Discussion
A messenger RNA is 222 nucleotides long including the initiator and termination codons. The number of amino acids in the protein translated from this mRNA is :
Answer: C
The protein has 73 amino acids.
Each codon is three nucleotides, so the mRNA stretch described holds:
\[ \begin{aligned} \frac{222}{3} &= 74 \text{ codons} \\ 74 - 1 &= 73 \text{ amino acids} \end{aligned} \]
The initiator codon AUG codes for methionine and is counted, but the termination codon codes for no amino acid; it is read by a release factor that ends translation. So one is subtracted for the stop codon only.
74 is the number of codons, the trap of forgetting that the stop codon adds nothing. 221 and 111 come from counting nucleotides or pairs instead of triplets.
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