histone

If DNA is a very long thread, histones are the spools it winds around. These small, positively charged proteins give the genome something firm to coil onto, turning a floppy molecule meters long into an orderly, compact bundle that fits inside the nucleus.

Histones come in several types. Four of them — called H2A, H2B, H3 and H4 — assemble in pairs into an eight-protein core, and about 147 base pairs of DNA wrap roughly twice around this core to form a nucleosome. A fifth type, H1, sits outside and helps clamp the DNA in place and draw nucleosomes into tighter fibers. Because histones carry a positive charge and DNA's backbone is negatively charged, the two attract and bind tightly.

Histones are not merely passive scaffolding. Their tails can receive chemical modifications — such as methylation or acetylation — that loosen or tighten how the DNA is packed and thereby help decide which genes are accessible. This makes histones central players in gene regulation and in the field of epigenetics, where the same DNA sequence can behave differently depending on how its histones are marked.