3-5
Biology 22
Problems in Gene Structure and Function
3. Four auxotrophic mutant strains of E. coli (1, 2, 3, 4) are each blocked at a
different step of the same metabolic pathway. This pathway involves
compounts W, X, Y and Z. Each of these compounds is tested for its ability to
support growth of mutant strains, with the results given below. (+ = growth,
-- = no growth).
Compound Used
as Supplement
W X
Y
Z
Mutant 1
+
-+
+
Mutant 2
+
-+
-Mutant 3
+
+
+
+
Mutant 4
-- -+
--
a. Give the order of the compounds in the metabolic pathway and indicate the
specific step that is blocked in each mutant strain.
b. Several sets of double muations were produced. For example, double mutant
1,2 has mutant forms for both enzyme 1 and enzyme 2. Pairs of these double
mutations were combined in partial diploids. Would a partial diploid consisting
of each of the following pairs of mutations grow on minimal medium, without
supplementation with W, X, Y and Z?
1. 1, 3 and 2, 4
2. 3, 4 and 2, 3
3. 1, 4 and 2, 3
4. Amino acids 1-5 were compared for an E. coli metabolic enzyme from a wild type
strain and a strain with a double frameshift mutation having one nucleotide deletion
followed by one nucleotide insertion. The results are shown below.
Wild Type
NH2-
1
MET
2
HIS
3
ALA
4
VAL
5
LYS
-COOH
Double
NH2SER THR GLN LEU LYS -COOH
Mutant
a. Determine the base sequence of the mRNA for both the wild type and double
mutant polypeptides.
b. Using your wild type mRNA sequence, show which base was deleted and the
site from which it was removed.
c. Using your mutant sequence, show which base was added and where it was
inserted. (There may be more than one possible solution.)
3-6
Biology 22
Problems in Gene Structure and Function
5. The sequence of eight amino acids of a protein from the bacterium E. coli is given
below:
11
12
13
14
15
16
17
18
NH
2
Amino Acid
TRP GLU THR SER VAL PHE ARG GLY -COOH
Original Strain
After exposing these bacteria to a mutagen, one mutant was isolated with
different amino acids in this region of the protein. The mutation resulted from a
change in a single nucleotide, either a substitution, insertion or deletion. What
is the cause of the mutation? What is the sequence of codons for the original
strain?
Amino Acid
MUTANT
NH2-
11
GLY
12
ARG
13
HIS
14
PRO
15
SER
16
LEU
17
GLU
18
VAL
-COOH
6. Nine mutants of the rII region in bacteriophage T4 DNA were analyzed for
complementation. The results are given in the table below where
+ = complementation and -- = no complementation. Determine the number of cistrons
represented by these mutations and indicate which mutations are found in each
cistron.
1
2
3
4
5
6
7
8
9
1
-+
+
+
-+
+
-+
2
-+
-+
+
-+
+
3
-+
+
-+
+
-4
-+
+
-+
+
5
-+
+
-+
6
-+
+
-7
-+
+
8
-+
9
--