Apentix/AP Biology/Unit 2: Cell Structure and Function/Question
AP Biologymediummcq1 pt

A protein carrying a nuclear localization signal is translated by free ribosomes in the cytosol. How does the finished protein enter the nucleus?

A.It dissolves directly through the two lipid bilayers of the envelope
B.Importins bind the signal and ferry the protein through nuclear pore complexes, with Ran-GTP providing direction
C.Its signal sequence inserts it into the rough ER, and vesicles deliver it to the envelope
D.The envelope disassembles during interphase to admit cytosolic proteins wholesale

Explanation

Core Concept

Finished proteins cross the nuclear envelope only through nuclear pore complexes, large gated channels built from nucleoporins. An importin recognizes the nuclear localization signal, docks the cargo at the pore, and moves it into the nucleoplasm. The small GTPase Ran sets the traffic direction: Ran-GTP inside the nucleus releases the cargo, and GTP hydrolysis resets the cycle. Passive diffusion through the pore is limited to small molecules, so an untagged protein of this size would be excluded.

Correct Answer

BImportins bind the signal and ferry the protein through nuclear pore complexes, with Ran-GTP providing direction

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