Final answer:
After a signal sequence enters the RER, the signal sequence is cleaved by signal peptidase. This is followed by the folding of the polypeptide into its tertiary structure, as part of its protein synthesis and processing.
Step-by-step explanation:
After a signal sequence on a nascent polypeptide enters the lumen of the Rough Endoplasmic Reticulum (RER), a series of steps occur to ensure proper protein synthesis and processing. The correct answer to what happens after the entry into the RER is A) Cleavage of the signal sequence by signal peptidase. This cleavage occurs after the Signal Recognition Particle (SRP) detaches from the growing polypeptide chain, allowing translation elongation to resume within the translocation channel. Subsequently, the signal peptidase catalyzes the co-translational hydrolysis of the signal peptide, which leads to the separation of the signal sequence from the rest of the protein. Following this, the polypeptide can begin to fold into its functional tertiary structure within the RER.