A researcher observes that a microtubule is growing rapidly at one end. Which end is likely growing, and why?

• A. The plus end, because it is more stable and less dynamic.
• B. The minus end, because it is closer to the microtubule organizing center.
• C. The minus end, because it has a higher concentration of GTP-bound tubulin.
• D. The plus end, because it has a GTP cap that stabilizes the addition of new tubulin dimers.

Answer: (D)