Holliday Model of DNA Recombination

Holliday Model of DNA Recombination which predicts about the crossover and non cross over. Provide base for ds break model.

1. Holliday Model Muhammad Saeed Siddiqui BOT-19-27

2. Holiday Model of Recombination Discovered by Robin Holliday in 1964 in E. Coli Explain heteroduplex formation and gene conversion during recombination. Although it has been supplanted by double strand break model, it provides a base for understanding. It illustrates the critical steps of pairing of homologus duplexes, formation of a heteroduplex , formation of recombination joints, branch migration and Resolution.

3. Duplex Double stranded DNA molecule, generated by base pairing between complimentary single strands. Belong to same source Heteroduplex Double stranded DNA molecule, generated by base pairing between complimentary single strands. Double strands derived from different parental duplex molecule. Occurs during genetic recombination.

4. Steps Alignment Nick formation ( endo -nucleases) Strand invasion and ligation ( heteroduplex formation) Holliday junction/ Holliday fork/ Holliday intermediate/ chi-structure. Branch migration Junction is resolved Recombination

5. Alignment

6. Nick Formation Nick Endo-Nucleases cuts single strand of each homologus DNA at the same position 5’ 5’ 5’ 5’ 3’ 3’ 3’ 3’

7. Chain Invasion Helicase and SSB Protein, which open the strand and migrate each open strand to the other DNA molecule. Then Ligase enzyme attaches both open strands. 5’ 3’ 3’ 5’ 3’ 5’ 5’ 3’

8. Migration Branch migration occurs which forms the heteroduplex . 5 ’ 3’ 3’ 5’ 3’ 5’ 5’ 3’

9. Resolve of Junction Duplex pulled away from one another. And rotate at 180 degree. Resolvase ( RuvC ), Ligase , gyrase and Pol. 5’ 3’ 3’ 5’ 3’ 5’ 3’ 5’

10. Animation

11. Vertical Cut and Horizontal Cut Non-Crossover Crossover