Apentix/AP Biology/Unit 2: Cell Structure and Function/Question
AP Biologyhardmcq1 pt

A secreted enzyme fails to enter the endoplasmic reticulum after researchers delete its N-terminal hydrophobic signal sequence. Which step was disrupted?

A.SRP binding of the emerging peptide and docking of the ribosome at the ER translocon
B.Mannose-6-phosphate tagging in the cis Golgi that marks lysosomal enzymes
C.Clathrin-coated vesicle budding from the trans Golgi network
D.ATP-driven phosphorylation of the enzyme inside the mitochondrial matrix

Explanation

Core Concept

Cotranslational targeting begins when the signal recognition particle binds the emerging hydrophobic signal peptide and pauses translation. The SRP-ribosome complex then docks at the SRP receptor beside the Sec61 translocon, and growth resumes directly into the ER lumen. Deleting the signal removes the molecular address, so the ribosome finishes the protein free in the cytosol and the enzyme is never threaded into the ER. Later steps such as Golgi glycosylation presuppose ER entry and cannot rescue the defect.

Correct Answer

ASRP binding of the emerging peptide and docking of the ribosome at the ER translocon

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