CTools021611lecture17

CTools021611lecture17 - Biology 172 Lecture 17: Wednesday,...

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Lecture 17: Wednesday, February 16, 2011 Biology 172 Today’s Outline Announcements   DNA Replication - summary The Central Dogma Transcription - Basic Features - Initiation - Elongation - Termination RNA Processing - cap, tail, splicing Genetic code •  Discussion This Week: Central Dogma Quiz on Lectures 14, 15, 16
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Main Points from Monday Models for DNA Replication - Conservative model - Semi-conservative model - Dispersive model Meselson-Stahl Expt DNA Replication - Basic Features - Initiation - Elongation - Termination DNA Repair
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Synthesis of leading and lagging strands during DNA replication Parental DNA 5 3 Leading strand 3 5 3 5 Okazaki fragments Lagging strand DNA pol III Template strand Leading strand Lagging strand DNA ligase Template strand Overall direction of replication
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Leading strand Primase DNA pol III Sliding clamp
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5 3 Parental DNA 3 5 Overall direction of replication DNA pol III Replication fork Leading strand DNA ligase Primase OVERVIEW Primer DNA pol III DNA pol I Lagging strand Lagging strand Leading strand Leading strand Lagging strand Origin of replication Elongation Phase of DNA Replication Because parental DNA strands are antiparallel and synthesis only occurs in one direction, one new strand is made continuously and one new strand is made discontinuously Helicase Single-Strand Binding Proteins
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Termination of DNA Replication Ends of linear DNA (euks) – requires special enzyme (telomerase) to maintain proper length of telomeres Circular DNA (proks) and between oris (euks) – replication forks meet & ligase joins ends
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Telomeres and telomerase
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Proofreading and repair of DNA Initial error rates in DNA replication are rare (one in 100,000 base pairs) During replication, DNA polymerases ‘proofread’ each nucleotide against the template Mismatches can be recognized by other enzymes and fixed DNA molecules are subject to constant damage from radiation, including UV light
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(a) DNA polymerase III adds a mismatched base… (b) …but notices the mistake and corrects it. Figure 14.14
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UV irradiation damages DNA Ultraviolet light causes dimerization of adjacent thymines ( get ‘thymine dimers’) Base-pairing (H-bonds) to the opposite DNA strand is lost, causing a bulge in the double helix Skin cancers (carcinoma) caused by UV damage to DNA of epidermal cells
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Figure 14.16 Repaired damage 5 3 5 3 Damaged bases NUCLEOTIDE EXCISION REPAIR
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CTools021611lecture17 - Biology 172 Lecture 17: Wednesday,...

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