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The correct answer is a. DNA grows in the 5’ → 3’ direction because the hydroxide group on the 3’ carbon will attack the phosphate group on the 5’ carbon, thus resulting in a phosphodiester bond formation between the 5’ carbon and the 3’ hydroxyl (it helps if you draw this out). The nucleotide-amino acid bond forms on the 3’ carbon, causing an ester to form between these two molecules (again draw it out). When one amino acid attacks another on a neighboring tRNA, it attacks at the carboxy-terminus, thus relieving the ester bond and leaving the amino-terminus free.
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