<P> The ribosome molecules translate this code to a specific sequence of amino acids . The ribosome is a multisubunit structure containing rRNA and proteins . It is the "factory" where amino acids are assembled into proteins . tRNAs are small noncoding RNA chains (74 - 93 nucleotides) that transport amino acids to the ribosome . tRNAs have a site for amino acid attachment, and a site called an anticodon . The anticodon is an RNA triplet complementary to the mRNA triplet that codes for their cargo amino acid . </P> <P> Aminoacyl tRNA synthetases (enzymes) catalyze the bonding between specific tRNAs and the amino acids that their anticodon sequences call for . The product of this reaction is an aminoacyl - tRNA . In prokaryotes, this aminoacyl - tRNA is carried to the ribosome by EF - Tu, where mRNA codons are matched through complementary base pairing to specific tRNA anticodons . Aminoacyl - tRNA synthetases that mispair tRNAs with the wrong amino acids can produce mischarged aminoacyl - tRNAs, which can result in inappropriate amino acids at the respective position in protein . This "mistranslation" of the genetic code naturally occurs at low levels in most organisms, but certain cellular environments cause an increase in permissive mRNA decoding, sometimes to the benefit of the cell . </P> <P> The ribosome has three sites for tRNA to bind . They are the aminoacyl site (abbreviated A), the peptidyl site (abbreviated P) and the exit site (abbreviated E). With respect to the mRNA, the three sites are oriented 5' to 3' E-P-A, because ribosomes move toward the 3' end of mRNA . The A-site binds the incoming tRNA with the complementary codon on the mRNA . The P - site holds the tRNA with the growing polypeptide chain . The E-site holds the tRNA without its amino acid . When an aminoacyl - tRNA initially binds to its corresponding codon on the mRNA, it is in the A site . Then, a peptide bond forms between the amino acid of the tRNA in the A site and the amino acid of the charged tRNA in the P site . The growing polypeptide chain is transferred to the tRNA in the A site . Translocation occurs, moving the tRNA in the P site, now without an amino acid, to the E site; the tRNA that was in the A site, now charged with the polypeptide chain, is moved to the P site . The tRNA in the E site leaves and another aminoacyl - tRNA enters the A site to repeat the process . </P> <P> After the new amino acid is added to the chain, and after the mRNA is released out of the nucleus and into the ribosome's core, the energy provided by the hydrolysis of a GTP bound to the translocase EF - G (in prokaryotes) and eEF - 2 (in eukaryotes) moves the ribosome down one codon towards the 3' end . The energy required for translation of proteins is significant . For a protein containing n amino acids, the number of high - energy phosphate bonds required to translate it is 4n - 1 . The rate of translation varies; it is significantly higher in prokaryotic cells (up to 17 - 21 amino acid residues per second) than in eukaryotic cells (up to 6 - 9 amino acid residues per second). </P>

What is the role of the a-site in translation
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