Final answer:
The AUG codon is known as the start codon, initiating protein synthesis and therefore is the first codon that begins a polypeptide sequence, coding for methionine.
Step-by-step explanation:
The AUG codon, which specifies the amino acid methionine, is known as a start codon. This codon plays a crucial role at the beginning of an mRNA transcript by signaling the initiation of protein synthesis. Methionine is generally the first amino acid incorporated into a newly formed polypeptide chain, setting the reading frame for translation. Even though methionine is included initially, it is often enzymatically removed from the polypeptide chain later during the maturation process. The reading frame is essential as it dictates how sets of three nucleotides are grouped into codons, which ultimately determine the sequence of amino acids in a protein.