AUG is the start codon (also called the initiator codon). It has a dual function: it tells the ribosome where to begin translation, and it also codes for the amino acid methionine. There are three stop codons, UAA, UAG and UGA, and these do not code for any amino acid, so they end protein synthesis. Memory hook: "AUG Always yoU Go" to start, and "U Are Away, U Are Gone, U Go Away" for the three stops.
The ribosome reads mRNA in triplets. AUG (green) starts translation and adds methionine; the ribosome reads codon by codon until it hits a stop codon (red: UAA, UAG or UGA), which codes for no amino acid and ends the protein.
Your doubts, answered
Is AUG a start codon, or does it code for methionine?
It does both. This is the confusing part. NCERT says AUG has a dual function. It codes for the amino acid methionine (met), AND it also acts as the initiator (start) codon that tells the ribosome to begin translation. So the same codon does two jobs. In a NEET question, if the option says AUG is only a start codon, that is incomplete; the full truth is start + methionine.
Does AUG code for phenylalanine?
No. This is a direct NEET trap (NEET 2023). AUG codes for methionine, not phenylalanine. Phenylalanine is coded by UUU and UUC. So the statement 'AUG codes for methionine and phenylalanine' is FALSE. Remember: AUG = Methionine only, plus the start job.
What are the 3 stop codons?
The three stop (terminator) codons are UAA, UAG and UGA. They do not code for any amino acid. When the ribosome reaches any of these, it stops making the protein. Memory trick: U-A-A, U-A-G, U-G-A all start with U. Say 'U Are Away, U Are Gone, U Go Away'.
How many of the 64 codons do not code for amino acids?
There are 64 codons in total. 61 codons code for the 20 amino acids. The remaining 3 codons (UAA, UAG, UGA) do not code for any amino acid; they are stop codons. So 61 code + 3 stop = 64. NEET loves this exact number split.
Is a stop codon read by a tRNA?
No. Stop codons (UAA, UAG, UGA) have no matching tRNA anticodon and code for no amino acid. Instead, release factors recognise the stop codon and end translation. This is why the polypeptide is released at the stop codon.
⚠️ The NEET trap ✗ AUG codes for methionine and phenylalanine. ✓ AUG codes for methionine only (and also acts as the start/initiator codon). Phenylalanine is coded by UUU and UUC, not AUG. 🧠 NEET 2023 set this exact trap. AUG = Methionine + Start. If a phenylalanine gets attached to AUG in an option, it is wrong. Phe belongs to UUU/UUC.
Real NEET questions
NEET 2016
Which one of the following is the starter codon?
A · AUG ✓
B · UGA
C · UAA
D · UAG
Solution: AUG is the initiator (start) codon and also codes for methionine. UGA, UAA and UAG are the three stop (terminator) codons, so they cannot start translation. NCERT: 'AUG has dual functions. It codes for Methionine (met), and it also acts as initiator codon. UAA, UAG, UGA are stop terminator codons.'
NEET 2023
Statement I: The codon 'AUG' codes for methionine and phenylalanine. Statement II: 'AAA' and 'AAG' both codons code for the amino acid lysine. Choose the correct answer.
A · Statement I is correct but statement II is false
B · Statement I is incorrect but statement II is true ✓
C · Both Statement I and Statement II are true
D · Both Statement I and Statement II are false
Solution: Statement I is FALSE: AUG codes for methionine and acts as the initiator codon, but it does NOT code for phenylalanine (phenylalanine is coded by UUU/UUC). Statement II is TRUE: AAA and AAG both code for lysine, which shows the code is degenerate. So Statement I incorrect, Statement II correct.
NEET 2023
The salient features of genetic code are: (A) The code is palindromic (B) UGA acts as initiator codon (C) The code is unambiguous and specific (D) The code is nearly universal. Choose the most appropriate answer.
A · (A) and (B) only
B · (C) and (D) only ✓
C · (B) and (D) only
D · (A) and (C) only
Solution: (C) and (D) are correct: the code is unambiguous/specific and nearly universal. (A) is wrong (the code is not palindromic). (B) is wrong because UGA is a STOP codon, not an initiator; the initiator codon is AUG. This is why (C) and (D) only is the answer.
Solved Molecular Basis of Inheritance NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
