Final answer:
The AUG codon serves as a start codon and encodes for the amino acid methionine, but it does not act as a stop codon or encode glutamine.
Step-by-step explanation:
The two functions of the AUG codon are that it acts as a start codon and it encodes the amino acid methionine. The AUG codon initiates the process of translation, establishing the reading frame for protein synthesis. It not only signals the start but also adds methionine, the first amino acid, to the growing polypeptide chain.
However, if AUG appears in the middle of the mRNA sequence, its role is solely to specify the addition of a methionine residue within the protein. It is important to note that AUG does not act as a stop codon nor does it encode for the amino acid glutamine.