Final answer:
Passive diffusion drives the nuclear import cycle.
Step-by-step explanation:
The nuclear import cycle is propelled by passive diffusion. As proteins approach a nuclear pore, interaction with nuclear pore fibrils triggers pore opening. Through passive diffusion, proteins traverse the nuclear envelope's double membrane, accumulating in the nucleus driven by a concentration gradient.
This mechanism relies on the inherent molecular motion of proteins, operating without the need for external energy input. The process underscores the nuanced interplay of molecular dynamics and cellular structures, facilitating the seamless transit of proteins across nuclear boundaries to regulate essential functions within the nucleus.