Nucleosome is associated with __________ molecules of histones.

The correct answer is Option (2) → Nine

Note: The given answer is as per the official NTA Answer Key. However, as per NCERT a nucleosome is built on a histone octamer of eight histone molecules, which is Option (4), as explained below.

The linker histone H1 sits where the DNA enters and leaves the octamer; counting it with the octamer gives 8 + 1 = 9. NCERT does not describe H1, so by the book the number is eight.

Why the answer given by NTA is incorrect

Option (1) Four — Incorrect. The octamer is made of four kinds of histones, but each kind is present twice. So the core has eight molecules, not four.

Option (2) Nine — Correct as per NTA; however, it is not correct as per NCERT. Nine is reached only by adding the H1 linker histone to the octamer. NCERT counts only the octamer.

Option (3) Two — Incorrect. Two histone molecules are far too few to hold 200 bp of DNA. The core has eight.

Option (4) Eight — Correct as per NCERT. The histone octamer is the core of the nucleosome.

NCERT Extract:
Packaging of DNA Helix – “… In eukaryotes, this organisation is much more complex. There is a set of positively charged, basic proteins called histones. A protein acquires charge depending upon the abundance of amino acids residues with charged side chains. Histones are rich in the basic amino acid residues lysine and arginine. Both the amino acid residues carry positive charges in their side chains. Histones are organised to form a unit of eight molecules called histone octamer. The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called nucleosome (Figure 5.4 a). A typical nucleosome contains 200 bp of DNA helix.”