Drug blocks anaphase by inhibiting spindle function

Steps:

• Mitosis stages: prophase (1, chromosomes condense), prometaphase/metaphase (2, chromosomes align via spindle), anaphase (3, chromatids separate and move to poles), telophase (4, nuclei reform).
• Drug binds spindle, preventing chromatid separation and poleward movement, which occurs in anaphase.
• Cells entering mitosis complete prophase (spindle forms but drug doesn't block yet) and reach metaphase (alignment possible).
• Process arrests at metaphase; anaphase and telophase cannot proceed without separation.

Why A is correct:

• Prophase and metaphase rely on early spindle assembly, not chromatid pulling, allowing completion before the drug's effect halts progression.

Why the others are wrong:

• B: Includes anaphase (3), blocked by drug's prevention of chromatid separation.
• C: Anaphase (3) only cannot occur due to spindle inhibition.
• D: Includes anaphase (3) and telophase (4), both requiring prior chromatid movement.

Final answer: A