Final answer:
A stop-transfer sequence, which is a hydrophobic domain within the polypeptide chain, correctly describes the transmembrane sequence that prevents further translocation of the peptide through the ER membrane channel during protein synthesis.
Step-by-step explanation:
The correct description of the transmembrane sequence that blocks further translocation of the peptide through the channel in the ER membrane during synthesis is the stop-transfer sequence. This sequence is a hydrophobic domain within the polypeptide chain that gets trapped in the fatty acid interior of the membrane, thereby halting the translocation process. Proteins that need to span the membrane multiple times have several stop-transfer sequences. During the process of synthesis, a signal peptide guides the ribosome to the rough endoplasmic reticulum (RER), and translation begins. As the new peptide elongates, it is processed and moved into the RER. If the growing polypeptide contains a stop-transfer sequence, it prevents the peptide from passing completely through the membrane, hence becoming part of the membrane itself.