Chromatin is the substance of a chromosome and consists of a complex of DNA and protein in eukaryotic cells. It can be made visible by staining (thus the name). The nucleic acids are generally in the form of double-stranded DNA. The major proteins involved in chromatin are histone proteins. In a eukaryotic cell, nearly all DNA is found compacted in chromatin. DNA is packaged into chromatin both to constrain the size of the molecule and to allow the cell to control expression of the chromatin packaged genes.

There are three levels of chromatin organization:

1. nucleosome - beads on a string
2. 30 nm condensed chromatin fiber
3. chromosome

Sperm chromatin is an exception to the above. During spermiogenesis, the male germ cell's chromatin is remodelled into a more tightly packaged, compact structure. This process is associated with the cessation of transcription and involves nuclear protein exchange. The histones are mostly displaced, and replaced by protamines, small, arginine-rich proteins.

See also

• chromosome
• chromatid
• genetics

Reference

• van Holde KE. 1989. Chromatin. New York: Springer-Verlag. ISBN 0387966943.

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