H1 and Chromatin √Treatment of chromatin with trypsin(胰岛素)or high salt buffer removes histone H1 This treatment leaves chromatin looking like beads-on-a-string” The beads named nucleosomes Core histones form a ball with DNA wrapped around the outside DNA on outside minimizes amount of DNA bending -H1 also lies on the outside of the nucleosome 13-6
13-6 H1 and Chromatin ✓ Treatment of chromatin with trypsin (胰岛素) or high salt buffer removes histone H1 ✓ This treatment leaves chromatin looking like “beads-on-a-string” ✓ The beads named nucleosomes – Core histones form a ball with DNA wrapped around the outside – DNA on outside minimizes amount of DNA bending – H1 also lies on the outside of the nucleosome
Nucleosome Structure Central(H3-H4)2 core attached to H2A-H2B dimers Grooves on surface define a left-hand helical ramp -a path for DNA winding DNA winds almost twice around the histone core condensing DNA length by 6-to 7-X Core histones contain a histone fold: 3 a-helices linked by 2 loops Extended tail of abut 28%of core histone mass Tails are unstructured
13-7 Nucleosome Structure ✓ Central (H3-H4)2 core attached to H2A-H2B dimers ✓ Grooves on surface define a left-hand helical ramp – a path for DNA winding – DNA winds almost twice around the histone core condensing DNA length by 6- to 7-X – Core histones contain a histone fold: • 3 a-helices linked by 2 loops • Extended tail of abut 28% of core histone mass • Tails are unstructured
The30-nm Fiber(30nm纤丝) Second order of chromatin folding produces a fiber 30 nm in diameter -in a solution of increasing ionic strength This condensation results in another six-to seven-fold condensation of the nucleosome itself √Four nucleosomes condensing into the 30-nm fiber form a zig-zag structure 13-8
13-8 The 30-nm Fiber (30nm纤丝) ✓ Second order of chromatin folding produces a fiber 30 nm in diameter – in a solution of increasing ionic strength – This condensation results in another six- to seven-fold condensation of the nucleosome itself ✓ Four nucleosomes condensing into the 30-nm fiber form a zig-zag structure
Formation of the 30-nm Fiber √Two stacks(摞)of nucleosomes form a left-handed helix -Two helices of polynucleosomes Zig-zags of linker DNA √/Role of histone H1? -30-nm fiber can't form without H1 -H1 crosslinks to other H1 more often than to core histones 13-9
13-9 Formation of the 30-nm Fiber ✓Two stacks (摞) of nucleosomes form a left-handed helix – Two helices of polynucleosomes – Zig-zags of linker DNA ✓Role of histone H1? – 30-nm fiber can’t form without H1 – H1 crosslinks to other H1 more often than to core histones
Higher Order Chromatin Folding 30-nm fibers account for 30-nm most of chromatin in a fiber typical interphase(间期) nucleus v√Further folding is required in structures such as the mitotic(有丝分 裂)chromosomes Model favored for such higher order folding is a series of radial loops 13-10
13-10 Higher Order Chromatin Folding ✓ 30-nm fibers account for most of chromatin in a typical interphase (间期) nucleus ✓ Further folding is required in structures such as the mitotic (有丝分 裂) chromosomes ✓ Model favored for such higher order folding is a series of radial loops