GENETIC RECOMBINATION.

Classifications of Genetic Recombination
  • Three Classifications:
    • Homologous Recombination: Involves genetic exchange between two DNA molecules sharing extended regions with nearly identical sequences.
    • Site-specific Integration: Occurs at specific DNA sequences, allowing certain DNA segments to jump between chromosome locations.
    • DNA Transposition: Refers to short DNA segments that can move from one chromosome site to another.
Introduction to Genetic Recombination
  • Germline Eukaryotic and Prokaryotic Cells: Both can exchange homologous DNA segments.
  • Prokaryotic Exchange: Includes genetic information exchange among bacteria or viruses, promoting genetic diversity.
  • Site-Specific Integration: Example: attB site where specific recombination occurs.
Recombination of Phage T2 in E. coli
  • T2 Phage Variants:
    • h-: Infects both E. coli strains, leaving a clear plaque.
    • h+: Infects only certain E. coli strains, resulting in a cloudy plaque.
    • r-: Causes rapid lysis, larger plaque.
    • r+: Causes slow lysis, smaller plaque.
  • Observations: Parental phenotypes dominate, but some plaques are recombinant.
Recombination of Phage Lambda λ and the Meselson-Weigle Experiment
  • Experiment Setup (1961): Involved light and heavy λ phages grown in C- media.
  • Recombination: Observed in infected bacteria leading to the formation of light phages from heavy phage mixtures.
Homologous Recombination in Eukaryotes
  • Occurs at Meiosis: Specifically between homologous segments of DNA.
  • Chiasma Formation: Involves crossing over of parental chromosomes resulting in recombinant chromosomes.
  • Tetrad Formation: Four chromosomes after replication during meiosis.
Importance of Genetic Recombination
  • Evolutionary Significance: Enables the separation of beneficial and detrimental mutations within populations, facilitating evolutionary change.
  • Testing Alleles: Allows for the testing of new alleles on a population level rather than just individuals.
Holliday Model of Homologous Recombination
  • Mechanism Description: Described by Robin Holliday, involves homologous chromosomes aligning and undergoing strand invasion and exchange.
  • Holliday Junction: A four-way cross-strand intermediate formed during recombination.
  • Polarity of Strands: Only strands of like polarity participate in the exchange.
Mechanism of Holliday Junction Formation and Resolution
  • Formation Process: Involves nicking of homologous strands and crossover which can migrate through branch migration.
  • Resolution: Essential for separating chromatids, involves cleaving the junction at specific points.
  • Branch Migration: Facilitated by ATP-dependent enzymes moving DNA past the junction.
Double-Strand Break Repair through Homologous Recombination
  • RecBCD Complex:
    • Initiation: Edits DNA to create 3' overhangs on the damaged strand.
    • Recruitment of RecA: Binds to the single-stranded DNA to form a nucleoprotein filament for homologous pairing.
RecA and Strand Invasion Process
  • Filament Formation: RecA polymerizes on single-stranded DNA, facilitating the search for double-stranded homologous DNA.
  • Strand Exchange: Binding leads to the spooling in of double-stranded DNA while unwinding occurs to facilitate strand invasion.
RuvABC Complex in Holliday Junction Resolution
  • RuvA and RuvB: Enzymes that drive branch migration of Holliday Junctions.
  • RuvC: Cleaves Holliday junctions to resolve them into separate strands.
  • Possible Outcomes: Can produce different recombinant products via vertical or horizontal cleavage.
Key Proteins in Eukaryotic Homologous Recombination
  • Rad51: Eukaryotic equivalent of RecA; critical in forming nucleoprotein filaments for homologous pairing.
  • BRCA1/BRCA2: Contribute to DNA damage repair and tumor suppression, involved with homologous recombination processes.