6 - Heterochromatin and epigenetic control of gene expression, Grewal & Moazed

6 - Heterochromatin and epigenetic control of gene expression, Grewal & Moazed

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Article 6 Heterochromatin and Epigenetic Control of Gene Expression Abstract -epigenetically heritable domains of heterochromatin: -control the structure and expression of large chromosome domains -are required for proper chromosome segregation -assembly process of chromosome domainsoccurs in steps: -has many rounds of of histone modification through silencing complexes that spread along the chromatin fibre by self-oligomerization and association with modified histone amino-terminal tails Introduction - folding of DNA in chromatin regulate processes such as gene expression and chromosome dynamics - local chromatin regulation is at a single gene, on the globla scale its at chromosome domains or the entire chromosome – can both at the nucleosome (the fundamental unit of chromosome folding) -heterochromatin aka ‘silent chromatin’ -chromosomes have 2 types of domains: 1) euchromatin 2) heterochromatin - -Heterochromatin domains: inaccessible by DNA binding factors and are transcriptionally silent -Euchromatin domains: accessible by DNA binding factors, and are transcriptionally active -large heterochromatin surrounds functional chromosome structures like centromeres and telomeres -small heterochromatin structures are found throughout the chromosome -CENTROMERES AND HETEROCHROMATIN: - heterochromatin plays a central role in centromere function -heterochromatin proteins are associated with DNA repeats that surround centromeres -heterochromatin also stabilizes repetitive DNA sequences at centromeres, telomeres, and elsewhere in the genome -through inhibiting recombination b/w homologous repeats -REGULATION OF GENE EXPRESSION: -heterochromatin-like structures are involved in the stable inactivation of developmental regulators (ie. homeotic gene clusters) -heterochromain inactivates one of the 2 ‘X’ chromosomes in female somatic cells
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-PROPERTIES OF HETEROCHROMATIN: -it is suitable for processes that require stable maintenance of expression states over long periods: 1) the heterochromatic state is epigenetically and stably inherited through many cell divisions 2) assembly and spreading of heterochromatin from its origin site to surrounding DNA regions allows transition from sequence specific genetic control to sequence-independent epigenetic control Factors in Heterochromatin Assembly -histones and their post-translational modifications play an important role in the assemble of heterochromatin -in eukaryotes: -DNA is assembeled with histones to form a nucleosome -DNA is wrapped about 2 turns around an 8-unit complex composed of 2 of each of H2A, H2B, H3, H4 -the amino-terminus of histones have many post-translational modifications:
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This note was uploaded on 02/05/2012 for the course BIOLOGY 3u3 taught by Professor Dr.rollo during the Spring '11 term at McMaster University.

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6 - Heterochromatin and epigenetic control of gene expression, Grewal & Moazed

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