An Hfr strain of genotype his + leu + met + str s is mated with a female strain
ID: 57953 • Letter: A
Question
An Hfr strain of genotype his+ leu+ met + strsis mated with a female strain of genotype his- leu- met- strr. At various times, the culture is shaken vigorously to separate mating pairs. The cells are then plated on agar of the following three types.
Agar type
Str
His
Leu
Met
1
+
+
+
-
2
+
+
-
+
3
+
-
+
+
a. What donor genes are being selected on each type of agar?
b. The following table shows the number of colonies on each type of agar for samples taken at various times after the strains are mixed. Use this information to determine the order of genes his, leu, and met.
Time of sampling (minutes)
# of colonies on agar of type
1
2
3
0
0
0
0
5
0
0
0
7.5
100
0
0
10
200
0
0
12.5
300
75
0
15
400
150
0
17.5
400
225
50
20
400
250
100
25
400
250
100
Agar type
Str
His
Leu
Met
1
+
+
+
-
2
+
+
-
+
3
+
-
+
+
Explanation / Answer
a.) What donor genes are being selected on each type of agar?
Answer: Agar type 1 --- Met+
Agar type 2 --- Leu+
Agar type 3 --- His+
Answer b). The colonies are first found to appear on plate Agar type 1 which selects for Met+ ,so you can conclude Met should be first.
Colonies next appear on plate Agar type 2 which selects for Leu+ which shows Leu comes next.
Colonies lastly appear on plate Agar type 3 which selects for His+ which shows His comes last.
So the order of genes would be Met - Leu-- His.