Answers to Concept Checks
- 1. B → A → C
- 2. The amino acid sequence (primary structure) of the protein
- 3. b
- 4. d
- 5. The initiation codon in bacteria encodes N-formylmethionine; in eukaryotes, it encodes methionine. Termination codons do not specify amino acids.
- 6. c
- 7. The Shine–Dalgarno sequence
- 8. a
- 9. b
WORKED PROBLEMS
Problem 1
A series of auxotrophic mutants were isolated in Neurospora. Examination of fungi containing these mutations revealed that they grew on minimal medium to which various compounds (A, B, C, D) were added; growth responses to each of the four compounds are presented in the following table. Give the order of compounds A, B, C, and D in a biochemical pathway. Outline a biochemical pathway that includes these four compounds and indicate which step in the pathway is affected by each of the mutations.
| Compound | ||||
|---|---|---|---|---|
| Mutation number | A | B | C | D |
| 134 | + | + | – | + |
| 276 | + | + | + | + |
| 987 | – | – | – | + |
| 773 | + | + | + | + |
| 772 | – | – | – | + |
| 146 | + | + | – | + |
| 333 | + | + | – | + |
| 123 | – | + | – | + |
Solution Strategy
What information is required in your answer to the problem?
The order of compounds in a biochemical pathway; for each mutation, which step in the pathway is affected by the mutation.
What information is provided to solve the problem?
For each mutant, whether it grew on minimal medium to which compounds A, B, C, and D were added.
For help with this problem, review:
The One Gene One Enzyme Hypothesis in Section 15.1.
Solution Steps
Hint: Group the mutations by which compounds allow growth.
| Mutation | Compound | ||||
|---|---|---|---|---|---|
| Group | Number | A | B | C | D |
| I | 276 | + | + | + | + |
| 773 | + | + | + | + | |
| II | 134 | + | + | – | + |
| 146 | + | + | – | + | |
| 333 | + | + | – | + | |
| III | 123 | – | + | – | + |
| IV | 987 | – | – | – | + |
| 772 | – | – | – | + | |
436
Mutants in group I will grow if compound A, B, C, or D is added to the medium; so these mutations must affect a step before the production of all four compounds:
Hint: If a compound is added after the block, it will allow the mutant to grow; if a compound is added before the block, it will have no effect.
Group II mutants will grow if compound A, B, or D is added but not if compound C is added. Thus, compound C comes before A, B, and D; and group II mutations affect the conversion of compound C into one of the other compounds:
Group III mutants allow growth if compound B or D is added but not if compound A or C is added. Thus, group III mutations affect steps that follow the production of A and C; we have already determined that compound C precedes A in the pathway, and so A must be the next compound in the pathway:
Finally, mutants in group IV will grow if compound D is added but not if compound A, B, or C is added. Thus, compound D is the fourth compound in the pathway, and mutations in group IV block the conversion of B into D:
Problem 2
A template strand in bacterial DNA has the following base sequence:
5′-AGGTTTAACGTGCAT-3′
What amino acids are encoded by this sequence?
Solution Strategy
What information is required in your answer to the problem?
The list of amino acids encoded by the given sequence.
What information is provided to solve the problem?
The DNA sequence of the template strand.-
The 5′ and 3′ ends of the template sequence.
-
The amino acids encoded by different codons (Figure 15.10).
For help with this problem, review:
The Degeneracy of the Code in Section 15.2.
Solution Steps
To answer this question, we must first work out the mRNA sequence that will be transcribed from this DNA sequence:
Recall: The mRNA is antiparallel and complementary to the DNA template strand.
| DNA template strand: | 5′-AGGTTTAACGTGCAT-3′ |
| mRNA copied from DNA: | 3′-UCCAAAUUGCACGUA-5′ |
An mRNA is translated 5′→3′; so it will be helpful if we turn the RNA molecule around with the 5′ end on the left:
mRNA copied from DNA: 5′-AUGCACGUUAAACCU-3′
The codons consist of groups of three nucleotides that are read successively after the first AUG codon; using Figure 15.10, we can determine that the amino acids are
437
Problem 3
The following triplets constitute anticodons found on a series of tRNAs. Name the amino acid carried by each of these tRNAs.
- a. 5′-UUU-3′
- b. 5′-GAC-3′
- c. 5′-UUG-3′
- d. 5′-CAG-3′
Solution Strategy
What information is required in your answer to the problem?
The amino acid carried by each tRNA.
What information is provided to solve the problem?
-
The sequence on the anticodon of each tRNA.
-
The amino acids encoded by different codons (Figure 15.10).
For help with this problem, review:
The Degeneracy of the Code in Section 15.2.
Solution Steps
To solve this problem, we first determine the codons with which these anticodons pair and then look up the amino acid specified by the codon in Figure 15.10. For part a, the anticodon is 5′–UUU–3′.
Recall: Codons are antiparallel and complementary to the anticodons.
According to the wobble rules, U in the first position of the anticodon can pair with either A or G in the third position of the codon, so there are two codons that can pair with this anticodon:
Hint: Consult the wobble rules in Table 15.2
| Anticodon: | 5′-UUU-3′ |
| Codon: | 3′-AAA-5′ |
| Codon: | 3′-GAA-5′ |
Listing these codons in the conventional manner, with the 5′ end on the right, we have:
| Codon: | 5′-AAA-3′ |
| Codon: | 5′-AAG-3′ |
According to Figure 15.10, both codons specify the amino acid lysine (Lys). Recall that the wobble in the third position allows more than one codon to specify the same amino acid; so any wobble that exists should produce the same amino acid as the standard base pairings would, and we do not need to figure the wobble to answer this question. The answers for parts b, c, and d are:
| b. Anticodon: | 5′–GAC–3′ |
| Anticodon: | 3′–CAG–5′ |
| Codon: | 5′–GUC–3′ encodes Val |
| c. Anticodon: | 5′–UUG–3′ |
| Anticodon: | 3′–GUU–5′ |
| Codon: | 5′–CAA–3′ encodes Gln |
| d. Anticodon: | 5′–CAG–3′ |
| Anticodon: | 3′–GUC–5′ |
| Codon: | 5′–CUG–3′ encodes Leu |