7.3 Recombination and Homologous Repair

Homologous recombination is a crucial process in DNA repair and genetic diversity. It swaps DNA segments between similar molecules, fixing double-strand breaks and shuffling alleles during meiosis. This mechanism ensures genomic stability and creates new gene combinations.

The repair process involves several steps, from resecting broken DNA ends to resolving recombination intermediates. Compared to non-homologous end joining, homologous recombination is more accurate but slower. It's vital for maintaining genetic stability and driving evolution.

Homologous Recombination and DNA Repair

Homologous recombination in DNA

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• Homologous recombination exchanges DNA segments between two similar or identical DNA molecules (sister chromatids or homologous chromosomes)
  • Occurs during meiosis facilitates genetic diversity in offspring
  • Occurs during DNA repair maintains genomic stability by accurately repairing damaged DNA
• Repairs double-strand breaks (DSBs) in DNA caused by factors such as ionizing radiation or replication errors
• Generates new combinations of alleles through crossing over during meiosis
  • Crossing over involves physical exchange of DNA segments between homologous chromosomes
  • Contributes to genetic variation within a population by shuffling alleles on chromosomes

Double-strand break repair process

1. Resection of broken DNA ends creates 3' single-stranded DNA overhangs
   • Nucleases (MRE11, EXO1) digest 5' ends leaving 3' overhangs
2. Single-stranded DNA invades homologous DNA template forming displacement loop (D-loop)
   • Recombinase proteins (RAD51) facilitate strand invasion and homology search
3. DNA synthesis occurs using the homologous template to restore missing sequence
4. Recombination intermediates resolved through either:
   • Double Holliday junction (dHJ) pathway
     1. Formation of double Holliday junctions between invading and template strands
     2. Resolution of junctions by structure-specific endonucleases (GEN1, MUS81-EME1)
     3. Leads to crossover or non-crossover products depending on resolution orientation
   • Synthesis-dependent strand annealing (SDSA) pathway
     1. Newly synthesized DNA strand dissociates from template
     2. Reanneals with the other broken end
     3. Results in non-crossover products only

Repair mechanisms: NHEJ vs HR

• Non-homologous end joining (NHEJ)
  • Directly ligates broken DNA ends without requiring homologous template
  • Can occur throughout the cell cycle (G1, S, G2, M phases)
  • Error-prone may introduce small insertions, deletions, or substitutions at repair site
  • Useful for quickly repairing breaks to prevent more extensive damage
• Homologous recombination (HR)
  • Uses homologous DNA template (sister chromatid or homologous chromosome) to repair DSBs
  • Primarily occurs during S and G2 phases when sister chromatid available as template
  • Considered error-free as it uses homologous template for accurate repair
  • Takes longer than NHEJ but results in more precise repair
• Both mechanisms important for maintaining genomic stability in different contexts

Recombination impact on genetic variation

• Recombination during meiosis critical for generating genetic diversity
  • Occurs during prophase I when homologous chromosomes pair and form synaptonemal complexes
  • Programmed double-strand breaks formed by SPO11 enzyme and repaired by HR
  • Crossing over exchanges genetic material between homologous chromosomes
• Crossing over generates new combinations of alleles on recombinant chromosomes
  • Shuffles existing genetic variation to create novel genotypes (eye color, hair color)
  • Breaks up linkage between alleles on same chromosome
• Increases genetic diversity of gametes (sperm, eggs) and offspring
  • Each gamete contains a unique combination of recombinant chromosomes
• Genetic variation essential for population adaptation and evolution
  • Provides raw material for natural selection to act upon
  • Allows populations to respond to changing environments (climate change, new pathogens)
• Recombination key driver of genetic variation in sexually reproducing species (humans, animals, plants)