Biochemistry 3107: Week 4 Tutorial

1. Biochemistry 3107:Week 4 Tutorial Phil Williams octobon@hotmail.com 777-6878 HSC4333

2. Hershey-Delbruck Experiment • E. coli was infected with two strains of bacteriophage T2. • One strain is h-r+; • One strain is h+r-

3. Hershey-Delbruck Experiment • h locus determines whether the phage can grow on a particular strain of E. coli • phage that are h- can infect the strain • phage that are h+ cannot infect the strain • r locus is a gene that determines whether the phage will lyse the host cells rapidly or slowly • phage that are r- will lyse the host cells rapidly • phage that are r+ will lyse the host cell slowly

4. Hershey-Delbruck Experiment • two strains of E. coli were used in the experiment • strain 1 supports growth of h- phage but not h+ phage • strain 2 supports the growth of both phage

5. Hershey-Delbruck Experiment • Bacteriophage are plated on a lawn of bacteria that consists of a mixture of both strains of E. coli • Expect two different phenotypes: • Both strains infected: Clear plaques • Infect only one of the two strains: Turbid plaques

6. Hershey-Delbruck Experiment

7. Hershey-Delbruck Experiment Proves: • If an E. coli cell was infected at the same time with two genetically different bacteriophage, the resulting phage population included recombinant phage types as well as the original parental phage types • The Nobel Prize in Physiology or Medicine 1969

8. Meselson-Weigle Experiment • First demonstrated that both strands of each molecule must be broken and then rejoined in 1961

9. Meselson-Weigle Experiment • Infected E. coli cells at the same time with phage from two different stocks of bacteriophage lambda. • Bacteriophage lambda c- mi- • Heavy isotopes of carbon 13C and nitrogen 15N • Bacteriophage lambda c+ mi+ • Light isotopes of carbon and nitrogen

12. Meselson-Weigle Experiment • Phenotypes of the intermediate density phage were recombinant phages • c- mi+ found near the band of "heavy“ non- recombinant phage • c+ mi- found near the band of "light" non- recombinant phage.

13. Meselson-Weigle Experiment • These results can only be explained if recombination between the two parental phage involves breakage and rejoining of both DNA strands

14. Recombination Model

15. Recombination Model With Branch Migration

16. Holliday Model • Holliday Model

17. Holliday Model

18. Double Strand Break Model

19. Double Strand Break Model

20. Double Strand Break Model

21. Question 16 On Midterm