Learning Objective

Explain the mechanism of site-specific recombination, including the roles of attachment sites and restriction endonucleases, and describe how it differs from homologous recombination.

Site-specific recombination is the integration of one DNA molecule into another at a specific site, even when the two DNA molecules share no extended sequence homology, except for a short sequence known as an attachment, integration, or insertion site.

Mechanism

• Requires specific restriction endonucleases to recognize the attachment sites.
• Recombination occurs precisely at these small sequences, rather than through general homology.
• Because it integrates the entire DNA molecule instead of exchanging segments, the final DNA molecule is a combination of the two original molecules.

Special Note

• Transposition is the site-specific integration of transposons.

Major Biological Roles

• Integration of a fertility factor to convert an F⁺ plasmid into an Hfr cell.
• Integration of temperate phage DNA into a bacterial chromosome to create a prophage.
• Movement and insertion of transposons within the genome.

Key Features

• No need for long stretches of homologous DNA.
• Generates stable integration of plasmids, phages, or other genetic elements.
• Critical for:
  • Phage integration into bacterial genomes (e.g., λ phage → prophage)
  • Integration of episomes or mobile genetic elements

Activity