CHAPTER 10 Homologous recombination at the molecular level

1
CHAPTER 10 Homologous recombination at the
molecular level
2
1.All DNA is recombinant DNA. 2.Genetic exchange
works constantly to blend and rearrange
chromosomes, most obviously during meiosis, when
homologous chromosomes pair prior to the first
nuclear division. 3.During this pairing, genetic
exchange between the chromosomes occurs. This
exchange, classically termed crossing over, is
one of the results of homologous recombination.
3
OUTLINE

• Models for Homologous Recombination
• Homologous Recombination Protein Machines
• Homologous Recombination in Eukaryotes
• Mating-Type Switching
• Genetic Consequences of the Mechanism of
  Homologous Recombination

4
Two models for Homologous Recombination

• 1.The Holliday Model
• 2.The Double-Strand Break Repair Model

5
The Holliday Model Illustrates Key Steps in
Homologous Recombination
10-1 Holliday model through the steps of branch
migration
6
10-2 Holliday junction cleavage
7
Key words of Holliday Model

• Holliday junction
• Branch migration
• Heteroduplex DNA
• Crossover product
• Patch product

8
The Double-Strand Break Repair Model More
Accurately Describes Many Recombination Events
10-3 DSB repair for homologous recombination
9
Duble-Stranded Breaks Arise by Numerous Means and
Initiate Homologous Recombination
10
3.Homologous recombination protein machines
11
The RecBCD Helicase/Nuclease Processes Broken DNA
Molecules for Recombination
10-5 Steps of DNA processing by RecBCD
12
RecA Protein Assembles on Single-stranded DNA and
Promotes Strand Invasion
10-6 Substrates for RecA strand exchang
13
10-7 Polarity of RecA assembly
14
Newly Base-Paired Partners Are Established Within
the RecA Filament
RecA Homologs are present in all organisms
15
Other Proteins

• RuvAB Complex Specifically Recognizes Holliday
  Junctions and Promotes Migration
• RuvC Cleaves Specific DNA Strands at the Holliday
  Junction to Finish Recombination

16
Homologous Recombination In Eukaryotes

• Homologous Recombination Has Additional Functions
  in Eukaryotes
• Homologous Recombination Is Required for
  Chromosome Segregation during Meiosis(nondisjuncti
  on,meiotic recombination)
• Programmed Generation of Double-Stranded DNA
  Breaks Occurs during Meiosis(Spo11 protein )

17
10-8 DNA dynamics during meiosis
18
10-9 Mechanism of cleavage by Spo11
19

• MRX Protein Processes the Cleaved DNA Ends for
  Assembly of the RecA-like Strand-Exchange
  Proteins
• Dmc1 Is a RecA-like Protein that Specifically
  Functions in Meiotic Recombination
• Many Proteins Function Together to Proote Meiotic
  Reconbination (recombination factories)

20
10-10 Overview of meiotic recombination pathway
21
10-11Dmc1-dependent recombination occurs
preferentially between nonsister homologous
chromatids
22
Mating-Type Switching
10-12 Genetic loci encoding mating-type
information
23

• Mating-Type Switching Is Initiated by a
  Site-Specific Double-Strand Break (HO
  endonuclease )
• Mating-Type Switching Is a Gene Conversion Event,
  Not Associated with Crossing Over(SDSA)

24
10-13 Recombination model for mating-rype
switchingsynthesis-dependent strand annealing
(SDSA)
25
Genetic Consequences of the Mechanism of
Homologous Recombination

• Exchang or Conversion
• Gene Conversion Occurs because DNA Is Repaired
  during Recombination

26
10-14 Comparion of the genetic and physical maps
of a typical region of a yeast chromosome
27
10-15 Mismatch repair of heteroduplex DNA within
recombination intermediates can give rise to gene
conversion
28
The end ?????? 200431060053.???