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Topic 12, Chromatin and chromosome structure.ppt.edu

Topic 12, Chromatin and chromosome structure.ppt.edu -...

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Topic 12—Chromatin and Chromosome structure
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Learning Objectives Be able to define the following terms: histones, chromosome, chromatin, nucleosome, acetylation, antibodies Be able to explain how DNA can be compacted (via histones and other interactions) Be able to explain how modification of histone proteins can lead to changes in chromatin structure and gene expression Be able to describe how the ChIP assay works and how to interpret the results
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Overview DNA needs to be compact, because the length of total DNA is much too long to fit into a single cell Histone proteins are used to both compact the DNA and to control access of genes in the DNA to transcription (gene expression)
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Chromatin only forms in eukaryotic, not prokaryotic cells
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Size of a typical human cell nucleus=5 μ m or 0.000005m (about 2.05m)
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2.05m long, but fits into 0.000005m? 410,000x compacted to fit into a nucleus That is pretty compact!
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We still don’t fully understand the entire process….. Packing ratio refers to how compact the DNA is relative to DNA which is not compacted at all
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DNA exists in different configurations, depending upon the cell cycle What controls the changing conformation of the DNA?
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Metaphase chromosome As cell divides, DNA must be evenly partitioned between the 2 daughter cells DNA needs to be condensed before cell division. Why? If DNA is not condensed, it may break as it is pulled apart
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Centromere : DNA repeats where spindle fibers attach (for pulling & separation) Telomere : DNA repeats that protect the end of a chromosome
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Level 1: naked DNA, no compaction
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Level 2: DNA associates with histone octomers
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Nucleosome = DNA + histone About 1.6-2 twists of DNA per nucleosome (organism-specific)
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Charge analysis of the primary amino acid sequence of a histone protein - charge + charge C-terminus – globular (DNA wrapping) N-terminus – linear (available for modification)
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Linker regions: areas of DNA between histone octomers
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