Q.What is a nucleosome? Describe how DNA is wound around a histone octamer to form this structure.

DNA is negatively charged because of its phosphate backbone; histones are positively charged because they are rich in basic amino acids (lysine and arginine). Opposite charges let DNA wrap tightly around the histone octamer.

• DNA: the sugar–phosphate backbone has many phosphate groups, each contributing a negative charge, so DNA carries a net negative charge.
• Histones: these are basic proteins rich in the positively charged amino acids lysine and arginine, giving them a net positive charge.

The electrostatic attraction between negatively charged DNA and positively charged histones allows DNA to coil around the histone octamer, forming nucleosomes — the fundamental packaging unit of chromatin. Therefore the charges on DNA and histones are negative and positive respectively — option (c).

Based on NCERT/CBSE Class 12 Biology DNA packaging (chromatin/nucleosome).

(a) Lysine and Arginine.

Histones are basic nuclear proteins rich in the amino acids lysine and arginine.

Histones are small, positively charged (basic) proteins that form the core around which DNA wraps to make nucleosomes, the basic unit of chromatin packaging. Their strongly basic character comes from an abundance of the amino acids lysine and arginine, both of which carry positively charged side chains at physiological pH. This positive charge allows histones to bind tightly and neutralise the negatively charged phosphate backbone of the DNA double helix, enabling the DNA to be condensed and organised within the nucleus.

(d) 54 base pairs — the linker DNA connecting two successive nucleosomes.

A nucleosome's core DNA (146 bp) plus its linker DNA (about 54 bp) gives the ~200 bp nucleosome repeat length.

In chromatin, DNA is packaged into nucleosomes — each core consists of about 146 base pairs of DNA wound roughly twice around a histone octamer (two molecules each of H2A, H2B, H3 and H4). Successive nucleosomes are joined by a short stretch of free 'linker DNA' associated with histone H1, and the combined core-plus-linker length (the nucleosome repeat unit) averages about 200 base pairs. Subtracting the 146 bp wound around the core leaves the linker DNA at roughly 54 base pairs.