What role do histones play in DNA packaging?

• A. They form nucleosomes that package DNA and regulate access.
• B. They catalyze base excision repair.
• C. They unwind DNA during replication as helicases.
• D. They are primary enzymes for transcription.

Answer: (C)

Histones are the proteins around which DNA winds to form nucleosomes, the basic units of chromatin. This packaging compacts DNA inside the nucleus and also controls how accessible the DNA sequence is for processes like transcription, replication, and repair. Each nucleosome has a core of histones H2A, H2B, H3, and H4, with almost 147 base pairs of DNA wrapped around them. The linker histone H1 helps fold those nucleosomes into higher-order structures. The tails of histone proteins can be chemically modified (for example, acetylation or methylation), altering how tightly DNA is packed and thereby regulating gene expression—more open chromatin generally means genes can be accessed and transcribed, while tighter packing represses access.

This role is distinct from other functions: base excision repair is carried out by specialized enzymes that remove damaged bases, helicases unwind DNA during replication, and transcription involves RNA polymerase. So the key idea is that histones form nucleosomes that package DNA and regulate access to it.