Solutions to Assignment 3 - BEL204

Clue to the assignment no.3

1. In these studies always look for the supplement that is + for all the mutants. In this case G. So, G is the final metabolite. Then work backward, like supplement that gives ++ and so on.
Sequence is
5 4 2 1 3
E -------- A -------- C--------- B -------- D ---------- G

2. Here, either trpB or trp A is closer to cys
In the first case, recipient is cys+trpA-. So, it may be cys+ trpA- trpB+ or cys+trpB+ trpA- and incoming DNA fragment is cys- trpA+ trpB- or cys- trpB- trpA+. In either case a double crossover event will form prototroph (all +ve).
In the second case, recipient is cys-trpB-. Now, depending upon the order among cys, trpA and trpB, the donor fragment will be cys+trpA-trpB+or cys+trpB+trpA-. Corresponding recipients will be cys-trpA+trpB- or cys-trpB- trpA+. In the first case where trpA is closer to cys, 4 crossover events will be required to get the prototroph, while 2 crossover events will be needed if trpB is close to cys. So, the sequence is cys-trpB-trpA.