The substance formed by DNA wrapped around histone proteins is chromatin. Chromatin is the DNA–protein complex that packages long DNA molecules inside a cell while helping regulate access to genetic information.
Its basic repeating unit is the nucleosome. In a typical nucleosome, about 146–147 base pairs of DNA wrap around a histone octamer containing two copies each of histones H2A, H2B, H3, and H4. Linker DNA connects neighboring nucleosomes, producing the often-described “beads-on-a-string” arrangement. Further folding and organization produces the larger chromatin structure found in chromosomes.
Chromatin is more than passive packaging. Looser euchromatin generally makes DNA more accessible for transcription, replication, and repair, while denser heterochromatin is often associated with reduced gene activity. Chemical modifications of histone tails, including acetylation and methylation, can alter chromatin compaction and help regulate which genes are accessible.
A common mix-up is answering “nucleosome.” A nucleosome is the basic DNA-and-histone unit within chromatin, whereas chromatin is the broader material made from DNA, nucleosomes, and additional associated proteins. A chromatid, by contrast, is one of the duplicated copies of a chromosome.