Question

4. (2 pts) The following illustrates a set of Benzer's "A" cistron in the rll region) deletion mutants with the boundary of each mutant numbered: (Note that the lines indicate the region deleted in a particular mutant). Four rll point mutations of the bacteriophage T4 were crossed with the seven deletion mutants where (+) indicates the appearance of r+recombinants and (-) indicates the lack of r+ recombinants among the progeny. 1 2 1364 386 168 2695 PT153 57 8 10 9 H&& 184 0 1 2 3 4 5 6 7 8 9 10 11 Deletion Mutant- 168 1695 Mutant # 1364 386 PT153 H88 1 84 Indicate the region, for example, 0 to 1, 1 to 2,2 to 4, etc. of the "A" cistron where each of the mutants map:

Answer 1

Recombination is a process of genetic exchange that is mostly guided by a region of homology between the recombining DNA sequences. In the given question, one needs to analyze the pattern of appearance or lack of recombinants when deletion mutants (Benzer’s A cistron) are recombined with rII mutations of bacteriophage T4.

Therefore, a (+) would indicate presence of homology in the region, whereas a(-) indicates lack of homology and hence no recombination.

For finding out the exact regions, we should look for deletion mutants that do not show recombination with the bacteriophage T4 rII mutants and mark the regions that is deleted in the deletion mutants.

For example, consider mutant r1, it shows recombination with all deletion mutants except 1695 (this has a deletion in region 7 onwards). This indicates that r1 region of homology is present in region 7 onwards. However, r1 recombines with deletion mutants PT153 (deletion region 8 to 10), H88 (deletion region 9 onwards), 184 (deletion region 11 onwards). All these observations indicate that r1 has a mutation in the region 7 to 8. This is deduced by removing the overlap regions between the (+) recombinants and (-) recombinants.

Using this logic we can complete the table

Mutant #	Region	Explanation
r1	7 to 8	(-) for 1695 (region 7 onwards) but positive for PT153 (8 to 10), H88 (9 onwards), 184 (11 onwards)
r2	1 to 2	(-) for 1364 (1 to 6) but (+) for 386 (2 to 4), 168 (3 to 5)
r3	8 to 9	(-) for 1695 (7 onwards), (-) for PT153 (8 to 10) but (+) for H88 (9 onwards) and 184 (11 onwards)
r4	3 to 4	(-) for 1364 (1 to6), (-) for 168 (3 to 5) but (+) for 386 (2 to 4)