Homologous Recombination Exchange between pair of homologous DNA segments Genetic exchange during meiosis DNA repair

Strand Invasion Base pairing between complementary strands of two different duplexes Heteroduplex

RecA Protein Catalyzes Strand Invasion Associates invading strand with DNA duplex being invaded Transient three-stranded structure Heteroduplex formation

Double Strand Break Repair Associated sister chromatids Strand invasion by broken chromosome DNA synthesis using unbroken chromosome as template

Meiotic Recombination Double-strand break Strand invasion Holliday junction Resolution either regenerates original duplexes or generates crossover

Holliday Junction Isomerization Resolution by cutting strands

Gene Conversion Divergence in allele distribution Occurs at sites of DNA synthesis following strand invasion and heteroduplex region at crossover sites Mismatch repair at heteroduplex

Preventing Inappropriate Homologous Recombination Potential recombination between repeated sequences Aborted due to excessive mismatch Deletions or gene conversions if allowed to occur

Site-specific Recombination Mobile genetic elements: moved to nonhomologous sites encode enzymes that catalyze mobilization recognition of short sequences at ends

DNA-Only Transposons Transposase recognizes sequences at ends Both ends cut and inserted at new site Cut-and-paste or replicative pathways

Retroviruses RNA genome Reverse transcription (RNA→DNA) Integration of DNA Transcription (DNA→RNA)

Retroviral-Like Retrotransposons Retroviral trans-position mechanism Integration of DNA via RNA intermediate

Nonretroviral Retrotransposons Complex of endonuclease, reverse transcriptase, and transposon RNA nicks target DNA Free 3’-OH as primer for first strand Comprise large proportion of human genome