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Unformatted text preview: Bio Final Test Review Chpt. 15 Why do eukaryotic cells require a nucleus as a separate compartment when prokaryotic cells can manage perfect well without? • Eukaryotic gene expression is more complicated than prokaryotic gene expression • In particular, prokaryotic cells do not have introns that interrupt the coding sequences of their genes, so that an mRNA can be translated immediately after it is transcribed, without a need for further processing • In fact, in prokaryotic cells ribosomes start translating most mRNAs before transcription is finished • This would have disastrous consequences in eucaryotic cells, because most RNA transcripts have to be spliced before they can be translated • The nuclear envelope separates the transcription from the translation processes in space and in time • A primary RNA transcript is held in the nucleus until it is properly processed to form an mRNA, and only then is it allowed to leave the nucleus so that ribosomes can translate it Explain how an mRNA molecule can remain attached to the ER membrane while individual ribosomes translating it are released and rejoin the cytosolic pool of ribosomes after each round of translation. • An mRNA molecule is attached to the ER membrane by the ribosomes translating it • This ribosome population, however, is not static; the mRNA is continuously moved through the ribosome • Those ribosomes that have finished translation dissociate from the 3’ end of the mRNA and from the ER membrane, but the mRNA itself remains bound by other ribosomes, newly recruited from the cytosolic pool, that have attached to the 5’ end of the mRNA and are still translating the mRNA • Depending on its length, there are about 10-20 ribosomes attached to each membrane-bound mRNA molecule • A common pool of ribosomes is used to synthesize both the proteins that stay in the cytosol and those that are transported into membrane-enclosed organelles, including the ER : o Ribosomes that are translating cytosolic proteins remain free in the cytosol o For proteins that are bound for the ER, a signal sequence on the growing polypeptide chain directs the ribosome to the ER membrane o Many ribosomes bind to each mRNA molecule, forming a polyribosome o At the end of each round of protein synthesis, the ribosomal subunits are released and rejoin common pool in the cytosol o mRNA molecule encoding a protein with an ER signal sequence, the polyribosome becomes riveted to the ER membrane by the growing polypeptide chains, which have become inserted into the membrane Start-Stop-Signals-Other Shit for Determining Arrangement of a Transmembrane Protein in the Lipid Bilayer • Start-Transfer Sequence : An internal signal sequence that is used to start the protein transfer, and is never removed from the polypeptide o Serves to initiate translocation, which continues until a stop-transfer sequence is reached • Stop-Transfer Sequence : the transfer process is halted by an additional sequence of hydrophobic amino acids...
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- Fall '07