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tRNA is a type of molecule used by cells to transport amino acids from the cytosol to ribosomes for use in protein synthesis. Cells re-use individual tRNA molecules rather than synthesizing new tRNA molecules each time one is used. A class of enzymes called aminoacyl-tRNA synthetases bind specific free cytosolic amino acids to specific tRNA molecules. This endergonic reaction is powered by ATP.

To begin, a specific synthetase's active site is shaped in such a way that an ATP molecule and a specific free amino acid can fit. The active site, in its initial conformation, does not have a region for a tRNA molecule to fit.

Which of the following must occur before a tRNA molecule can become involved in a reaction?

A

The tRNA molecule must undergo a conformational shape change so as to fit into the active site with the ATP molecule and the amino acid.

B

The tRNA must first be bound to the ATP molecule via dehydration synthesis.

C

The enzyme must use the ATP to phosphorylate the amino acid, which will induce a shape change in the active site of the enzyme that then allows the entry of the tRNA molecule.

D

A separate enzyme must first cleave the 3' end of the tRNA molecule so that it can chemically bind with the amino acid.

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