Chromatin

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Chromatin is the combination of DNA, histone, and other proteins that makes up chromosomes. It is found inside the nuclear envelope of eukaryotic cells. It is divided between heterochromatin (condensed) and euchromatin (extended) forms.[1][2] The functions of chromatin are to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis and meiosis, and to control gene expression and DNA replication. Changes in chromatin structure are affected by chemical modifications of histone proteins, such as methylation and acetylation, and by other DNA-binding proteins.

Contents

Basic structure

In simple terms, there are three levels of chromatin organization (Fig. 1):

These structures do not occur in prokaryotic cells. Examples of cells with more extreme packaging are spermatozoa and avian red blood cells.

During spermiogenesis, the spermatid's chromatin is remodelled into a more spaced-packaged, widened, almost crystal-like 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).

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