Conceptual analysis
A nonsense mutation in a transporter gene
- Unit 6
- 4 points
- About 10 minutes
You can use a calculator on this question, just like on exam day.
A short question about a biological system that gets disrupted. You describe and explain the normal process, then predict what the disruption causes and justify that prediction. On the exam: Question 4 of 6 in Section II; about 10 minutes suggested.
The question and its sources
In a species of yeast, gene T codes for a 600-amino-acid membrane protein that transports a sugar into the cell. A mutant yeast strain has a single-nucleotide substitution in gene T. The substitution changes the mRNA codon for amino acid 45 from UGG to UGA (Table 1). Mutant cells make a much shorter version of the protein and cannot take up the sugar.
Table 1. Selected mRNA codons
| mRNA codon | Amino acid |
|---|---|
| UGG | Tryptophan (Trp) |
| UGA | Stop |
| UAA | Stop |
| GGU | Glycine (Gly) |
| GGC | Glycine (Gly) |
| GGA | Glycine (Gly) |
| GCU | Alanine (Ala) |
| GAU | Aspartic acid (Asp) |
Source: Standard genetic code (excerpt)
Suggested time: 10 minutes
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Part (a)
1 pointDescribe the role of tRNA in translation.
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Part (b)
1 pointExplain how the substitution in the mutant strain results in a nonfunctional protein.
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Part (c)
1 pointA second mutant strain has a different single-nucleotide substitution in gene T that changes an mRNA codon from GGU to GGC. Using Table 1, predict the effect of this mutation on the yeast's ability to take up the sugar.
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Part (d)
1 pointJustify your prediction in part (c).
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