Final answer:
Methionine (Met) is often the first amino acid added to a polypeptide chain because the codon AUG not only codes for methionine but is also the initiation codon for protein synthesis.
Step-by-step explanation:
The first amino acid added to a polypeptide chain during protein synthesis is methionine (Met). This is because the codon AUG not only codes for methionine but it also serves as the initiation codon. During translation, the process where the genetic code from mRNA is translated into a sequence of amino acids, AUG is recognized by the translation machinery as the start signal, and hence methionine is the first amino acid incorporated into the nascent polypeptide chain. While methionine can also appear elsewhere within a protein sequence, specified by internal AUG codons, it is always the first one when the codon is at the start of the mRNA.
In the context of protein synthesis, it's also important to note that the genetic code is both degenerate and universal. For example, while glycine has four codons (GGU, GGC, GGA, and GGG), tryptophan and methionine have only one codon each. After the initial methionine is added to the starting end (N-terminus) of a polypeptide, it is often removed enzymatically before the complete protein is folded and functional.
Therefore, the correct answer to the question of what is often the first amino acid added to a polypeptide chain is B) Methionine (Met).