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slides-14_1 - #14 Eukaryotic transcription (IV) 1.Chromatin...

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#14 Eukaryotic transcription (IV) 1.Chromatin structure 2.Nucleosome positioning 3.Histone modifications 4.Chromosome remodeling
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Chromatin, chromatid, and chromosome Genomes exist as chromatins before or after cell division (interphase) but as chromatids or chromosome during cell division 1. Chromatin structure chromosomes chromatin
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Euchromatin : less condensed chromatin, light color under microscope, actively transcribed genomic regions Heterochromatin : highly condensed chromatin regions, such as centromeres and telomeres, which are not actively transcribed. Euchromatin and heterochromatin
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Nucleosomes are the basic structural units of chromatins 10 nm 35-85 kb per loop
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Nucleosome and core histones Nucleosomes are DNA-histone particles. A core nucleosome consists of 147 bp DNA wraped around core histone octamer (8-molecule complex). Core histones are H2A (~13 kD) H2B (~14 kD) H3 (~15 kD), H4 (~11 kD). The core nucleosomes are linked by about 50bp linker DNA and linker histones H1 (~21kD). Histones are basic proteins rich in Arg and Lys, bind tight to negatively charged DNA.
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Summary of chromatin structure Linker histone H1 Core nucleosome 2x(H2A, H2B,H3, H4) 147bp DNA (10 nm fiber)
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How chromatin structure affects transcription activity of genes? 1. The position of core nucleosome on DNA affects binding of transcription factors to the specific DNA regions and transcription activity 2. Various chemical modifications of histone N-terminal tails affect the structure of nucleosome and transcription activity 3. The degree of chromatin condensation (i.e. how tightly is chromatin packed) affect transcription, e.g. less condensed euchromatins transcribe more than highly condensed heterochromatins, completely condensed chromosomes do not transcribe at all
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2. Nuleosome positioning 1. The location of nucleosomes may be affected by the number of linker histone H1 , histone modification, and chromosome remodeling proteins 2. An active promoter is free of nucleosome 3. Core nucleosome localization on DNA is dynamic (nucleosome "moves"), the location of core nucleosomes can be changed by histone modification and chromosome remodeling , which is controlled by histone modifying enzymes and chromosome remodeling proteins, respectively.
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(1) Linker histone (H1) may inhibit transcription Nucleosomes are connected by histone H1, also called linker histone. H1 often represses
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This note was uploaded on 02/20/2009 for the course LS 3 taught by Professor Lin during the Spring '06 term at UCLA.

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slides-14_1 - #14 Eukaryotic transcription (IV) 1.Chromatin...

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