Final answer:
Methionine is the common starting amino acid for polypeptide chains.
Step-by-step explanation:
The common starting amino acid for polypeptide chains is methionine (Met).
When a polypeptide chain is synthesized, it starts with a specific amino acid called the initiator amino acid. In most cases, the initiator amino acid is methionine, which is recognized as the start codon (AUG) in the messenger RNA (mRNA) during translation.
For example, the polypeptide chain for the protein insulin starts with methionine. The one-letter abbreviation for methionine is M.