Lecture 7 - 5:20 - Biology 172 Lecture 7 Friday Todays...

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1 Lecture 7: Friday, May 20, 2011 Biology 172 Today’s Outline Announcements 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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DNA Structure - the three-dimensional shape of DNA is a double helix (Watson and Crick, 1953) What information contributed to the discovery of the double helix structure? DNA composition X-ray diffraction data
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3 What does DNA do? 1. Copy Function - Faithfully copied during cell division DNA Replication 2. Expression Function - Encodes cellular products that determine phenotype RNA Protein Transcription Translation Structure of DNA
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Figure 4.9 DNA FORMS A TEMPLATE FOR ITS OWN SYNTHESIS. 1. If the hydrogen bonds between complementary base pairs are broken, the DNA helix can separate. 2. Each strand of DNA can serve as a template for the formation of a new strand. Free nucleotides attach according to complementary base pairing. 3. When the new strands polymerize to form a sugar- phosphate backbone, secondary structure is restored. New New Old Old Conclusion: The original molecule has been copied. Each copy has one strand from the original DNA molecule and one new strand.
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Semi-conservative model Conservative model Dispersive model Fig14.5 The Meselson-Stahl experiment tested three models of DNA replication
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Fig14.5 The Meselson-Stahl experiment
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7 Fig14.8 The Meselson-Stahl experiment Semi-conservative model - consistent with all data Conservative - eliminated after 1st generation Dispersive - eliminated after 2nd generation
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8 Figure 4-9 DNA FORMS A TEMPLATE FOR ITS OWN SYNTHESIS. 1. If the hydrogen bonds between complementary base pairs are broken, the DNA helix can separate. 2. Each strand of DNA can serve as a template for the formation of a new strand. Free nucleotides attach according to complementary base pairing. 3. When the new strands polymerize to form a sugar- phosphate backbone, secondary structure is restored. New New Old Old Conclusion: The original molecule has been copied. Each copy has one strand from the original DNA molecule and one new strand.
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9 Today’s Outline Chromosome Structure 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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10 New strand 5 end Phosphate Base Sugar Template strand 3 end 5 end 3 end 5 end 3 end 5 end 3 end Nucleoside triphosphate DNA polymerase Pyrophosphate - new nucleotides are added to an existing chain by phosphodiester bonds - catalyzed by DNA polymerase - requires a template - requires dNTPs - proceeds in the 5’ to 3’ direction Basic Features of DNA Replication
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11 DNA Synthesis Proceeds in Two Directions A chromosome being replicated
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(a) A chromosome being replicated 0.25 μ m Origin of replication Replication proceeds in both directions Old DNA New DNA Replication fork Replication bubble Old DNA New DNA Replication proceeds in both
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