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Molecular Basis of Heredity: Part 2. Genomes

Author(s): Raye L. Alford, PhD

Chromatin and DNA Packaging

If the DNA contained within a single human cell were laid out lengthwise, with each chromosome arranged end to end, the DNA would be 5.8 feet, or almost two meters, long. However, it would be very thin, only ~24 angstroms wide. It would take 26 million DNA molecules arranged side by side to measure 1 inch wide.

To fit into the microscopic nucleus of a human cell, the length of DNA must be packaged very precisely. In the first level of packaging, the DNA double helix is wound tightly around proteins called histones. Histones are made up of eight component proteins and often are referred to as histone octamers. The packaging of DNA around histones gives DNA a beads-on-a-string appearance. Histone-packaged DNA results in a 10 nanometer (nm) fiber: remember, the native DNA double helix is only 24 angstroms wide, so this 100 angstrom, or 10 nm, fiber is four times wider, and quite a bit shorter than the native double helix would be.

The histone-packaged DNA is further wound into 30 nm fibers. These fibers are then packaged into loops of DNA believed to be anchored to the cytoskeleton within the nucleus of the cell. DNA that is completely packaged is referred to as chromatin.