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Practice MCAT Question - Q66 - Answer!

During which stage(s) of translation is the enzyme peptidyl transferase active?

I. Initiation
II. Elongation
III. Termination

A.  I and II
B.  II and III
C.  I, II, and III
D.  I and III

The correct answer is (B). Peptidyl transferase is needed to: a) attach a newly arrived amino acid at the ribosome’s P site to the existing chain of amino acids already present at the A site; b) detach the tRNA holding the amino acid chain at the A site, once the chain has been attached to the tRNA at the P site; and c) attach the water molecule to the amino acid chain at the P site when a terminator codon is reached. The first two functions are part of elongation, while the third function is part of termination.

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