Lec18 - Lecture 18 Eukaryotic Genome 3 Chromatin Structure...

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Lecture 18 Eukaryotic Genome 3 - Chromatin Structure II Lodish 6th ed. Chapter 6.6
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MITOSIS : Chromatin over entire chromosomes is highly compacted/condensed to reach the highest level of condensation at Metaphase to facilitate chromosome segregation in daughter cells. INTERPHASE : Never transcribed genes (in a specific cell type) and centromeric and telomeric DNA regions (in all cells) are highly condensed/compacted into high order chromatin structures ->Heterochromatin . Transcribed genes (or genes ready to be transcribed) are less compact ->Euchromatin . DNA regions/genes are most decondensed during transcription (10 nm fibers) and are “recompacted” after their transcription is turned off. (note DNA repair and replication also need chromatin decondensation) Chromatin structure and condensation levels are regulated in the cell
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Heterochromatin (dark masses) Euchromatin (white/empty space) Plasma membrane Nuclear envelope Electron microscopy visualization of heterochromatin in interphase Lodish 6th ed. Fig. 6-33
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Chromatin structure is regulated by two general classes of enzymes 1. Histone-modifying enzymes that add (or remove) covalent groups on histones (mostly on histone tails). Histone acetyltransferases (HATs) and deacetylases (HDACs) Histone methyltransferases (HMTs) kinases and phosphatases poly-ADP-ribose polymerases ubiquitin ligases others?.. These modifications alter the interactions of the histone tails (which are basic and positively charged, -NH3 + ) with the negatively-charged DNA phosphate backbone, with neighboring nucleosomes, and with other histone-tail interacting proteins (e.g. ATP-dependent remodelers, see below). 2. ATP-dependent nucleosome remodeling enzymes . Use the energy of ATP hydrolysis to assemble, disassemble, or slide/move nucleosomes along DNA (by altering histone-DNA contacts). These enzymes are important for (i) chromatin/nucleosome assembly and (ii) for nucleosome disruption/mobilization to allow access of DNA-binding proteins (e.g. proteins from the transcription, DNA repair and replication machineries) to their target genomic sites. Histone modifying enzymes(1) and ATP-dependent nucleosome remodelers (2) work in concert to alter chromatin structure.
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Histone AcetylTransferases (HATs) and Histone Deacetylases (HDACs) Lysine( ε )-NH 3 + acetyl-CoA CoA S C CH3 O Histone Lysine( ε )-NH C CH3 CoA SH Histone O HATs HATs were originally classified in 2 Types: HAT-A: Predominantly nuclear. Acetylate nucleosomal histones within chromatin. Roles in transcription and DNA repair on chromatin. HAT-B: Predominantly cytoplasmic. Acetylate newly synthesized free histones in cytoplasm. Role in nuclear transport and deposition of histones on DNA (i.e. chromatin assembly).
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