[Structure of chromatin. 2: levels of organization of DNA in the nucleus. Highly organized structures].
In the interphase nucleus, the 30 nm fiber, whose characteristics were discussed in the first part of this review, is condensed 10-15 fold to form loops or domains of chromatin with a DNA packing ratio of 600. In the metaphase chromosomes, the packing ratio is 12000. The existence of loops in the interphase nucleus has been suggested by experiments from electronic microscopy, sedimentation and nuclease digestion. These loops of 20-200Kb are attached at their bases to a non-histone protein network, that is the nuclear matrix or scaffold whose major component is the topoisomerase II. There is disagreement regarding the arrangement of the loops in metaphase chromosomes and several models have been proposed. Six of these models are presented. A number of data suggest that the sequences of attachment (SARs/MARs) define functional units containing related genes. Thus, the compartmentation of the genome in topographically independent domains might ensure the fidelity of epigenetic transmission of the chromatin structure, and it might help to control gene expression. In the interphase nucleus, the chromosomes coexist with other domains involved in major nuclear functions such as transcription, replication or post-transcriptional modifications and whose compartmentation is not always evident.
Santisteban
M S
MS
University of Virginia, HSC, School of Medicine, Department of Microbiology, Charlotesville 22908, USA.
D002503:Centromere / Q000737:chemistry / Q000235:genetics; D002843:Chromatin / Q000737:chemistry* / Q000235:genetics; D002875:Chromosomes / Q000737:chemistry / Q000235:genetics; D004247:DNA / Q000737:chemistry* / Q000235:genetics; D004250:DNA Topoisomerases, Type II / Q000235:genetics; D006801:Humans; D007399:Interphase / Q000502:physiology*; D008677:Metaphase / Q000502:physiology*; D015530:Nuclear Matrix / Q000235:genetics; D016615:Telomere / Q000737:chemistry / Q000235:genetics
D002843:Chromatin; D004247:DNA; D004250:DNA Topoisomerases, Type II
La structure de la chromatine. 2e partie: niveaux d'organisation de l'ADN dans le noyau. Structures hautement organisées.