The DNA of a eukaryotic cell must be constrained within the confines of the nucleus. In human cells, six billion base pairs are contained on the forty-six chromosomes of double-stranded DNA. This DNA has a total length of 1.8 meters, yet it must fit into a nucleus with an average diameter of 6 micrometers. This feat is accomplished in part by the packaging of DNA into chromatin, a condensed complex of DNA, histones, and nonhistone proteins.
The basic unit of chromatin is the nucleosome. The nucleosome is composed of approximately 146 base pairs of DNA wrapped in 1.8 helical turns around an eight-unit structure called a histone protein octamer. This histone octamer consists of two copies each of the histones H2A, H2B, H3, and H4. Nucleosomes form arrays along the DNA. The space in between individual nucleosomes is referred to as linker DNA, and can range in length from 8 to 114 base pairs, with 55 base pairs being the average. Linker DNA interacts with the linker histone, called H1, and there are equal numbers of nucleosomes and H1 histone molecules in the chromatin. A nucleosome particle bound to a single molecule of H1 is termed a chromatosome.
Higher-order chromatin structure can be visualized microscopically as fibers 10 and 30 nanometers in diameter. The 10-nanometer fiber can be observed under conditions of low ionic strength. This fiber resembles beads on a string, and is in fact a string of nucleosomes. The structure does not require the presence of histone H1. It is unclear if the 10-nanometer fiber exists in vivo or if it is just an artifact of chromatin unfolding during extraction in vitro.
Cellular chromatin usually exists as a 30-nanometer fiber. It can be seen under conditions of higher ionic strength. The presence of the linker histone H1 is required for the formation of this fiber, as it helps promote compaction and condensation. The 30-nanometer fiber is formed into a coil, but its exact structure has not been determined.
During cell division, chromosomes condense to an even greater extent. The mechanism by which the 30-nanometer fibers are packed into the highly condensed, organized structure of the mitotic chromosome is unclear. The compaction of chromosomes during cell division is accomplished in part by the organization of chromatin into large, looped structures that are attached at their bases to a protein scaffold. This scaffold remains intact even if the DNA is experimentally removed. It is possible that this scaffold is responsible for maintaining the shape of the chromosomes.
- Eukaryotic Chromosome - Heterochromatin Versus Euchromatin
- Eukaryotic Chromosome - Basic Organization
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