Question

What is usually the first amino acid inserted into a new polypeptide chain?

1) Methionine
2) Alanine
3) Glycine
4) Proline

Answer 1

Methionine is usually the first amino acid inserted into a new polypeptide chain, coded by the initiation codon AUG, but it is often enzymatically removed before the polypeptide chain is completed.

Step-by-step explanation:

The first amino acid inserted into a new polypeptide chain is usually methionine. This is due to the codon AUG, which not only codes for methionine but also serves as the initiation codon for protein synthesis. As proteins are synthesized, the initial methionine is typically removed enzymatically before the completion of the polypeptide chain, indicating that most mature proteins do not actually start with a methionine residue.

Answer 2

Methionine is typically the first amino acid inserted into a new polypeptide chain due to the AUG codon, which signals the start of protein synthesis, but it is frequently removed after the polypeptide is synthesized. The correct answer is 1.

Step-by-step explanation:

The first amino acid inserted into a new polypeptide chain during protein synthesis is methionine. The codon AUG serves two functions. First, within the mRNA sequence, it encodes for the amino acid methionine.

More importantly, at the beginning of an mRNA, this AUG codon acts as the initiation signal for protein synthesis, securing methionine's place as the first amino acid in the nascent polypeptide chain.

This initial methionine is often removed enzymatically before the polypeptide chain is fully synthesized; therefore, the majority of proteins ultimately do not start with methionine after processing. The correct answer is 1.