Lecture 10 - 5:27 - Lecture 10: Friday, May 27, 2011...

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Unformatted text preview: Lecture 10: Friday, May 27, 2011 Biology 172 Todays Outline Chromatin structure Enhancers DNA binding motifs DNA methylation Photosynthesis- Overview- Light Reactions- Calvin Cycle Announcements In eukaryotes, DNA is wrapped tightly around protein to form chromatin . Chromatin contains nucleosomes repeating, beadlike structures consisting of DNA wrapped twice around histone proteins Eukaryotic DNA must undergo a series of physical changes before transcription can take place. Multiple levels of chromatin packing in euks 1. Nucleosomes (10-nm fiber) beads on a string basic unit of DNA packing beads = histone proteins have amino tail projecting outward string = double strand of linker DNA Interactions between histone tails of one nucleosome, linker DNA, and adjacent nucleosomes cause coiling 30-nm fibers form loops attached to a chromatin scaffold of non-histone proteins to make 300-nm fibers 2. Higher levels = 30 nm and 300 nm fibers Multiple levels of chromatin packing in euks Chromosome Structure Levels of Eukaryotic DNA Packaging Nucleosome basic unit of DNA packing Packing of looped domains into fibers and into condensed chromosomes Figure 18-2 Nucleosomes in chromatin Nucleosomes DNA Nucleosome structure Linker DNA H1 protein attached to linker DNA and nucleosome DNA Group of 8 histone proteins Nucleosome In some cases, nucleosomes may be grouped into 30-nanometer fibers. 30 nm Particular genes are always in the same place 3. Further packing produces metaphase chromosomes Multiple levels of chromatin packing in euks How Is Chromatin Altered? ATP-dependent chromatin-remodeling complexes . Histone acetyl transferases (HATs) add acetyl groups ( acetylation ) or methyl groups ( methylation ) to histones. This decondenses the chromatin and allows gene expression. Histone deacetylases (HDACs) remove the acetyl groups from histones to allow chromatin condensation and turn off gene expression. Decondensed chromatin allows access to RNA polymerase Histone Acetylation = covalent addition of acetyl groups (-COCH 3 ) to neutralize positive charge of lysines in histone tails Some important regulatory elements are Near the promoter Eukaryotic Promoters TATA-binding protein ( TBP ) binds to the TATA box Basal Transcription complex TATA-binding protein ( TBP ) binds to the TATA box Multi-step process--- 60 proteins including RNA polymerase assemble around TBP Some are associated directly with TBP, some are not Some important regulatory elements are Far from the promoter Enhancers can be more than 100, 000 bases away from the promoter Enhancers occur in all eukaryotes Enhancers are unique regulatory sequences that can influence gene expression from a long distance upstream, downstream, or within the gene they regulate....
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Lecture 10 - 5:27 - Lecture 10: Friday, May 27, 2011...

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