Antirepression, Potentiation and Activation of Promoters in Reconstituted Chromatin
The amount of genetic information that higher eukaryotes need to propagate through generations is huge compared with that of prokaryotes. Consequently, large amounts of genomic DNA have to be packaged into the relatively small mucleus of a eukaryotic cell. The Greek word for “packaging” is the composite word “συσκευάζω” (συν + σκευάζω) which means “prepare together”. Accordingly, packaging of DNA has to meet two seemingly opposing requirements: first, DNA must be condensed (by a factor of approximately 4 × 105) in order to fit the limited nuclear volume, and second, it needs to be “prepared” in such a way that it can still function as a substrate for the nuclear machineries that replicate, transcribe, recombine and repair the genome. These two prerequisites are met by the chromatin organization of eukaryotic genomes. Chromatin is a loosely defined term describing the association of histones and non-histone proteins with DNA in a nucleus. The unit of chromatin is the nucleosome: an octamer of histones formed by the combination of an H3 and H4 histone heterotetramer and two heterodimers of histones H2A and H2B around which DNA winds in approximately two turns. A fifth type of histone, most commonly histone HI, interacts with the nucleosome body and the linker DNA that connects neighbouring nucleosomes. Arrays of nucleosomes are further folded into higher-order structures.
KeywordsTranscription Start Site Chromatin Structure High Mobility Group Heat Shock Factor Gaga Factor
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