Eukaryotic chromosome is packaged in hierarchical levels, mediated by various proteins.
DNA duplex→Nucleosome → Chromatin →Chromosome
We talked about DNA duplex and chromatin; now it’s time to meet the basic unit of chromatin structure–nucleosome.
- Definition: nucleosome is the chromatin subunit that consists of DNAand a set of eight histone core proteins(complex of (H2A)2(H2B)2(H3)2(H4)2, →octamer. More loosely with one molecule of H1 )
Comparison
The proteins protect the DNA from the action of micrococcal nuclease.
Micrococcal Nuclease is an endo–exonuclease that preferentially digests single-stranded nucleic acids The enzyme is also active against double-stranded DNA and RNA and all sequences will be ultimately cleaved.(wikipedia)
Digestion with nuclease leads to the loss of H1, yielding a very resistant structure consisting of 146 bp of DNA associated very tightly with the histone octamer. The structure, known as the nucleosome core, is structurally very similar whatever the source of the chromatin.
- Adjacent nucleosome is connected by a varied length (10-100 bp, average 55bp)of DNA, called “linker DNA”
- One molecule of linker histone H1 binds to the linker DNA between nucleosome.
H1 also acts to stabilize the point at which the DNA enters and leaves the nulceosome core.
↑Chromatins at different packaging levels
30 nm chromatin fiber: condensed form
10 nm chromatin fiber : less-condensed form, like a thread of beads
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Summary of Nucleosome Structure:
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Higher Order Structure
The organization of chromatin at the highest level seems rather similar to that of prokaryotic DNA(see THE BLUEPRINT OF LIFE [7]: PROKARYOTIC CHROMOSOME STRUCTURE OF DNA). Even the size of the loops is approximately the same, up to aorund 100 kb of DNA, although there are many more loops in a eukaryotic chromosome.
The loops are constrained by interaction with a protein complex known as the nuclear matrix. The DNA in the loops is in the form of 30 nm fiber, and the loops form an array about 300 nm across.
”Solenoid model “of 30-nm chromatin fiber
6 nucleosomes per turn
“Zigzag model” of 30-nm chromatin fiber
6 nucleosomes per turn, longer linker DNA may be required
