Q.What are histones? What are their functions?

Histones are small, basic (positively charged) proteins that, together with DNA, non-histone proteins and RNA, make up chromatin -- the thread-like nucleoprotein material of the interphase nucleus.

• Being basic proteins, histones carry a net positive charge, which lets them bind closely to the negatively charged phosphate backbone of DNA.
• This tight, charge-based binding is what allows the very long DNA molecule (about two metres in a single human cell, distributed among 46 chromosomes) to be condensed and packaged so it fits within the nucleus. …

The interphase nucleus (the nucleus of a cell that is not dividing) contains highly extended, elaborate nucleoprotein fibres called chromatin, along with the nuclear matrix and one or more nucleoli. Chromatin itself is made up of four components: DNA, histones, non-histone proteins and RNA.

Histones are specifically described as basic proteins -- meaning they carry a net positive charge.

A single human cell has approximately a two-metre-long thread of DNA distributed among its 46 (23 pairs of) chromosomes. It is this close histone-DNA association that lets such a long molecule be condensed and packaged to fit within the small volume of the nucleus, and be condensed still further into the structured, visible chromosomes that appear during cell division. …

Method: Naming the Mechanism, Not Just the Outcome

When a question asks for the "function" of a structural molecule like a histone, a definition alone (basic protein, part of chromatin) is not enough -- you need to name the actual mechanism by which it does its job, because that mechanism IS the function being asked about. Start by recalling what chromatin is built from, then narrow in on the specific physical arrangement histones create with DNA.

The key fact to anchor the answer on is the histone octamer: eight histone molecules clustering together, with DNA winding around this cluster to form a nucleosome. Once you have this concrete structural picture, the function follows directly and specifically, rather than needing a vague word like "organise": a positively charged protein neutralising a negatively charged DNA strand, and doing so in a repeating, space-saving pattern that compacts an enormo …