The centromère is the site of constriction of the Chromosome whose functions are essential with the life of the cells Eucaryotes, in particular (1) to connect both Chromatides sisters between them, the replication and the anaphase, (2) to ensure the proteinic assembly of the Kinétochore at its level, him even responsible for (3) the equitable segregation of the chromatides sisters between the two cells girls during the cellular division. The centromères of the majority of the organizations eucaryotes are composed of repeated DNA and carry particular epigenetic marks.
The composition in DNAThe sequence of DNA repeated of the centromères enormously varies from one species to another. Generally the DNA is repeated there out of tandem on several kilobases. At the Man, the repeated DNA is mainly DNA alpha-satellite, of which the unit reason (monomeric) made 170 nucleotides length. These monomers are repeated a very great number of times. Therefore they are named " higher Order repeat" (HOR). These HOR are aligned in an uninterrupted way in the same orientation at very important distances, energy of 40kb with 5Mb. These HOR are very homogeneous on the level of their sequence: they divide 99.8% of identity of sequence, whereas the monomers within these HOR divide only between 60 and 80% of identity of sequence. This organization is presented figure 2. In addition to this organization in very homogeneous repeated DNA and inintterompue, particular sequences seem to be of primary importance for the assembly of the complex of the kinétochore, which gives to the centromère its essential function during the cellular division: capacity with ségréger equitably chromatides in the cells files. It is the case of the " limp CENP-B" (CENP-B box), long of 17pb, able to bind protein CENP-B, necessary to the assembly of this proteinic complex. The presence of these particular sequences of fastener, present in each HOR, as well as the organization in DNA highly repeated was characteristics of the human centromères, and good number of other species eucaryotes. However, of the recent observations on the formation of néo-centromères in absence of DNA alpha-satellite incite to think that the sequence of DNA to the centromère is not only the person in charge of its function.
Epigenetic marksThe epigenetic marks are the whole of the reversible modifications and héritables which can take place on the Chromatine. These marks can be chemical modifications of the Histones (methylation, acetylation, phosphorylation…) or of the DNA, or of variable of histones. It is the case on the level of the centromère, since Histone H3 is replaced by one of its variable, protein CENP-A. This protein CENP-A is found in all the human centromères, like in the néo-centromères. This variable of histone, as well as many other epigenetic modifications, give a structure particular to centromeric chromatin. It seems looser than the Hétérochromatine (which prohibits the transcription because of sound too high degree of compaction) and more compact than the Euchromatine.
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