Playlists containing Powerdesigner 15.2 License Key LINK.Users who reposted Powerdesigner 15.2 License Key LINK.Users who like Powerdesigner 15.2 License Key LINK.These two delays introduced by the elongation factor cause most incorrectly bound tRNA molecules (as well as a significant number of correctly bound molecules) to leave the ribosome without being used for protein synthesis, and this two-step mechanism is largely responsible for the 99.99% accuracy of the ribosome in translating proteins. Although this lag is believed to be the same for correctly and incorrectly bound tRNAs, an incorrect tRNA molecule forms a smaller number of codon-anticodon hydrogen bonds than does a correctly matched pair and is therefore more likely to dissociate during this period. A second lag occurs between EF-Tu dissociation and the full accommodation of the tRNA in the A site of the ribosome. In other words, GTP hydrolysis selectively captures the correctly bound tRNAs. The rate of GTP hydrolysis by EF-Tu is faster for a correct codon-anticodon pair than for an incorrect pair hence an incorrectly bound tRNA molecule has a longer window of opportunity to dissociate from the ribosome before the irreversible step of chain elongation occurs. The first delay is the time required for GTP hydrolysis. The elongation factor introduces two short delays between codon-anticodon base pairing and polypeptide chain elongation these delays selectively permit incorrectly bound tRNAs to exit from the ribosome before the irreversible step of chain elongation occurs. The initial codon recognition, however, triggers the elongation factor to hydrolyze its bound GTP (to GDP and inorganic phosphate), whereupon the factor dissociates from the ribosome without its tRNA, allowing protein synthesis to proceed. Although the bound elongation factor allows codon-anticodon pairing to occur, it prevents the amino acid from being incorporated into the growing polypeptide chain.
Charged tRNAs enter the ribosome bound to the GTP-form of EF-Tu. In addition to helping move translation forward, EF-Tu is thought to increase the accuracy of translation by monitoring the initial interaction between a charged tRNA and a codon (see Figure 6-66).
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