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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?


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.


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


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.


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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