What is the initiation codon for every mRNA strand?

The initiation codon for every mRNA strand is AUG, which codes for the amino acid methionine. This codon serves as the starting point for translation, the process through which proteins are synthesized from the mRNA template. The presence of the AUG codon signals the ribosome to begin translating the mRNA into a polypeptide chain.

In eukaryotic organisms, the recognition of the AUG start codon leads to the incorporation of methionine as the first amino acid of the newly synthesized protein. In prokaryotes, while the same initiation codon is recognized, the initiator tRNA may be a modified form, N-formylmethionine.

Other codons listed have different roles in translation or code for different amino acids. For example, UAC codes for tyrosine, CUG codes for leucine, and GUA codes for valine, but none of these codons serve as the starting point for translation in proteins. Therefore, understanding AUG as the universal initiation codon is crucial for grasping the basic mechanisms of protein synthesis.