Biology 1002B Study Guide - Quiz Guide: Microtubule, Dynein, Mitophagy
Prokaryotes
• In the absence of a nuclear membrane , and since the mRNA is mature,
transcription and translation can occur simultaneously
Ribosomes
• Soluble protein that will remain in cytosol, will be translated using free
cytosolic ribosomes
o Pool of available subunits
• What about proteins to be secreted or transmembrane protein?
o Vesicles are used
Co-translation
• Occurs at the same time as translation is going on
• mRNA goes to the ER where a channel or pore is to allow it across the
membrane
o Use a signal peptide (pattern of amino acids at the end terminus)
o Protein in the cytosol are fishing for polypeptides emerging from
ribosomes with signal peptides.
• Signal recognition proteins
• Attach to the signal peptide (ribosome is still attached) until the
protein can guide it to the ER.
• Signal recognition particle binds to a receptor to ensure that the
ribosome is correctly lined up with a part of the membrane
(translocating protein-translocate things from outside of the ER to
the inside of the ER)
• Signal peptide is transferred over to the translocating protein
• End terminal is anchored to a protein that is translocating into the
mb
• Protein is transcribed into the ER
• Peptidase: Cleaver protein, cuts the signal peptide off and the
protein is now free inside the lumen of the ER.
o Chemical modifications are done in the ER
Transmembrane proteins
• In addition to the signal sequence to direct translation to ER, there is a stop-
sequence that halts co-translational import
• Translation is completed, but not import; resulting in a transmembrane
protein
o Peptidase will clear the signal sequence and when translation is complete
the translated protein will change conformation and release… the
transmembrane protein is then available for the next polypeptide.
o Tyrosine kinase receptor is now in the ER, fully translocated proteins will
vesicle and will go to the Golgi
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