Translation elongation and termination
- Created by: natasha8sherry
- Created on: 12-01-14 12:33
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- Translation elongation and termination
- ribosome processes along the mRNA, adding AAs to the nascent polypeptide chain
- arrival of next AA in the sequence and its addition through formation of polypeptide bond causes ribosome to move along to the next codon
- called TRANSLOCATION
- aided by elongation factors (EFs)
- called TRANSLOCATION
- arrival of next AA in the sequence and its addition through formation of polypeptide bond causes ribosome to move along to the next codon
- 3 important sites
- A site
- acceptor site - where the aminoacyl tRNA lands charged with the next AA to be added
- P site
- peptidyl-tRNA site, occupied by the most recent aminoacyl tRNA
- E site
- exit site - where the tRNA due to leave the ribosome following delivery of its associated AA is temporarily located
- located primarily in the large subunit
- but A P E sites can only form when 2 subunits are associated
- A site
- translation initiation
- initial tRNA carrying methionine AA is located to P site
- next tRNA joins, entering the A site
- aided by elongation factor EF1a bound to a molecule of GTP
- after GTP hydrolysis, a peptide bond is formed between the AAs
- reaction catalysed by the ribozyme activity of the rRNA itself
- translocation occurs
- aided by EF2 and hydrolysis of the GTP molecule it carries
- initiator tRNAiMet has now lost its methionine and is moved to the E site, the 2nd tRNA shifts to the P site, allowing a new incoming charged tRNA to enter the A site
- polypeptide synthesis ends with translation termination
- release factors recognise stop codons
- help to release the nascent polypeptide from the mRNA
- when ribosome reaches a stop codon, eRF1 docks near the A site in association with eRF3
- GTP hydrolysis promotes cleavage of the complete polypeptide from the last tRNA and dissociation from the ribosome
- ribosome subunits then free to begin another round of translation
- GTP hydrolysis promotes cleavage of the complete polypeptide from the last tRNA and dissociation from the ribosome
- release factors recognise stop codons
- polypeptide synthesis ends with translation termination
- (methionine is anticodon for AUG)
- next tRNA joins, entering the A site
- initial tRNA carrying methionine AA is located to P site
- signal recognition particles
- used for proteins that are destined for secretion from the cell or for insertion into cellular membranes
- RNA-protein complex which binds to a short signal sequence present at the N-terminus of such proteins and mediates their targeting
- signal sequence = short order of about 12 hydrophobic AAs
- binds to signal sequence while the protein is still being translated
- leads to arrest in translation and targeting of the ribosome-polypeptide-SRP complex to the endoplasmic reticulum
- SRP interacts with the membrane-bound SRP receptor on the ER membrane and transfers the polypeptide-ribosome complex onto a translocon protein in the ER membrane
- the translocon serves as a pore, allowing the newly-synthesised protein to be inserted within the membrane
- the remaining section of translation then takes place through the pore, allowing the protein to either be inserted into the ER membrane
- this requires a 2nd signal sequence along the polypeptide called a topogenic sequence
- or into the ER lumen, where it can be modified and sent along the rest of its cellular journey before reaching its target destination
- the remaining section of translation then takes place through the pore, allowing the protein to either be inserted into the ER membrane
- the translocon serves as a pore, allowing the newly-synthesised protein to be inserted within the membrane
- SRP interacts with the membrane-bound SRP receptor on the ER membrane and transfers the polypeptide-ribosome complex onto a translocon protein in the ER membrane
- leads to arrest in translation and targeting of the ribosome-polypeptide-SRP complex to the endoplasmic reticulum
- ribosome processes along the mRNA, adding AAs to the nascent polypeptide chain
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