A nucleosome forms when DNA wraps around a core of eight histone proteins.
The histone core contains two copies each of histones H2A, H2B, H3, and H4. About 147 base pairs of DNA wind around this octamer in roughly 1.65 turns. The resulting unit is a fundamental building block of chromatin, the DNA–protein material in eukaryotic chromosomes.
A fifth histone, H1, commonly associates with linker DNA between nucleosomes and helps organize higher-order chromatin. The “beads-on-a-string” description refers to nucleosomes connected by linker DNA, although chromatin in living cells has more complex and dynamic organization.
Nucleosomes do more than package DNA into a smaller space. Their position and chemical modifications can influence whether cellular machinery can access nearby genes. They should not be confused with ribosomes, which translate RNA into proteins, or centrosomes, which help organize microtubules.