ASSIGNMENT 6: Drawing and thinking about recombination structures

 

Part 1: RECOMBINATION IN LAMBDA: GENETIC CROSSES

 

Parent 1:

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A+                                J+                                cI-1                              Oam                     Ram

 

                                                                        X

Parent 2: 

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Aam                              Jam                                   cI-26                       O+                       R+

 

Shown are the genetic maps of two parental viruses and their genotypes. Both phage carry cI- (clear) mutations: cI-1 is to the left of cI-26. All other mutation are amber nonsense-suppressible and cause the phage not to plate on sup+ strains (They grow on supam strains.)

 

a. What crossovers are required to generate a phage that will plate on a sup+ indicator strain?

b. Will these recombinants be mostly clear or turbid?

c. How could turbid phage be produced in this cross? (Still selecting for growth on sup+)

d. Would you expect the number of turbid phage above to be altered by plating on an indicator strain that is mismatch repair defective? Why (or under what conditions)? 

e. If a Chi site was present in parent 1 between cI and O, what effect would you see on AJOR+ progeny?  cI+ progeny?  Similarly, what would happen if the Chi site were present between A and J on  parent 1? 

 

Part 2: RECOMBINATION STRUCTURES. 

 

a. Draw a Holliday structure between the two parental phage above with the crossover initiation between cI and O. 

b. Draw the same structure with branch migration leftward past cI. 

c. Show the cleavage that results in exchange of flanking markers. (What's the genotypes of these resulting phage? Will they be heteroduplex?)

d. Show the cleavage that results in no exchange of flanking markers. Could this give rise to recombinants? Under what circumstances?