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Lecture Molecular biology (Fifth Edition): Chapter 22 - Robert F. Weaver

Chapter 22 - Homologous recombination. The main contents of this chapter include all of the following: The RecBCD pathway for homologous recombination, experimental support for the RecBCD pathway, meiotic recombination, gene conversion. | Molecular Biology Fifth Edition Chapter 22 Homologous Recombination Lecture PowerPoint to accompany Robert F. Weaver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Homologous Recombination Homologous recombination occurs between homologous chromosomes during meiosis The process scrambles the genes of maternal and paternal chromosomes resulting in nonparental combinations in the offspring Meiotic recombination forms physical links between homologous chromosomes that allow them to align properly during meiotic prophase so they separate properly during meiotic metaphase It also plays an important role in allowing cells to deal with DNA damage by recombination repair 22- 22.1 Homologous Recombination Pathways 22- RecBCD Pathway – Initial Binding RecBDC-sponsored homologous recombination in E. coli: DNA helicase activity unwinds the DNA toward a Chi-site Sequence 5’-GCTGGTGG-3’ Chi sites found on average every 5000 bp in E. coli genome | Molecular Biology Fifth Edition Chapter 22 Homologous Recombination Lecture PowerPoint to accompany Robert F. Weaver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Homologous Recombination Homologous recombination occurs between homologous chromosomes during meiosis The process scrambles the genes of maternal and paternal chromosomes resulting in nonparental combinations in the offspring Meiotic recombination forms physical links between homologous chromosomes that allow them to align properly during meiotic prophase so they separate properly during meiotic metaphase It also plays an important role in allowing cells to deal with DNA damage by recombination repair 22- 22.1 Homologous Recombination Pathways 22- RecBCD Pathway – Initial Binding RecBDC-sponsored homologous recombination in E. coli: DNA helicase activity unwinds the DNA toward a Chi-site Sequence 5’-GCTGGTGG-3’ Chi sites found on average every 5000 bp in E. coli genome RecBCD protein has ds- and ss-exonuclease activity ss-endonuclease activity Activities permit RecBCD to produce a ss-tail now coated by RecA protein 22- The RecBCD Pathway Schematic A well-studied pathway used by E. coli REPLACE WITH REVISED FIGURE 22.2 22- RecBCD Pathway – D Loop Invasion of a duplex DNA by a RecA-coated single-stranded DNA from another duplex that has suffered a double-stranded break Invading strand forms a D loop (displacement) Loop is defined by displaced DNA strand When tail finds homologous region, nick occurs in in D-looped DNA Nick allows RecA and ss-break create a new tail that can pair with gap in the other DNA Subsequent degradation of the D-loop strand leads to the formation of a branched intermediate 22- Holliday Junctions Branch migration in this intermediate yields a Holliday junction with 2 strands exchanging between homologous chromosomes Branch in the Holliday junction can migrate in either direction by breaking old base pairs and forming