Which one of the following codons performs a dual function?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → AUG
Option (1) UAG — Incorrect. UAG is a stop codon. It codes for no amino acid and only ends translation, so it has one job.
Option (2) AUG — Correct. Translation of an mRNA begins at AUG, so it is the start signal. The same codon also brings in the amino acid methionine, so every new protein starts with methionine.
Option (3) UUU — Incorrect. UUU codes for phenylalanine only. It is the book’s example of the code being nearly universal, from bacteria to humans.
Option (4) AUA — Incorrect. AUA codes for the amino acid isoleucine. It is not a start codon, so it has only one function.
NCERT Extract:
Genetic Code – …
