What is nucleosome?

The correct answer is Option (3) → Negatively charged DNA wrapped around the positively charged histone octamer

DNA is negatively charged because of its phosphate groups. A nucleosome holds about 200 bp of DNA.

Option (1) Positively charged DNA wrapped around the negatively charged histone octamer — Incorrect. The charges are reversed: DNA is negative and the histones positive.

Option (2) 200 bases of DNA strand — Incorrect. A nucleosome holds about 200 base pairs of DNA, but the histone core is what makes it a nucleosome.

Option (3) Negatively charged DNA wrapped around the positively charged histone octamer — Correct. Histones are rich in lysine and arginine, so the histone octamer is positively charged. The negatively charged DNA is wrapped around it, and this unit, holding about 200 bp of DNA, is the nucleosome.

Option (4) Densely packed heterochromatin — Incorrect. Heterochromatin is the densely packed, darkly stained, inactive region of chromatin, a higher level of packing.

Note: A similar question was also asked in CUET (UG) 2024 on 29-05-2024 (Slot 1).

NCERT Extract:
Packaging of DNA Helix – “… 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).”