Question

which of these two things happens first: The ribosome docks with the ER membrane or the ribosome binds mRNA encoding a transmembrane protein

Answer 1

first: The ribosome docks with the ER membrane

Ribosomes initiate translation in the cytosol Small subunit 5' mRNA +NH Large subunit Newly forming polypeptide +NH, OR (a) Ribosomes attach to ER membranes if they are synthesizing polypeptides destined for the endomembrane system or for export from the cell. As synthesis continues, the newly forming polypeptide is transferred across the ER membrane by COTRANSLATIONAL IMPORT. The completed polypeptide either remains in the ER or is transported via various vesicles to another compartment of the endomembrane system. (Integral membrane proteins are inserted into the ER membrane as they are made, rather than being released into the ER lumen.) (b) Ribosomes remain free in the cytosol if they are synthesizing polypeptides destined for the cytosol or for import into the nucleus, mitochondria, chloroplasts, or peroxisomes. When the polypeptide is complete, it is released from the ribosome and either remains in the cytosol or is transported into the appropriate organelle by POSTTRANSLATIONAL IMPORT. Polypeptide uptake by the nucleus occurs via the nuclear pores, using a mechanism different from that involved in posttranslational uptake by other organelles. mRNA СОО Coo- ER lumen +NH, +NHg +NH. Completed polypeptide in cytosol + NH3 - Completed +NH, polypeptide in ER + NH3 Endoplasmic reticulum (ER) Remains in cytosol Remains in ER Or is imported into an organelle Golgi complex Via nuclear pores Peroxisome OO 333333333333333 XXXXXXXXXXXX Nucleus PO Secretory vesicle Lysosome Plasma membrane Mitochondrion Chloroplast © 2012 Pearson Education, Inc.