Question

Which of the following is an initiation codon?

• A. AUG
• B. AUU
• C. UAG
• D. GAU

Hint:

Only one of these four codons both starts translation and sets the reading frame.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Protein synthesis (translation) on a ribosome always begins at a fixed triplet of mRNA bases called the initiation or start codon. This codon does two jobs: it marks where the ribosome should start reading, and it codes for the first amino acid of the chain.

Step 2: Recall the actual codon.
In almost all organisms, the start codon is AUG. It codes for the amino acid methionine (or a modified form, formyl methionine, in bacteria), which becomes the first amino acid of every newly made polypeptide before any later processing removes it.

Step 3: Why the other options are wrong.
AUU codes for isoleucine, an ordinary amino acid used inside a chain, not a signal to start reading.
UAG is one of the three stop codons (along with UAA and UGA); it tells the ribosome to stop, the opposite job of a start codon.
GAU codes for aspartic acid and has no start or stop role at all.

Final Answer:
AUG is the only codon among these that signals the ribosome to begin translation. \[ \boxed{\text{AUG}} \]