Chromatin
From Freepedia
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, hence its name, which literally means coloured material. The nucleic acids are generally in the form of double-stranded DNA. The major proteins involved in chromatin are histone proteins. 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. Changes in chromatin structure are affected mainly by methylation and acylation of the nucleous proteins. Chromatin structure is also of importance in DNA replication.
There are three major levels of chromatin organization:
- nucleosome - "beads on a string"
- 30 nm condensed chromatin fiber
- chromosome
Sperm cell chromatin is an exception to the above. During spermiogenesis, the spermatid'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.
Two distinct types of chromatin can be distinguished:
See also
Reference
- van Holde KE. 1989. Chromatin. New York: Springer-Verlag. ISBN 0387966943.
