Chapter 6 Molecular Biology of DNA Replication and Recombination Jones and Bartlett Publishers ©...
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Transcript of Chapter 6 Molecular Biology of DNA Replication and Recombination Jones and Bartlett Publishers ©...
![Page 1: Chapter 6 Molecular Biology of DNA Replication and Recombination Jones and Bartlett Publishers © 2005.](https://reader035.fdocuments.net/reader035/viewer/2022081501/56649edb5503460f94bead7f/html5/thumbnails/1.jpg)
Chapter 6
Molecular Biology of DNA Replication and Recombination
Jones and Bartlett Publishers © 2005
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A misinserted deoxynucleotide is excised by the proofreading exonuclease function
of DNA polymerase
Q1. Why is a mismatch a problem?
Q2. What is the consequence if proofreading doesn’t catch a mistake?
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Addition of a dideoxynucleotide to the 3’-OH end of a DNA chain terminates chain elongation
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The chain termination method of DNA sequencing
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DNA sequencing machines use fluorescent dideoxynucleotides
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A “shotgun sequencing” approach for sequencing of a large DNA molecule
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Analogs of normal deoxynucleotides useful as anti-AIDS agents
Dideoxynucleosides are used because they can cross cell membranes more easily than can dideoxynucleotides
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Excision repair of a mismatched base pair
Mismatch repair can result in ‘gene conversion’.
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Variations from normal 2:2 segregation (gene conversion) can result from mismatch repair
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Molecular details of recombination
• How are heteroduplexes formed as intermediate steps in recombination?
• First model that described molecular recombination was suggested by Holliday in 1964.
• Holliday junction
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Symmetric model of homologous recombination showing some of the intermediate
steps and structures
Note the 2 nicks in the same location in the two duplexes in
step (B). This is followed by strand
exchange and Holliday junction
formation (C)
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An asymmetric model of homologous recombination
Recombination is initiated by a cut in
only one of the 2 recombining duplexes
(A) followed by strand invasion (B)
and Holliday junction formation (C)
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A recombining molecule showing the parental and the recombinant duplexes joined at the Holliday junction
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Enzymes involved in recombination in E. coli
• Obvious ones: exonuclease, ligase
• Others: RecA
• RecBCD
• RuvC – Holliday junction-resolving enzyme
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Resolution of a Holliday junction can occur by 2 alternative cleavage pathways
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3D view of a Holliday junction
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Double-strand break & repair
• The current favored model for homologous recombination is the double-strand break and repair model.
• A broken duplex interacts with an intact duplex.
• Gaps and sticky ends on broken duplex invade intact duplex.
• Displacement results (D loop).
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Model for the recombinational repair of a double strand break in DNA