Question

Assuming that protein synthesis was under way when the radioactive amino acids were added, which of the following best describes how the radioactivity was distributed in one of the first molecules of protein X that was completely translated?

A) radioactive aa were randomly located throughout the molecule
B) radioactive aa were located only at one end of the molecule
C) radioactive aa were added at both ends, but not in the middle
D) radioactive aa were located in the middle, but not at either end

Answer 1

The radioactive amino acids would be distributed randomly throughout the newly synthesized protein X, due to the continuous process of polypeptide chain elongation in accordance with the mRNA template.

Step-by-step explanation:

Radioactive Labeling in Protein Synthesis. When radioactive amino acids are added to a cell culture during protein synthesis, the assimilation of these amino acids into the polypeptide chain follows the sequence directed by the messenger RNA (mRNA). Therefore, as amino acids are added sequentially from the amino (N) terminus to the carboxyl (C) terminus, newly synthesized proteins will incorporate the radioactive amino acids throughout the entire length of the polypeptide chain as it is elongated. If we assume that the radioactive amino acids were uniformly available from the moment they were introduced until the completion of translation, the entire molecule of protein X would be labeled, not just at the ends or in the middle.

Based on the principles of the central dogma of molecular biology, the correct distribution would be that the radioactive amino acids were randomly located throughout the molecule (Option A). This is due to the continuous process of elongation during translational synthesis, where each peptide bond formation is a potential site for incorporation of a radioactive amino acid. Hence, the radioactivity is expected to be equally spread across the length of the synthesized polypeptide.