins that bind to specific sequences of DNA; work with chromatin-remodelling complexes

Histones enzymes RNA polymerase An enzyme that links together the growing chain of RNA nucleotides during transcription, using a DNA strand as a template. Histone acetyltransferase (HAT) recruit transcriptional co-activators, act in large complexes, recruit RNA polymerase, turn on gene expression Lead to the uncoiling of chromatin Histone deacetylase (HDAC) - proteins that function to remove acetyl groups from histones, which closes chromatin conformation and decreases gene expression Heterochromatin DNA that is densely packed around histones The genes in heterochromatin are generally inaccessible to enzymes and are turned off. Euchromatin DNA that is loosely packed around histones This DNA is more accessible to enzymes and the genes in euchromatin can be activated if needed. Methylation A chemical modification of DNA that does not affect the nucleotide sequence of a gene but makes that gene less likely to be expressed