Question

Shown below are the deletion maps of a series of rII- mutations. The deleted region is indicated as (.....) and the intact region as 1_ ..........) 3 ................) 4 5 (...........) A series of point mutations A-E is used in a coinfection experiment. Shown below are the results of those coinfections. Ability to produce wild-type progeny phage is indicated by (+), (o) indicates no wild- type progeny. 12345 1 + + + IO + + + + 100++ + + + + + O B + + + + D + + +1+0 E+++01+1 + Indicate the order that is most consistent with these data. A. CADBE B. ACBDE C. BADCE D. ABDEC E. CEADB
the answer is C. i need an explanation

Answer 1

The given coinfection experiments work as follows:

We co-infect the cells using a phage which contains deletion and another phage which contains the point mutation. If the point mutation is in a region other than the deleted region. This would mean that in the phage with the point mutation, the region that is deleted in the co-infected phage is intact. At the same time, in the phage that contains the deletion, the region which contains the point mutation in the other phage is intact. Therefore, the 2 phages will complement each other, and are able to produce wild-type progeny.

If the point mutation is in the same region as the deleted region, then both co-infected phages would be disrupted in the same regions, making them unable to complement each other and therefore unable to produce wild-type progeny.

Let us look at the given problem. We see that map 4 has a deletion at the very end. In co-infection experiments involving 4, we see that only E is unable to complement it. This means that E must be located at the very end.

Map 3 has a deletion at the very beginning. In co-infection experiments involving 3, both A and B are unable to complement it. So, A and B are located at the beginning, though the exact order of A and B needs to be determined.

Map 1 has a deletion that is slightly away from the very beginning and involves the latter half of the region that is seen to be deleted in map 3. Co-infection experiments using 1 also show that only A is unable to complement it. Therefore, A must be located in the latter half of the deleted region in 3 which means that B is at the very beginning, immediately followed by A.

The deletion in map 5 immediately follows the one in map 1. Also, only point mutation D is unable to complement it. Therefore, D must immediately follow A.

The deletion in map 2 immediately follows the one in map 5 and only C is unable to complement it. Therefore, C must immediately follow D.

So, the correct order has to be BADCE.