MBB331_Week10-Fall-2011

Condensation in 30nm fiber fiber 30nm fiber solenoid

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Unformatted text preview: linked by 2 loops DNA wraps twice around core H4 basic tail exposed on side of core, H4 tail interacts with H2A & B acidic regions of neighboring particle ­ mediates nucleosome X­linking 22 CHROMATIN PACKAGING CHROMATIN • Increasing ionic strength Increasing results in increased chromatin condensation chromatin • Nucleosome string Nucleosome eventually condenses to 30nm fiber 30nm • DNA condensed ~7 fold in DNA nucleosome ---> add’nal ~7 fold condensation in 30nm fiber fiber • 30nm fiber: solenoid or 30nm irregular zig-zag? 23 Weaver 2002 Molecular Biology Ed. 2 CHROMATIN: 30 nm SOLENOID? CHROMATIN: • 30nm fiber solenoid 30nm model: Nucleosome string coiled into hollow tube (solenoid) tube • Model consistent with Model X-ray diffraction data X-ray • Inconsistent with Inconsistent irregular structure observed by electron microscopy Weaver 2002 Molecular Biology Ed. 2 24 25 CHROMATIN: 30nm FIBER • 30nm zigzag 30nm ribbon model: alternating reg & irregular zigzag string of nucleosomes nucleosomes • Regardless of Regardless model, 30nm fiber further looped into chromatin domains domains Transcription factors must act on DNA in chromatin 26 CHROMATIN & TRANSCRIPTION: CHROMATIN PRE-EMPTIVE MODEL PRE-EMPTIVE • Pre-emptive model: histones & Pre-emptive transcription factors battle for DNA possession possession • Histones displaced at replication, Histones permits...
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