CHROMOSOMES
Chromosome consists of two identical parts called sister chromatids. Arms of sister chromatids are held together by centromere. The position of the centromere for each chromosome is strictly definite. After colouring centromeres look more dense and compact comparing to the arms of sister chromatids. These dense chromosome regions with intensive colouring are called heterochromatic. It is customary to call other, non-heterochromatic, regions of chromosomes euchromatic.
End-regions of chromosomes are called
telomers. Often they are also heterochromatic.
Besides chromatin, a complex of double-chain DNA, chromosomes of eukaryotic cells contain five histone proteins (H1, H2A, H2B, H3, H4). Successive chromatin segments, 145 base pairs long, are wound on nucleosome cores. Core is a histone octamer consisting of H2A, H2B, H3, and H4 histones. It contains two molecules of each kind. DNA molecule winding 1,75 times around the nucleosome core forms superhelix. The fifth histone, H1, is not included into the composition of the nucleosome core and does not participate in the process of winding of DNA on the histone octamer. It contacts with the DNA in the sites where the double helix goes in and out of the nucleosome core. In the structure like this (nucleosome) 168 base pairs of a spiral DNA are associated with one histone octamer and a molecule of the H1 histone.
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Supplementary information
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