Eukaryotic Chromosomes

30nm Fiber (Solenoid)

When chromatin consists of only nucleosomes, we observe the structure that is referred to as beads on a string. Nucleosomes can be further condensed into a structure called 30nm fiber or 30nm solenoid (the name comes from the fact that a nucleosome is about 10nm in diameter and the fiber is three nucleosomes in width). The formation of the 30nm fiber involves the histone protein H1 and some epigenetic changes that are covered in the section on gene regulation. This structure is thought to be what exists in heterochromatin and is associated with a lack of gene expression. Knowledge of this structure comes from observations on extracted chromatin in vitro and there is still some uncertainty about whether chromatin actually exists in this structure in the nucleus.

You can see a diagram of this here.

This video will also show you the organization of chromatin.