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Lecture 2-4-08

Lecture 2-4-08 - 3 RNA is removed and replaced by RNA...

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2/4/08 Mitsley and Ris: DNA amount is constant but gametes have ½ amt A=T C=G (Chargaff’s rule) DNA Structure Purines (cytosine and thymine, one ring) pair with pyrimidines via hydrogen bonds Backbone = deoxyribose+ phosphate 5’ pairs to 3’ (“antiparallel”) Major and minor grooves DNA Replication Conservative model: parental double helix remains intact and 2 nd , all-new copy is made Semiconservative model : two strands of parental molecules separate, and each functions as a template for synthesis of a new complementary strand Dispersive model : Each strand of both daughter molecules contains a mixture of old and newly synthesized parts; ‘patchwork’ Grow bacteria in N 15 for 20 minutes; enough for 1 replication – harvest bacteria and look at replicated DNA Many origins of replication in eukaryotic chromosomes – forms replication bubbles Only add to the 3’ end! 1. Primase puts down RNA primer so nucleotides match the DNA 2. DNA polymerase III now builds on the 3’ end of the RNA primer
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Unformatted text preview: 3. RNA is removed and replaced by RNA polymerase I 4. helicase uncoils strands, primase puts down RNA primer, DNA polymerase III builds on chain on leading RNA strand (from 3’ of DNA on), builds Okazaki fragments on lagging strand (5’ on) Okazaki fragments: 1. RNA primase forms RNA primer 2. Primase released 3. DNA polymerase III synthesizes strand until it meets the previous one 4. DNA polymerase I replaces RNA primer 5. DNA ligase ‘stitches’ the pieces together DNA polymerase III has an error rate of about 1 in 10,000 base pairs Final error rate is ~ 1 in a billion Poly III can do approx 1000 bp/second in E. coli – slower in eukaryotes Rotates at about 3000 revolutions per minutes Telomeres Take a sequence like GGGATT – it’s repeated around 2500 times in humans Telomerase: preserves length of chromosomes on repeated sequences...
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