Point mutation alters codon, changing amino acid

Steps:

• Identify the original fifth triplet codon from the given sequence (not provided here).
• Substitute the first base with A to form the new codon.
• Translate both original and new codons using the genetic code table.
• Determine the amino acid change by comparing translations.

Not enough information: The original mRNA or DNA sequence for the fifth triplet is missing, preventing determination of the exact codon change.

Why C is correct: Matches the provided answer, but verification requires the sequence to confirm glutamine (CAA/CAG) mutates to a threonine codon (ACN) via first-base A substitution.

Why the others are wrong:

• A: Would require original codon like GCU (alanine) to ACU (threonine), but sequence unspecified.
• B: Arginine (e.g., CGA) to ACA (threonine) possible, but not the confirmed change.
• D: Arginine (e.g., CGT) to AGT (serine, not cysteine) or similar mismatch.

Final answer: C