Question

A mutant bacterial cell has a defective aminoacyl synthetase that attaches a lysine to tRNAs with the anticodon AAA instead of the normal phenylalanine. During protein synthesis:

• A. none of the proteins in the cell will contain phenylalanine.
• B. lysine will partially replace phenylalanine at certain positions.
• C. the cell will compensate for the defect by attaching phenylalanine to tRNAs with lysine-specifying anticodons.
• D. lysine will replace phenylalanine at all amino acids positions.

Hint:

Ribosomes check the anticodon, NOT the amino acid attached to the tRNA.

Answer 1

The Correct Option is B

Step 1: Concept
The fidelity of translation depends on the correct charging of tRNA by aminoacyl-tRNA synthetases.

Step 2: Meaning
If a tRNA is "mischarged," the ribosome will incorporate whatever amino acid is attached to that tRNA based on the mRNA codon.

Step 3: Analysis
The codon for Phenylalanine (Phe) is UUU, which corresponds to the anticodon AAA. If the enzyme for Phe mistakenly attaches Lysine (Lys) to these tRNAs, the ribosome will insert Lys whenever it sees a UUU codon. However, this is described as a specific defect in an enzyme, and usually, multiple tRNAs and enzymes exist. This leads to a competition where some UUU codons might get the right amino acid and others get the wrong one.

Step 4: Conclusion
Lysine will be incorporated where Phenylalanine should be, but since the defect is specific to that charging event, it leads to partial replacement. Final Answer: (B)