Nucleosome model

Chromatin structure:

•  A repeating unit of histones and DNA. There are five types of histone proteins — H1, H2A, H2B, H3and H4. Four of them (H2A, H2B, H3and H4) occur in pairs to produce histone octamer, called nu body or core of nucleosome.
• Strands of DNA wrapped around histoneoctamers forming nucleosomes, which to be organized into   chromatin, the building block of a chromosome. 
• The basic function of chromatin is packaging of Dna into a smaller volume to fit in  the cell. Nucleosomes are interconnected by linker Dna and  form a 10 nm fibre like structure called beads on strings.
• The diameter of the alpha helix structure of DNA is 2nm, the diameter afterhaving wrapped around the first histone is 11nm. That string wraps around another histone forming a 30nm chain, and so forth until you reach the chromosomal structure of roughly 700nm.

Q.1What will happen if histones are depleted from a metaphase chromosome and viewed under a transmission electron microscope?

a. 30 nm chromatin fibres will be observed
b. 10 nm chromatin fibres will be observed
c. A scaffold and a huge number of loops of DNA fibres will be observed
d. A huge number of loops of DNA fibres without scaffold will be observed

Ans. C
2.Histones are small basic proteins that assemble with DNA molecules to form chromosomes. There are five histones, including H1, H2A, H2B, H3 and H4, in eukaryotic cells. Which of the following structural features of chromosomes is associated with Histone H1?
(A) Telomere
(B) Nucleosome fiber (10-nm fiber)
(C) 30-nm fiber
(D) Looped domains
(E) Centromere
Ans. B


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