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BIOL 3110
Peter Cheung

Nov 5, 2009 Fiber Structure -The 10nm fiber has two sets of H2a, H2b, H3, H4 as its core particles, there is approximately 146 bp of DNA wrapped around every 1.65 turns; the linker DNA found in this fiber is about 54bp long. -The H1 particle is not required for the formation of the core of the 10nm fiber but it is required to be present as a linker between the other core particles; formation of this fiber is in low salt concentration, if high salt concentration and H1 is present then it will create the 30nm fiber. -H1 binds to the linker DNA between the nucleosomes and also has an affinity for itself, when near another H1 particle it will pull itself together to form a multi- strain structure (solenoid fig. 29.25) -In the solenoid structure there are 6 nucleosomes per turn of the helix; it is now known that the 30nm fiber is actually a double solenoid structure -The H1 particle forms the 30nm fiber by condensing the DNA and it is also a general repressor of transcription. -During replication and transcription the DNA needs to be unwound in that region for the polymerases to read the template; once this process begins the nucleosomes “fall” away from the helix. Controlling Chromatin Structure -The actual of process of how genes unpacked themselves for reading has been unknown for some time and was only recently discovered. -Chromatin remodelling is the general process by which genes are opened for reading (fig.30.4), the three basic forms is: sliding, spacing, and displacing. This is done via large protein complexes in conjunction with ATP hydrolysis. -The first of these complexes was discovered in yeast (SWI/SNF complex) and is responsible for the controlling of ~120 genes (~2% of all genes in yeast), the main enzyme used in yeast chromatin remodelling is Sui 2 subunit-ATPase. Histone Modification -H3 and H4 N-terminal tails protrude out of the core particle, they are flexible and alteration via enzymes (fig.30.9) -Acetylation of the lysine groups removes the +ve charge. -Methylation does not remove any charge, lysine and arginine can undergo mono, di, tri, methylation. -Phosphorylation occurs at the –OH on the 5’ of serine and th10onine, this modification imparts a negative charge onto the DNA. Ser of H3 is the one that is phosphorylated when chromosomes condense at mitosis. -Most modified sites are done so only in a single way; modification at one site may activate or inhibit modification at another
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