Nov 5, 2009
-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.
-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