[Campbell Biology P.287] What significant event primarily marks the transition from Prophase... | Practice Question

What significant event primarily marks the transition from Prophase to Prometaphase in an animal cell, as described in the provided text?

  • A: Chromatin fibers become tightly coiled and condense into discrete chromosomes.
  • B: The mitotic spindle begins to form.
  • C: The nuclear envelope fragments, allowing spindle microtubules to invade the nuclear area.
  • D: The nucleoli disappear.

Explanation

The description for Prometaphase clearly states: 'The nuclear envelope fragments. The microtubules extending from each centrosome can now invade the nuclear area.' This is the defining event distinguishing it from prophase.