Cell division concept check — In mitosis, which one of the following statements is incorrect? Provide careful reasoning about anaphase mechanics, spindle fibers, and chromosome movement to the metaphase plate.

Introduction / Context:
Mitosis ensures accurate chromosome segregation into two daughter nuclei. This question probes mechanistic understanding of spindle microtubules, especially how kinetochore and polar microtubules contribute differently to anaphase movement and metaphase alignment.

Given Data / Assumptions:

• Eukaryotic spindle has three main classes of microtubules: kinetochore, polar (overlap/ interpolar), and astral.
• Anaphase A refers to chromatid-to-pole movement; Anaphase B refers to spindle pole separation.
• Motor proteins (kinesins and dynein) and microtubule dynamics drive chromosome motion.

Concept / Approach:
Kinetochore microtubules attach to centromeric kinetochores. At anaphase onset, cohesin is cleaved and kinetochore microtubules shorten by depolymerization while maintaining attachment, drawing sister chromatids toward poles (anaphase A). Polar microtubules primarily push poles apart (anaphase B) via plus-end sliding by kinesin-5 and other motors; they do not directly pull chromatids.

Step-by-Step Solution:

Verification / Alternative check:
Live-cell imaging and laser ablation experiments show chromosome-to-pole movement correlates with kinetochore fiber depolymerization and motors at kinetochores and poles; polar microtubule sliding contributes to spindle elongation, not the primary mechanism of chromatid disjunction.

Why Other Options Are Wrong:

• a: Correct outcome of mitosis is genetic equivalence.
• c: Correct description of anaphase A mechanics.
• d: Correct; kinesins/dynein guide congression to the metaphase plate.
• e: Correct structural organization of the spindle apparatus.

Common Pitfalls:
Confusing anaphase A with anaphase B; assuming any pushing force automatically equals chromatid separation; overlooking the role of cohesin cleavage at anaphase onset.

Final Answer:
During anaphase, sister chromosomes separate primarily because polar microtubules from opposite poles push against each other to pull chromatids apart.