Which amino acids are present in the histone octamer that contribute…

The correct answer is Option (4) → Lysine and arginine

DNA is negatively charged because of its phosphate groups. It is packed around histones, which are positively charged because they are rich in two basic amino acids. Opposite charges hold the DNA tightly on the histone octamer.

Option (1) Methionine and proline — Incorrect. Neither is a basic amino acid; neither carries a positive charge in its side chain.

Option (2) Valine and Arginine — Incorrect. Arginine fits, but valine is not basic.

Option (3) Methionine and lysine — Incorrect. Lysine fits, but methionine is not basic.

Option (4) Lysine and arginine — Correct. Histones are rich in the basic amino acids lysine and arginine, whose side chains carry positive charges. The positively charged histone octamer binds the negatively charged DNA, giving stable packaging in 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.”