L16 - NEW PROBLEM SET ON THE WEB Chapter 6 pages: 359-366...

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NEW PROBLEM SET ON THE WEB Chapter 6 pages: 359-366 Chapter 12 pages: 704-712 Chapter 19 pages: 1131-1144; 1150-1154;1165-1172; 1178-1194 • No ofFce hours tomorrow.
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Histones are responsible for the most basic level of chromosome organization--the nucleosome--discovered in 1974. A) Chromatin from an interphase nucleus appears in the electron microscope like a thread 30 nm thick. B) Here the chromatin has been experimentally unpacked or decondensed after being isolated to show the nucleosomes. DNA and chromosome structure
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DNA is very long and very thin. Sheer forces could easily break it. Must be accurately replicated and exactly split between daughter cells at division. Packaging of DNA is very important. Histones package DNA Histones - small basic highly conserved proteins. H2A H2B H3 H4 2 of each of these proteins - eight total - form a nucleosome. This is called the octameric core. DNA wraps around the nucleosome - beads on a string 2 DNA coils - take up 146 base pairs Approximately 50 base pairs between nucleosomes The width of the coil here is 10 nM.
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The structure of a nucleosome core particle as determined by X-ray diffraction analyses of crystals. Each histone is colored with the DNA double helix in light gray Histones package DNA. They form an octomer around which DNA wraps. This is called a nucleosome
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Nucleosomes are usually packed together into a compact chromatin fiber. Several models have been proposed to explain how nucleosomes are packaged into a 30 nm fiber: zig-zag and solenoid 30 nm fber From interphase Unpacked 30 nm fber
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Mitotic chromosomes are formed from chromatin in its most condensed state Packaging into the 30 nm fiber doesn't account for all of DNA packaging, but it is a start. The reality is that most interphase chromosomes are too tangled for their structures to be visible. So scientists have focused on the compaction of mitotic chromosome. 5 cm of DNA is compacted to 5 µ M in metaphase chromosome.
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Condensins play a role in stabilizing the loops. Model for how this looping could occur Model for how this looping could occur A) Anti-body staining see punctate along chromosome axis B) Immunogold electron microscopy in cross-section
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profoundly affect chromatin structure One example is acetylation of lysines which neutralizes the positive charge of this amino acid, making it more difficult for the nucleosome to neutralize the negative charge on DNA (as described earlier as one possible function for histone H1). For the other histones, it makes them less attracted to DNA, potentially loosening the grip of histones on DNA and disrupting 30 nm fiber. This modification is associated with euchromatin (open chromatin) A second example is trimethylation of histone H3 at lysine 9 which is recognized by a heterochromatin specific protein, HP1, that create a compact form of chromatin- associated with heterochromatin that is not transcribed
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This note was uploaded on 03/28/2010 for the course MCD BIO 110 taught by Professor Hinck during the Fall '09 term at UCSC.

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L16 - NEW PROBLEM SET ON THE WEB Chapter 6 pages: 359-366...

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