Final answer:
The correct answer is D) glycine has a free n-terminus, where alanine has both c and n-terminals free, meaning the glycine is not yet part of the growing chain and has both terminals free, whereas alanine is part of the chain and only has a free C-terminus.
Step-by-step explanation:
If you are observing a 250 amino acid protein being translated and see that amino acid number 40, which is an alanine, is bound to the P site, and the next amino acid to be added is a glycine, it is important to understand the structure of the growing polypeptide chain. At this stage, the alanine at the P site is part of the growing polypeptide chain and, therefore, has its N-terminus linked to the previous amino acids in the chain. The glycine, represented by the incoming aminoacyl-tRNA to the A site, has both its N-terminus and C-terminus free since it has not yet formed a peptide bond with the alanine or any other amino acid. In peptide bond formation, the carboxyl group of the P-site amino acid (alanine) will bond with the amine group of the A-site amino acid (glycine).
The correct answer to the question is therefore: D) glycine has a free n-terminus, where alanine has both c and n-terminals free.