Final answer:
The signal sequence for translocation across the ER membrane is usually located at the start of the polypeptide. This signal sequence directs the protein to the ER as it emerges from the ribosome and is subsequently removed upon reaching its destination. The correct answer to the student's question is (c) start of the polypeptide.
Step-by-step explanation:
The signal sequence that marks a protein for translocation across the endoplasmic reticulum (ER) membrane is usually located at the start of the polypeptide. This signal sequence is a short tail of amino acids that acts as a 'train ticket,' directing the nascent protein to its specific cellular destination, such as the ER. As a protein is synthesized by ribosomes, the signal peptide at the N-terminal signal sequence emerges and interacts with the ER membrane. This sequence of 10-30 amino acid residues at the N-terminal end is recognized by cellular factors that facilitate protein transport and is later removed once the protein reaches its destination.
The correct option for the location of the signal sequence that directs a protein to the ER is therefore (c) start of the polypeptide. Understanding how proteins are accurately targeted to the ER is critical for grasping cellular function and protein sorting within cells which is part of the broader context of protein synthesis, modification, and targeting. Protein targeting is vital for the protein to carry out its function in the correct cellular compartment, and it is pivotal in maintaining cellular organization and homeostasis.