AUG Codon: Dual Role & Start Signal — NEET Biology
✅ Asked in NEET 2021Statement I: Codon ‘AUG’ codes for methionine and phenylalanine. Statement II: ‘AAA’ and ‘AAG’ both code for lysine. (NEET 2021)
Statement I: Codon ‘AUG’ codes for methionine and phenylalanine. Statement II: ‘AAA’ and ‘AAG’ both code for lysine. (NEET 2021)
Starter codon is: (NEET 2016 Phase 1)
Answer & NCERT explanation
Correct answer: B — Statement I is incorrect but Statement II is correct
Statement I is incorrect - AUG codes only for methionine, not phenylalanine (UUU/UUC codes for phenylalanine). Statement II is correct - AAA and AAG both code for lysine, demonstrating the degeneracy of genetic code. This shows that the genetic code is unambiguous (one codon = one amino acid) but degenerate (multiple codons can code for same amino acid). This is a key NCERT concept about genetic code properties.
Read more NCERT concept on the PYQ
AUG has dual functions. It codes for Methionine (met), and it also act as initiator codon.
AUG codes for methionine and uniquely acts as the start/initiator codon in protein synthesis. Students often mistakenly believe AUG only codes for methionine or forget its initiator function—these are actually two separate roles. The key difference: when AUG appears at the beginning of mRNA (5' end near ribosome binding site), it signals translation start and codes for N-formylmethionine in prokaryotes or regular methionine in eukaryotes; when AUG appears internally, it codes only for methionine. Remember: first AUG = start signal + methionine; internal AUG = just methionine. This dual function appears frequently in NEET because it tests both genetic code knowledge and translation mechanics.
What does NCERT say about AUG has dual functions?
Has this concept appeared in NEET?
Which chapter is this from?
Through deep analysis of NEET and NTA, 176 of the 180 ReNEET 2026 (June 21) questions were already in the MedicNEET question bank before the exam.
176/180MedicNEET's Biology question bank is built from the same NCERT lines NTA picks repeatedly. Not random MCQs — questions crafted exactly like NTA crafts them.