Part 9: Recombination and Mobile Elements

Genetic Shuffling

Recombination creates genetic diversity by exchanging DNA between molecules. Mobile genetic elements can move within and between genomes, driving evolution and sometimes causing disease. These processes shape genomes over evolutionary time.

Homologous Recombination

Exchange between similar or identical DNA sequences

The Holliday Model

Strand invasion → Branch migration → Holliday junction → Resolution

Key Proteins

  • • RecA/Rad51: Strand exchange
  • • RecBCD: Initiation (E. coli)
  • • BRCA1/2: Human HR
  • • RuvABC: Resolution

Functions

  • • DSB repair (high fidelity)
  • • Meiotic crossing over
  • • Replication fork restart
  • • Genetic diversity

Site-Specific Recombination

λ Phage Integration

  • • attP (phage) × attB (bacteria)
  • • Integrase catalyzes recombination
  • • Reversible: excision requires Xis
  • • Model for gene therapy vectors

V(D)J Recombination

  • • Generates antibody diversity
  • • RAG1/RAG2 proteins
  • • RSS sequences (12/23 rule)
  • • Essential for adaptive immunity

Mobile Genetic Elements

Transposons

DNA Transposons

"Cut and paste" mechanism. Transposase excises element, inserts at new site. Target site duplications.

Retrotransposons

"Copy and paste" via RNA intermediate. Reverse transcriptase. LTR and non-LTR types.

Impact on Genomes

~45%

Human genome from transposons

~17%

LINE elements (L1)

~11%

SINE elements (Alu)

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