Packing DNA
A human cell holds about 2 metres of DNA in a nucleus a few micrometres wide. It fits because proteins pack it in stages, from left to right here, ending in the X-shaped chromosome seen during cell division.
The double helix
The starting material is the DNA double helix, only 2 nm wide. Each chromosome is a single, unbroken DNA molecule.
Nucleosomes
DNA wraps about 1.7 turns around spools of eight histone proteins. Joined by linker DNA, these nucleosomes look like beads on a string, 11 nm wide.
Chromatin fiber
Nucleosomes stack and coil into a thicker fiber about 30 nm wide, shortening the DNA several times more.
Loops on a scaffold
The fiber is folded into loops anchored to a protein scaffold of condensin and other proteins. Further coiling of this looped fiber builds each chromatid.
Sister chromatids
Before division the DNA is copied, so the chromosome has two identical sister chromatids joined at the centromere. Kinetochores on the centromere are where spindle fibers attach to pull the sisters apart.
Arms and telomeres
The centromere splits each chromatid into a short p arm and a long q arm. Telomeres cap all four ends, protecting the genes from damage.