Question

1. What is homologous recombination? What is site-directed recombination? Compare and contrast the two processes. Address their principles, molecular mechanisms, and physiological functions.

Answer 1

Homologous Recombination is a type of recombination in which recombination occurs between two similar DNA molecules. eg gametogenesis events in humans

• dependent on the sequence homology
• steps in homologous recombination- it was first described by Robin Holliday and its model is thus called holliday model. in which single strand nicks are introduced at the same site on both parental molecules then the nicked strands are exchanged by complementary base pairing and ligation of two crosslinked strands produce holliday junction. the junction is cleaved at branching point and spliced products are sealed thus two recombined molecules are obtained.
• another type of homologous recombination is dsDNA break recombination.one of the two DNA molecules gets nicked DNA is resected from breaking point by an exonuclease that leaves 3' OH overhangs. one of the overhangs invades other duplex. the formation of heteroduplex generates D loops likewise other strand invades the Donor DNA duplex thus holliday junction is formed which is later cleaved and recombinant molecules are thus obtained.

Site-Directed Recombination as suggested by name; the recombination occurs at the specific sequence of DNA molecules. eg lambda phage integration.

• it is highly specific only short stretch recombine
• not dependent on the sequence homology
• the process initiated between two DNA which has the common specific site for recombination the
• well-studied model is the integration of lambda phage in E. coli genome. the recombination occurs between att sites called site for attachment which shares little homology.