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

3. 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.

4. • 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.

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

6. Alignment

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

8. 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’

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

10. 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’

11. Animation

12. Vertical Cut and Horizontal Cut
Non-Crossover Crossover