protein synthesis outline test one

protein synthesis outline test one - Mature mRNAs are...

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Mature mRNAs are selectively transported from the nucleus after transcription: Once mRNAs have been successfully transcribed, they must be shipped out of nucleus into cytoplasm to the translated into protein This picture details how mature mRNAs are selective exported from the nucleus B/c only a fraction of pre-mRNA that is synthesized becomes mature mRNA, there is a large amount of waste mRNA in nucleus that is potentially dangerous to cell Improperly processed RNA debris left in nucleus is eventually degraded by nuclear exosome. A selectivity mechanism is needed to allow only mature, correctly processed mRNA to leave the nucleus through nuclear pore complexes (NPCs) mRNA processing involves the loss of certain proteins and the gain of others, thereby signifying the successful completion of each of the different steps (acquisition of cap-binding complexes, exon junction complexes, poly-A binding proteins signify completion of capping, splicing, and poly-A binding). Properly processed mRNA also contains many hnRNP, which unwind hairpin helix structures to make splicing and other signals easier to read by other cellular structures. Sucessfully processed mature mRNA can only be guided through NPCs by the binding of a specific nuclear export receptor protein which may occur in concert with a 3’ cleavage and
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polyadenylation. The mRNA then undergoes a series of structural transitions culminating in formation of a curved fiber that enters nuclear pore complex, 5’ end first, then mRNA reaches cytosol and export receptor dissociates. Before CBT is lost it is subjected to a final check called “nonsence mediated decay Nonsence mediated decay : is an mRNA surveillance system that eliminates defective mRNAs before they can be efficiently translated into protein. This mechanism is used when cell determines that an mRNA molecule has a nonsense (stop) codon (UAA, UAG, or UGA) in the wrong place, a situation likely to arise when an mRNA has been improperly spliced. Mechanism begins as 5’ end of mRNA emerges from nuclear pore and it meets a ribosome that begins to translate it. As translation proceeds, the exon junction complexes (EJCs) bound to the mRNA at each splice site are displaced by moving ribosome; the normal stop codon will be within the last exon, so if ribosome reaches it and stalls, no more EJCs should be bound. IF there aren’t any, the mRNA passes inspection, if a premature stop codon is found the ribosome stalls and degrades the mRNA. This mechanism allows genetic recombination to occur more easily and safely b/c it weeds out any mRNAs that cannot produce a full length protein. Also important in cells in developing immune system where extensive DNA rearrangements occur that generate premature termination codes. Also plays a role in mitigating symptoms of many inherited human diseases. RNA Transport from Nucleus Can be Regulated:
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This note was uploaded on 03/20/2008 for the course CBIO 3400 taught by Professor Shen,kipreos during the Spring '08 term at University of Georgia Athens.

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protein synthesis outline test one - Mature mRNAs are...

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