Amino Acids & Proteins MCAT Practice Question
A 28-year-old man presents with progressive muscle weakness and cognitive decline. Genetic testing reveals a point mutation in the SELENOP gene that converts a tryptophan codon (UGG) to a stop codon (UAG). The patient's cells express an amber suppressor tRNA with an unusual anticodon that allows selenocysteine insertion at the UAG position. Analysis of the patient's SELENOP protein shows near-normal length and partial biological activity. Which of the following best explains these findings?
Answer choices
- AMissense mutation causing a conservative amino acid substitution that preserves the protein's structural fold
- BNonsense suppression through selenocysteine incorporation, allowing read-through of the stop codon and restoration of partial protein functionCorrect answer
- CNonsense mutation with complete loss of protein function due to premature termination and degradation by nonsense-mediated decay
- DFrameshift mutation causing a shift in the reading frame and production of an entirely different downstream sequence
- ESilent mutation with synonymous codon replacement that has no phenotypic consequence
- FNonsense suppression through selenocysteine incorporation, but the substitution introduces an incompatible residue that completely abolishes protein function
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