Compare and contrast the Holliday and the Double Strand Break models

Key steps shared by both:

Alignment of two homologous DNA molecules.
Introduction of breaks in the DNA
Formation of initial short regions of base pairing between the two recombining DNA molecules: strand invasion
Movement of the Holliday junction: branch migration
Cleavage of the Holliday junction: resolution

The Holliday model illustrates key steps in homologous recombination

The Holliday model very well illustrates the DNA strand invasion, branch migration, and Holliday junction resolution processes central to homologous recombination.

The double-strand break repair model more accurately describes many recombination events

1. Homologous recombination is often initiated by double-stranded breaks (DSB) in DNA, while Holliday model is initiate by the pair of aligned nicks.
2. DSB occurs relatively frequently, but the aligned nicks not.

DSB breaks Processing of the DSB to generate DNA with 3’ ss tails Strand invasion of 3’ end Second strand invasion and DNA repair synthesis from the 3’ ends Branch formation and migration

Double –stranded DNA  breaks arise by numerous means and initiate homologous recombination

1. DSBs in DNA arise quite frequently.
2. In bacteria, the major biological role of homologous recombination is to repair DSBs (Ionizing radiation, damage in the DNA template); DSBs also promotes genetic exchange.
3. In eukaryotic cells, homologous recombination is critical for DNA repairing, as well as for the process of chromosomal pairing during meiosis.

Here is an excellent website that explains the two in detail:

http://www.usask.ca/biology/genetics/recombination/recom.htm