33. Sydney Brenner and Francis Crick recombined the B genes from two mutant T4 p

Question

33. Sydney Brenner and Francis Crick recombined the B genes from two mutant T4 phages containing insertion or deletion mutations and created a phage with a B gene containing two mutations instead of one. They repeated this experiment with many mutant T4 phage. In some cases, the recombined gene restored normal function of the B gene (ability to infect multiple hosts). Brenner and Crick interpreted this result as evidence that the reading frame in the recombined B gene, disrupted by an insertion or deletion in one mutant B gene, was restored by an insertion or deletion mutation in the other mutant B gene. Below is a hypothetical wild-type gene sequence and four mutant gene sequences. Which pair of mutant gene sequences, when recombined, would restore the reading frame of the wild type gene? Of course, in between the mutations, the amino acids encoded by the recombined gene will be different than the wild type. Wild Type Mutant I Mutant I Mutant III Mutant IV CATCATCATCATCAT CATACATCATCATCAT CATTACATCATCAT CATCATCATATCAT CATCATCATTCAT a. Mutant I and Mutant II b. Mutant II and Mutant IV c. Mutant III and Mutant IV d. Mutant I and Mutant III

Explanation / Answer

Wild Type: CAT CAT CAT CAT CAT

Mutant 1: CAT ACA TCA TCA TCA T

There is insertion of A in mutant 1

Mutant III: CAT CAT CAT ATC AT

There is a deletion if C in Mutant 3 at position 10 (of Wild type). This is a suppressor mutation. A suppressor mutation counteracts or suppresses the effect of another mutation.

Mutant II show deletions as well as inversion. Mutant IV shows deletion of two bases C and A.

Two insertion or two deletions will not correct the reading frame. Only one insertion in one mutant and one deletion in second mutant will correct the reading frame after recombination.

Insertion or deletion of bases cause frameshifht mutation that alter the reading frame. However a compensatory mutation (insertion or deletion) will restore the proper reading framewith only a short stretch of gibberish information.

Recombination between mutant I and III will only restore the proper reading frame of wild type.

Right option is d. Mutant I and III

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