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Chromatin


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 which literaly means coloured material). 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. Changes in chromatin structure are effected mainly by methylation and acylation of the nucleosome proteins. Chromatin structure is also of importance with respect to the DNA replication.

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.

Figure 2: Different levels of DNA condensation. (1) Single DNA strand. (2) Chromatin strand (DNA with histones). (3) Condensed chromatin during interphase with centromere. (4) Condensed chromatin during prophase. (Two copies of the DNA molecule are now present) (5) Chromosome during metaphase.

Two distinct types of chromatin can be distinguished:

Chromatin is the extended form of DNA-protein complex present in the nucleus during interphase in eukaryotic cells. It can be made visible by staining (thus the name which literaly means coloured material). 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. Changes in chromatin structure are effected mainly by methylation and acylation of the nucleosome proteins. Chromatin structure is also of importance with respect to the DNA replication.

There are three levels of chromatin organization:

nucleosome - "beads on a string" 30 nm condensed chromatin fiber 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.


Figure 2: Different levels of DNA condensation. (1) Single DNA strand. (2) Chromatin strand (DNA with histones). (3) Condensed chromatin during interphase with centromere. (4) Condensed chromatin during prophase. (Two copies of the DNA molecule are now present) (5) Chromosome during metaphase. Two distinct types of chromatin can be distinguished:

Euchromatin Heterochromatin

See also

Reference

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