4. Chromosome structure

Figure 28: DNA is coiled around 8 histones and to form nucleosomes. These nucleosomes are tightened to form chromosomes Figure 29: DNA is coiled around 8 histones and to form nucleosomes. These nucleosomes are tightened to form chromosomes Figure 30: DNA is coiled around 8 histones and to form nucleosomes. These nucleosomes are tightened to form chromosomes

Figures 28-30: DNA is coiled around 8 histones and to form nucleosomes. These nucleosomes are tightened to form chromosomes

4.2 The eukaryotic chromosome

1st level of chromatin condensation: the nucleosome fiber

DNA is coiled around a group of 8 proteins called histones. DNA associates with histones to form the nucleosome. A nucleosome fiber is formed like a string of pearls.

2nd level of chromatin condensation: the thick fiber

Nucleosomes then are tightened into irregular zigzags. This is the thick fiber. DNA in the form of a nucleosome fiber or a thick fiber is called a euchromatin. In such a form, DNA becomes accessible.

3rd level of chromatin condensation: the heterochromatin

The heterochromatin is highly condensed DNA that is not accessible.