Answer:
Transmembrane are inserted into the rough endoplasmic reticulum membrane while they are still in their synthesis stage. They make there way to ER with the help of the signal sequence already present in the synthesizing protein. The signal sequence stops the process of translation and directs the ribosome, carrying the unfinished protein to dock with ER.Translation than restarts when the signal sequence dock with the ER and it occurs in the ER membrane. This process is necessary because it allows the passage of transfer of protein to cross the lipid bilayer, ER lumen facilitate protein folding and it rectifies the aberrant proteins and target them towards cytosol for destruction.