Read the passage and answer the following questions:
Translation refers to the process of polymerisation of amino acids to form a polypeptide. The order and sequence of amino acids are defined by the sequence of bases in the mRNA. The amino acids are joined by a bond which is known as a peptide bond. Formation of a peptide bond requires energy.
At which codon does the ribosome bind to the mRNA to initiate translation?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → AUG
The passage tells us that the order of amino acids is set by the sequence of bases in mRNA, but not where translation starts. A translational unit in mRNA is flanked by a start codon and a stop codon. The start codon is AUG; it codes for methionine and also acts as the initiator codon, and it is recognised only by the initiator tRNA.
Option (1) AUG — Correct. For initiation, the ribosome binds to the mRNA at the start codon AUG.
Option (2) UGA — Incorrect. UGA is one of the three stop codons (UAA, UAG, UGA). It ends translation; it does not start it.
Option (3) AGC — Incorrect. AGC is an ordinary codon that codes for an amino acid in the middle of a polypeptide. It has no role in initiation.
Option (4) ACG — Incorrect. ACG also codes for an amino acid. Only AUG acts as the initiator codon.
NCERT Extract:
Translation – “… A translational unit in mRNA is the sequence of RNA that is flanked by the start codon (AUG) and the stop codon and codes for a polypeptide. … For initiation, the ribosome binds to the mRNA at the start codon (AUG) that is recognised only by the initiator tRNA. The ribosome proceeds to the elongation phase of protein synthesis.”
Genetic Code – “… (v) AUG has dual functions. It codes for Methionine (met), and it also act as initiator codon. (vi) UAA, UAG, UGA are stop terminator codons.”