Question 1

What is a nucleosome?

Accepted Answer

A nucleosome is the basic repeating unit of chromatin and the first level of DNA packaging. It consists of about 200 base pairs of DNA wrapped roughly 1.65 turns around a histone octamer. Under the electron microscope, a chain of nucleosomes looks like 'beads on a string'.

Question 2

What is the composition of the histone octamer?

Accepted Answer

The histone octamer is made of eight histone proteins: two molecules each of H2A, H2B, H3 and H4. Two H3 and two H4 first form a central (H3–H4)₂ tetramer, and two H2A–H2B dimers attach on either side to complete the octamer. H1 is NOT part of the octamer.

Question 3

How many turns does DNA make around the histone octamer?

Accepted Answer

DNA makes approximately 1.65 left-handed superhelical turns around the histone octamer, which corresponds to about 147 base pairs of DNA in the nucleosome core. Including the linker DNA that connects neighbouring nucleosomes, roughly 200 base pairs are associated with each nucleosome.

Question 4

Why do histones bind DNA so tightly?

Accepted Answer

Histones are basic (positively charged) proteins, rich in the amino acids lysine and arginine. DNA carries a strong negative charge because of the phosphate groups in its backbone. The opposite charges attract, so the DNA wraps tightly around the positively charged histone core.

Question 5

What is the role of histone H1?

Accepted Answer

H1 is the linker histone. One H1 molecule binds where DNA enters and exits a nucleosome, sealing the wrap in place and helping the nucleosomes fold into the higher-order 30 nm chromatin fibre. H1 is associated with the nucleosome but is not one of the eight core histones of the octamer.

Question 6

How does 2 metres of DNA fit inside the nucleus?

Accepted Answer

DNA is packed in hierarchical levels. First it wraps around histone octamers to form nucleosomes ('beads on a string'), which coil into the 30 nm chromatin fibre, then into looped domains, and finally into the highly condensed metaphase chromosome. Overall the DNA is compacted roughly 10,000-fold, which lets about 2 metres of DNA fit inside a nucleus only ~10 µm across.

Question 7

Is the nucleosome an important NEET topic?

Accepted Answer

Yes. The nucleosome is part of the Class 12 chapter 'Molecular Basis of Inheritance' and is regularly tested in NEET. Common questions cover the octamer composition (2 each of H2A, H2B, H3, H4), the 1.65 DNA turns, the role of H1, and the difference between loosely packed euchromatin and densely packed heterochromatin.

Nucleosome — DNA Packaging & Chromatin

Practice

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