ID #2245

I don't understand what comes first, does the amino acid attach to the attachment site of the tRNA after the correct codon attaches to the anti-codon? Or does the amino acid that attaches to the attachment site determine the anti-codon, and therefore which codon attaches to it?

I think this short video might help clear up the confusion: 

The tRNA has two important regions (a) the anti-codon site and (b) the amino acid attachment site. As shown in one of the slides from lecture 19, the anti-codon is complementary and antiparallel to a codon found in mRNA. The codon in mRNA is what originally specifies the amino acid needed in the protein. Because the anti-codon of the tRNA is complementary to the codon in mRNA, a tRNA anti-codon can be "read" to figure out which amino acid was originally specified in the mRNA codon. Remember, you'll be provided a codon table that shows the amino acid that corresponds to each mRNA codon. 

In summary, the aminoacyl tRNA synthetase charges the tRNA with the amino acid specified by the codon of mRNA by "reading" the tRNA anti-codon first, and then covalently attaching the amino acid at the attachment site second. 

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