Question

What are two changes that occur in the process of chromatin remodeling?

Answer 1

A nucleosome consists of two copies of each of the core histone proteins H2A, H2B, H3, and H4. These histone proteins are involved in the structural organization of chromatin in eukaryotic cells and can undergo various post-translational modifications which can alter their interaction with DNA and nuclear proteins. The H3 and H4 histones, for example, have long tails protruding from the nucleosome which can be covalently modified at several sites. Modifications of the histone tails include methylation, acetylation, phosphorylation, ubiquitination, sumoylation, citrullination, and ADP-ribosylation occurring on the N-terminal tail domain, which results in remodeling of the nucleosome structure into an open conformation more accessible to transcription complexes (H2A can also be modified).

Combinations of these modifications are thought to constitute the so-called “histone code” which can be read and interpreted by different cellular factors leading to transcriptional activation or repression.2For example, a general characteristic of euchromatin is tri-methylation at lysine residues of histone H3 – lysine 4 (K4), lysine 36 (K36), and lysine 79 (K79) and a high level of histone acetylation, while heterochromatin is characteristically enriched in trimethylation of other histone H3 lysine residues K9, K20, and K27.