Chromatin: Difference between revisions

Chromatin undergoes various structural changes during a [[cell cycle]]. [[Histone]] proteins are the basic packers and arrangers of chromatin and can be modified by various post-translational modifications to alter chromatin packing ([[histone modification]]). Most modifications occur on histone tails. The positively charged histone cores only partially counteract the negative charge of the DNA phosphate backbone resulting in a negative net charge of the overall structure. An imbalance of charge within the polymer causes [[electrostatic]] repulsion between neighboring chromatin regions that promote interactions with positively charged proteins, molecules, and cations. As these modifications occur, the electrostatic environment surrounding the chromatin will flux and the level of chromatin compaction will alter.<ref name="doi.org"/> The consequences in terms of [[chromatin accessibility]] and compaction depend both on the modified amino acid and the type of modification. For example, [[histone acetylation and deacetylation|histone acetylation]] results in loosening and increased accessibility of chromatin for replication and transcription. Lysine trimethylation can either lead to increased transcriptional activity ([[H3K4me3|trimethylation of histone H3 lysine 4]]) or transcriptional repression and chromatin compaction ([[H3K9me3|trimethylation of histone H3, lysine 9]] or [[H3K27me3|lysine 27]]). Several studies suggested that different modifications could occur simultaneously. For example, it was proposed that a [[bivalent chromatin|bivalent]] structure (with trimethylation of both lysine 4 and 27 on histone H3) is involved in early mammalian development. Another study tested the role of [[H4K16ac|acetylation of histone 4 on lysine 16]] on chromatin structure and found that [[homogeneous]] acetylation inhibited 30&nbsp;nm chromatin formation and blocked [[adenosine triphosphate]] remodeling. This singular modification changed the dynamics of the chromatin which shows that acetylation of H4 at K16 is vital for proper intra- and inter- functionality of chromatin structure.<ref>Shogren-Knaak, M., Ishii, H., Sun, J. M., Pazin, M. J., Davie, J. R., & Peterson, C. L. (2006). Histone H4-K16 acetylation controls chromatin structure and protein interactions. ''Science, 311''(5762), 844–847. https://doi.org/10.1126/science.1124000</ref>