Holliday Model of Recombination

Holliday Model:

Robin Holliday (1964) proposed a model to explain general recombination. The complete pathway of DNA recombination needs interplay of following enzymes and proteins: Enzyme Endonulclease, DNA ligase, DNA-binding proteins, helix destabilization proteins and DNA unwinding proteins or DNA helicases. According to Holliday model, the following events occur in a sequential manner:

1. Recognition and alignment of homologous nucleotide sequences in two DNA duplexes of two homologous chromosomes (Synapsis).

2. Enzymes endonuclease cleaves single polynucleotide strands of each of the two parent DNA molecules at comparable positions (Breakage).

3. Segments of the single strands on one side of each cut are displaced from their complementary strands by their helix destabilization proteins.

4. The displaced strands then exchange pairing partners, base-pairing with the intact complementary strands of homologous chromosomes. This process is mediated by rec A protein. A homologous double helix is found promoting the displacement of a segment of one strand of double helix by the unpaired strand.

5. The cleaved strands are then covalently joined in recombination arrangements by the enzyme DNA ligase (Recombination).

6. By this union of cleaved segments an X-shaped recombination intermediate is formed. This is called “chi form” or Holliday intermediate or “Half chromatid chiasma“.

7. Holliday intermediate is cut in the bridge region to produce two independent duplexes. The cut might occur along east-west axis or along north-south axis. In the end two non-cross over strand having parental configuration and other two hybrids or recombinant strands are produced.