nucleosome

Picture a long string with beads threaded along it at regular intervals. In chromatin the string is DNA and each bead is a nucleosome — the basic repeating unit that gives packaged DNA its characteristic look down to the nanometer scale.

A nucleosome consists of about 147 base pairs of DNA wound nearly twice around a core of eight histone proteins (two copies each of H2A, H2B, H3 and H4). Stretches of linker DNA connect one nucleosome to the next, producing the famous “beads on a string” appearance seen under the electron microscope. This arrangement already shortens the DNA several-fold and is the first level of compaction; further folding stacks the nucleosomes into thicker fibers and ultimately into a chromosome.

Beyond packing, nucleosomes regulate access to the genome. Where nucleosomes sit tightly along the DNA, the underlying sequence is harder for the cell's reading machinery to reach; when they are repositioned or temporarily evicted, genes become available to be switched on. Cells actively slide, remove and reassemble nucleosomes, making them dynamic gatekeepers rather than fixed scaffolding.