NEET 2025 Zoology Question 146 Solution

Human diploid cells contain approximately 2.2 meters of DNA ($6.6 \times 10^9$ bp) that must be condensed to fit inside a nucleus of ~10 $\mu$m diameter:

• • DNA possesses a net negative electrical charge because of the phosphate groups in its phosphodiester backbone.
• • Packaging is facilitated by specialized basic proteins called histones.

Why histones carry a high positive charge density:

• • Histone proteins are enriched with the basic amino acids Lysine and Arginine.
• • At physiological pH (7.4), the amino side chains of lysine ($epsilon$-amino group) and arginine (guanidino group) are protonated ($-\text{NH}_3^+$ and $-\text{C(NH}_2)_2^+$), giving histones a strong positive charge.
• • Electrostatic attraction between the positively charged basic histone octamer and negatively charged acidic DNA forms the nucleosome core particle.

Histones are specifically enriched with the basic amino acids Lysine & Arginine, corresponding to Option (1).