Transfer RNAs Order Activated Amino Acids on the mRNA Template

Transfer RNAs Order Activated Amino Acids on the mRNA Template 

Transfer RNAs contain two crucial functional sites A site for attachment of the amino acids and a site that interacts with the mRNA during translation on the ribosome. At least one type of tRNA corresponds to each of the 20 types of amino acids that are incorporated into proteins. For accurate translation to occur, tRNAs must be distinguishable from one another by the molecules that recognize these specific types while still being recognizable to the molecules that interact with all tRNAs. A major challenge has been to understand how the similarities and differences in the structures of different tRNAs are related to the functions that 

Overview of reactions in protein synthesis. (aab aa2, aa3 = amino acids l, 2, 3.) Protein synthesis requires transfer RNAs for each amino acid, ribosomes, messenger RNA, and a number of dissociable protein factors in addition to ATP, GTP, and divalent cations. First the transfer RNAs become charged with amino acids, then the initiation complex is formed. Peptide synthesis does not start until the second aminoacyl tRNA becomes bound to the ribosome. Elongation reactions involve peptide bond formation, dissociation of the discharged tRNA, and translocation. The elongation process is repeated many times until the termination codon is reached. Termination is marked by the dissociation of the messenger RNA 

The unpaired loops of tRNA are named according to their unique structural features. Loop I varies in size from 7 to 11 unpaired bases and frequently contains the unusual base dihydrouracil it is designated the D loop. Loop II contains the three bases known as the anticodon and is therefore designated the anticodon loop. This portion of tRNA plays a 

Simplified diagram of mRNA structure, (a) Typical eukaryotic mRNA. An AUG start codon is located near the 5 end of the mRNA. The single reading frame ends with one of the three trinucleotide sequences that represents a stop codon. Frequently, but not always, the 5 end of the mRNA is capped, and the 3 end contains a poly(A) tail. The cap structure is described in figure 28.17. 

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