## **Core Concept**
The initiator codon, also known as the start codon, is a specific sequence of nucleotides that signals the beginning of a protein sequence. In both prokaryotes and eukaryotes, the genetic code is read in sequences of three nucleotides called codons. The core concept here involves understanding the genetic code and its role in initiating protein synthesis.

## **Why the Correct Answer is Right**
The correct initiator codon in both prokaryotes and eukaryotes is **AUG**, which codes for the amino acid methionine (Met) in eukaryotes and formylmethionine (fMet) in prokaryotes. The **AUG** codon serves as the start signal for protein synthesis in all organisms. In prokaryotes, the initiator tRNA recognizes the **AUG** codon and carries formylmethionine, which is a modified version of methionine. This distinction is crucial for the initiation of protein synthesis in prokaryotes.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because UAG is one of the three stop codons in the genetic code, signaling the termination of protein synthesis rather than its initiation.
- **Option B:** This option is incorrect because UGA is another stop codon, not involved in the initiation of protein synthesis.
- **Option D:** This option is incorrect because UAU codes for the amino acid tyrosine and is not involved in the initiation of protein synthesis.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while the initiator codon **AUG** is the same in both prokaryotes and eukaryotes, the amino acid it codes for differs: methionine in eukaryotes and formylmethionine in prokaryotes. This difference is significant for understanding the initiation of protein synthesis in different types of organisms.

## **Correct Answer:** . AUG