Final answer:
The AUG codon is called a start codon. It not only specifies the amino acid methionine but also initiates protein synthesis by setting the reading frame for translation in the mRNA.
Step-by-step explanation:
An AUG codon, which specifies the amino acid methionine, is often the first codon that begins a polypeptide sequence, and is called a start codon.
The AUG codon serves a dual function: in the middle of a sequence, it specifies methionine, and at the beginning of an mRNA strand, it signals the start of protein synthesis.
This is essential for establishing the proper reading frame for translation, where each set of three nucleotides is read as a codon, from the 5' end of the mRNA to a stop codon.
Interestingly, although methionine introduced by the AUG start codon is often the first amino acid in a new polypeptide, it is typically removed enzymatically before the polypeptide chain is completed, hence most mature proteins do not begin with methionine.