NEET 2021 Zoology Question 186 Solution

Eukaryotic genomic DNA is enormous in length (approx. 2.2 metres per diploid human nucleus) and must be compacted to fit inside a microscopic nucleus (~$10^{-6}\text{ m}$). This compaction is mediated by histones: - DNA carries a continuous negative charge along its polyanionic sugar-phosphate backbone. - Histones are a family of small, highly evolutionary conserved basic proteins that carry net positive charges under physiological pH. - The net positive charge arises because histones are unusually enriched with basic amino acid residues: Lysine ($\epsilon$-amino group, $\text{p}K_a \approx 10.5$) and Arginine (guanidinium group, $\text{p}K_a \approx 12.5$). - Because histones are basic proteins, their isoelectric point ($pI$) is high ($pI > 10$), and in solution they behave as bases (alkaline), never acidic.

- Statement (1): Histones assemble as a cylindrical hetero-octamer containing two copies each of H2A, H2B, H3, and H4 (8 molecules total), around which 146 base pairs of duplex DNA wrap 1.65 times (True). - Statement (3): Histones are exceptionally rich in Lysine and Arginine residues (True). - Statement (4): Lysine and Arginine side chains bear protonated amino/guanidino groups carrying positive charges (True). - Statement (2): Claiming histones have a slightly acidic pH is chemically and biologically false.