Elongation of the polypeptide chain begins when the anticodon of a second charged tRNA binds to the next mRNA codon in the vacant A site of the 70S initiation complex. The second charged tRNA is guided to the A site by elongation factors. The P and the A sites are very close together. This allows the carboxy terminus of the amino acid in the P site to react with the nucleophilic amino terminus of the amino acid on the tRNA in the A site to form a stable peptide bond. This reaction is catalyzed by peptidyl transferase, a ribozyme associated with the 50S subunit. The tRNA in the P site is now devoid of its amino acid and the tRNA in the A site is now a dipeptidyl tRNA with 2 amino acids. The 70S complex now moves along the mRNA toward its 3' end. This movement is called translocation and causes the tRNA in the P site to be displaced. The dipeptidyl tRNA in the A site moves into the P site so that another charged tRNA can move into the A site. Each translocation step is coupled to the hydrolysis of GTP. Elongation continues in this fashion until the stop codon is reached and the peptide chain is released from the ribosome.

Translation of proteins involves a mRNA, tRNA, ribosome, and energy. Click on the graphic to learn about translation with the Protein Synthesis animation.

Important points to remember are:
tRNAs have an inverted L structure
tRNAs have an anticodon loop that pairs with a mRNA codon
there is a tRNA for each amino acid
amino acids are attached to tRNAs via ester linkage to the 3'-OH by specific aaRS to form charged tRNAs
charged tRNAs pair with the mRNA codons in the P and A sites of the 70S initiation complex
peptide chain elongation proceeds when the attached amino acid of the tRNA in the P site forms a peptide bond with the amino acid of the tRNA in the A site
The crystal structure of tRNA elucidates other important points:
tRNAs all have an inverted L structure
tRNAs have conserved structural features: 5'P, D arm, anticodon arm, variable arm, TyC arm, 3' amino acid acceptor stem
tRNAs contain unusual bases
tRNAs are made up of a single chain of RNA yet have considerable tertiary structure arising from base pairing within the chain
there are several non-Watson-Crick base pairs that take part in cross-linking
tRNAs are narrow so as to be able to sit next to one another on adjacent mRNA codons in the P and A sites
the anticodon arm and amino acid acceptor stems are solvent accessible regions of the molecule

上一页  [1] [2]