Lecture10Fall09FINAL - MCB 250 Lecture 10 Chromosome...

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MCB 250 Lecture 10 Chromosome Structure DNA Replication - 1
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Beads on a String: the beads of the 10nm fiber are nucleosomes
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Eukaryotic Chromosomes are Packaged Around Histones Histones are small (11 - 22 kDa), very basic (20 -25% lys + arg) proteins. Four histone proteins (the “core histones) form a disk that the DNA wraps around The disk is made up of 2 copies each of H2A, H2B, H3, and H4. H1 is a “linker” protein between the “core” histones. Histone proteins are nearly identical in all eukaryotes, i.e., they are highly conserved. H4 from cows differs by 2 amino acids from H4 of peas so histones have not changed very much in the ~ 10 9 years since plants and animals diverged. Text, pp. 157 - 160
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Nucleosome dsDNA wraps around histone core ~1.65 times = Nucleosome or bead on a string (10 nm) 147 bp of DNA plus 20–60 bp linker DNA Text, Fig. 7–18 11nm
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Nucleosome Structure H3 (blue) H4 (green) H2A (yellow) H2B (red) Text, Fig. 7-20 Note N-terminal tails (NTDs, N-terminal domains). There are 8NTDs: 2 H3, 2H4, etc. About 25 - 30% of the total mass of the core histones is in the NTDs. NTDs extend out of the central disc and are unstructured in the crystal structure.
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Clicker Question Most of the interactions between histones and DNA in the nucleosome are between groups on the histones and either the DNA backbone or groups in the minor groove. Histones are DNA binding proteins. Are they sequence specific DNA binding proteins? a. Yes b. No
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Some of the contacts between 1 of the H3-H4 dimers and DNA in the nucleosome Text, Fig. 7-25
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H1 Histone and Histone Tails are Required to Form 30 nm Fibers Text, Figs. 7–29 and 7–31 H1 30 nm fiber The exact structure of the 30 nm fiber is not completely clear. This is one possibility. Note that the histone NTDs stabilize the 30nm fiber.
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nucleus. A possibility for higher order chromatin structure: Text, Fig. 7-32 The “chromosome scaffold” is presumably composed of a number of different non-histone proteins. Maybe TopoII is a major component. Loops of
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This note was uploaded on 02/17/2010 for the course MCB MCB 250 taught by Professor Miller during the Fall '09 term at University of Illinois at Urbana–Champaign.

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Lecture10Fall09FINAL - MCB 250 Lecture 10 Chromosome...

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