Several mutants are isolated, all of which require compound X for growth. The co
ID: 63570 • Letter: S
Question
Several mutants are isolated, all of which require compound X for growth. The compounds (A to F) in the biosynthetic patyway to G are known, but their order in the pathway is not known. Each compound is tested for its ability to support the growth of each mutant (1 to 6). In the following table, a plus sign indicates growth and a minus sign indicates no growth.
Compound tested
Mutant
A
B
C
D
E
F
X
1
-
-
-
-
-
+
+
2
-
-
-
+
+
+
+
3
+
-
+
+
+
+
+
4
-
-
-
-
-
-
+
5
+
-
-
+
+
+
+
6
-
-
-
-
+
+
+
A) What is the order of compounds A to F in the pathway?
B) At which point in the pathway is each mutant blocked?
Compound tested
Mutant
A
B
C
D
E
F
X
1
-
-
-
-
-
+
+
2
-
-
-
+
+
+
+
3
+
-
+
+
+
+
+
4
-
-
-
-
-
-
+
5
+
-
-
+
+
+
+
6
-
-
-
-
+
+
+
Explanation / Answer
A.
A mutant in biosynthetic pathway lacks one compound needed in the pathway and thus cannot make any of the compounds that come after the blocked step. Addition of compound before the block will not help because the mutant will still be unable to synthesize the rest of the compounds in the pathway.However, addition of compound after the block will resume the pathway because the mutant does have the enzymes to make these later compounds. So, in mutants with blocks at different steps in the pathway, the compounds that are used latest in the pathway will support the growth of the most mutants, and compounds that are used earliest in the pathway will allow the growth of the fewest mutants. A look at our table shows us that the compound supporting the fewest mutants is compound B, with no mutants supported, followed by C (one mutant), then by A, D, E, F and X.
So, order = B-C-A-D-E-F-X
2.
Between B - C (mutant 3)
Between C - A (mutant 5)
Between A - D (mutant 2)
Between D - E (mutant 6)
Between E - F (mutant 1)
Between F - X (mutant 4)
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