BIOL-UA 21 Chapter Notes - Chapter 7: Euchromatin, Transcription Preinitiation Complex, Elongation Factor
27 Mar 2016
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7. 5 molecular mechanisms of transcription repression and activation. The repressors and activators that bind to specific sites in dna and regulate expression of the associated protein-coding genes do so by three mechanisms. Regulatory proteins act with other proteins to modulate chromatin structure, inhibiting or stimulating the ability of general transcription factors to bind to promoters. Dna in eukaryotic cells is not free but associated with equal mass of protein in form of chromatin. Structural unit of chromatin is nucleosome, which is composed of 147 base pairs of dna wrapped around a disk-shaped core of histone proteins. Residues within the n-terminal region of each histone, and the c-terminal regions of histones h2a and h2b, called histone tails, extend from surface of nucleosomes and can be reversible modified. Modifications influence the relative condensation of chromatin and its accessibility to proteins required for transcription initiation. Activators and repressors interact with a large multiprotein complex called: the mediator of transcription complex called mediator.
