Conclusion

In eukaryotic cells, the DNA helix is associated with proteins of various kinds. These can be divided into two major groups, histone proteins and non-histone proteins.

There are five kinds of histone proteins, H1, H2A, H2B, H3, and H4. 2 copies of each of the proteins H2A, H2B, H3, and H4 combine to form an octamer which forms the structure known as a nucleosome.

In interphase cells, chromosomes are uncondensed. In order to undergo cell division, chromosomes must be condensed to approximately 1/500th of their original lengths. The first reduction in length is achieved by DNA coiling around nucleosomes. H1 binds to DNA between the nucleosomes.

The nucleosomes pack together to form a coiled structure known as a solenoid. The solenoid fiber is about 30 nm in diameter.

Solenoid fibers form a series of loops anchored by proteins that form a 300 nm fiber. It is these 300 nm fibers that appear to comprise the metaphase chromosome.

In interphase, some areas of the chromosome remain condensed; the genes in these regions are generally inactive.

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