Chromatin Packaging

In diploid human cells, about two meters of DNA is compressed inside a nucleus with a diameter of a few microns.

DNA, along with histone proteins, forms a complex called chromatin. Under a microscope, the extended chromatin resembles beads on a string. Each bead is called a nucleosome, comprising DNA wrapped around a core of histones. Being packaged into nucleosomes shortens the DNA length sevenfold.

After binding linker histones, the nucleosomes are stacked in a helical array, creating a shorter, thicker fiber with a diameter of 30 nanometers called the 30-nanometer fiber.

The widely accepted Solenoid model proposes that nucleosomes are arranged in a left-handed helix with six or more nucleosomes per turn, shortening the DNA length by 50-fold.

This arrangement ensures actively transcribed or replicated chromatin regions occur in an extended beads-on-a-string form while the rest exist in the 30-nm fiber form.

The 30-nanometer fibers then coil, forming 300-nanometer-long loops, which are further compressed into 250-nanometer-wide coils.

During metaphase, chromatin fibers form highly condensed chromosomes. After cell division, the chromosomes uncoil again.