Problem 1
The fox operon, which has sequences A, B, C, and D (which may represent either structural genes or regulatory sequences), encodes enzymes 1 and 2. Mutations in sequences A, B, C, and D have the following effects, where a plus sign (+) indicates that the enzyme is synthesized and a minus sign (−) indicates that the enzyme is not synthesized.
| Fox absent | Fox present | |||
|---|---|---|---|---|
| Mutation | Enzyme 1 | Enzyme 2 | Enzyme 1 | Enzyme 2 |
| in sequence | ||||
| No mutation | − | − | + | + |
| A | − | − | − | + |
| B | − | − | − | − |
| C | − | − | + | − |
| D | + | + | + | + |
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Is the fox operon inducible or repressible?
Indicate which sequence (A, B, C, or D) is part of the following components of the operon. Each sequence should be used only once.
| Regulator gene | __________ | Structural gene for enzyme 1 | __________ |
| Promoter | __________ | Structural gene for enzyme 2 | __________ |
Solution Strategy
What information is required in your answer to the problem?
Whether the fox operon is inducible or repressible.
Which sequence represents each part of the operon.
Hint: Review Figure 12.2 for a summary of operon structure.
What information is provided to solve the problem?
For each mutation, whether enzyme 1 and enzyme 2 are produced in the presence and absence of Fox.
For help with this problem, review:
Section 12.2.
Solution Steps
When no mutations are present, enzymes 1 and 2 are produced in the presence of Fox but not in its absence, indicating that the operon is inducible and that Fox is the inducer.
The mutation in A allows the production of enzyme 2 in the presence of Fox, but enzyme 1 is not produced in the presence or absence of Fox, so A must have a mutation in the structural gene for enzyme 1. With the mutation in B, neither enzyme is produced under any conditions, so this mutation most likely occurs in the promoter and prevents RNA polymerase from binding. The mutation in C affects only enzyme 2, which is not produced in the presence or absence of Fox; enzyme 1 is produced normally (only in the presence of Fox), so the mutation in C most likely occurs in the structural gene for enzyme 2. The mutation in D is constitutive, allowing the production of enzymes 1 and 2 whether or not Fox is present. This mutation most likely occurs in the regulator gene, producing a defective repressor that is unable to bind to the operator under any conditions.
| Regulator gene | D |
| Promoter | B |
| Structural gene for enzyme1 | A |
| Structural gene for enzyme 2 | C |
Problem 2
What would be the effect of a mutation that caused poly(A)-binding proteins to be nonfunctional?
Solution Strategy
What information is required in your answer to the problem?
A statement of the effect of a mutation that eliminates the function of the poly(A)-binding protein.
What information is provided to help solve the problem?
A mutation occurs in the gene that encodes poly(A) binding protein.
The mutation causes the poly(A) protein to be nonfunctional.
For help with this problem, review:
The Degradation of RNA in Section 12.3.
Addition of the Poly(A) Tail in Section 10.5.
Recall: The poly(A) tail affects the stability of mRNA.
Solution Steps
Degradation of mRNA from the 5′ end requires the removal of the 5′ cap and is usually preceded by the shortening of the poly(A) tail. Poly(A)-binding proteins bind to the poly(A) tail and prevent it from being shortened. Thus, the presence of these proteins on the poly(A) tail protects the 5′ cap, which prevents RNA degradation. If the gene for poly(A)-binding protein were mutated in such a way that nonfunctional poly(A)-binding protein was produced, the protein would not bind to the poly(A) tail. The tail would be shortened prematurely, the 5′ cap removed, and mRNA degraded more easily. The end result would be less mRNA and thus less protein synthesis.