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.