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Lecture 7 Chromosome structure 022218.pptx

Lecture 7 Chromosome structure 022218.pptx - REMINDER FIRST...

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REMINDER FIRST MID-TERM IS NEXT WEEK: THURSDAY, MARCH 1 TAKE ADVANTAGE OF MONDAY TUTORING HRS AT SKIRBALL LEARNING CENTER!
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Chromosome territories in a chicken cell nucleus. Each chromosome is marked with a distinct color. Each is confined to its own, well-defined territory. IMPORTANT POINTS REGARDING CHROMOSOME STRUCTURE: a) eukaryotic chromosomes are organized into chromatin b) chromatin is essential for chromosome structure c) chromatin state is important for gene expression: defines “euchromatin” and “heterochromatin”
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EUKARYOTIC CHROMOSOMES a) Eukaryotes have multiple linear chromosomes compacted in the nucleus b) The degree of compaction is enormous: 1.8 meters of DNA is packed into a 5 micrometer ( m = micron) nucleus (actual size of nucleus varies among cell types/organisms) . 1.8m = 1.8 x 10 6 m/5 m = 0.36 x 10 6 = 360,000 – fold compaction ! c) We do not fully understand how this is accomplished d) The DNA and associated proteins of a chromosome constitute the “chromatin” e) Proteins that organize chromosomes are essential and provide a mechanism for chromosome organization, condensation, segregation, and control of gene expression.
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Chromosome compaction overview Figure: Maeshima, K. et al. Chromosoma 123:225. 2014 String of nucleosomes is known as “10 nm fiber” – first described by Flemming in 1800’s Actual diameter of the “10nm fiber” is 11nm and includes: a. Histone octamer = “core” b. ~146bp , 2nm diameter DNA helix wrapped around this histone “core” c. Linker DNA , 20-80bp d. Overall repeat unit in the 10nm fiber includes ~ 165 to ~220bp of DNA, depending on linker length Some controversy about “regular” folding into 30nm fiber (solenoid) vs irregular folding
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NUCLEOSOME The nucleosome is responsible for the first level of compaction (~7X) Linked nucleosomes form the 10nm fiber, or ‘beads on string’
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NUCLEOSOME Figure 11.6 HISTONE CORE : two molecules of each histone H2A, H2B, H3, and H4 = 8 total proteins = “octamer” Fifth type of histone = H1 (see later); not part of the nucleosome All histone proteins are highly conserved among eukaryotes
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HISTONES 1. Small = 11-14kD (kiloDaltons), basic proteins, rich in Lys. 2. H1 is a bit larger than the others (23kD) 3. Histones neutralize the negative charges of the DNA so that it can be compacted (would otherwise tend to self-repel)
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NUCLEOSOME Nucleosome structure: Remember that the nucleosome is 11nm in diameter, and 5.7nm in thickness Figure 11.7 11nm 5.7nm Notice: there are two molecules of each histone in a nucleosome
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The DNA around the nucleosome (1.75 turns) is the ‘core DNA ’; ~146 bp (many investigators quote 147bp ) The DNA between nucleosomes is called the “linker DNA” Linker varies in length , both among organisms and among cell types in the same organism (~20bp to 80bp).
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Figure 11.6
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