Final answer:
The C-terminus of a polypeptide always contains a carboxyl group. It is the end of the polypeptide chain with a free -COOH group.
Step-by-step explanation:
The C-terminus of a polypeptide always contains a carboxyl group. The C-terminus is the end of the polypeptide chain where the last amino acid in the sequence has a free carboxyl group (-COOH). This is opposed to the N-terminus, which has a free amino group (-NH2). A peptide bond, which is a type of amide bond, links individual amino acids together. During protein synthesis, amino acids are added to the growing polypeptide chain at the C-terminus, with new amino acids being linked through peptide bonds by the removal of a water molecule, a process known as dehydration synthesis.
Therefore, the correct answer to the question 'The C-terminus of a polypeptide always contains?' is 1) a carboxyl group.