NEET 2025 Zoology Question 158 Solution

During prophase and prometaphase, microtubule organizing centers (centrosomes) assemble the bipolar mitotic spindle:

• • Spindle fibers are polymeric microtubules composed of $\alpha$- and $\beta$-tubulin heterodimers.
• • Kinetochore microtubules bind specifically to the proteinaceous trilaminar kinetochores assembled on centromeres of condensed chromosomes.
• • During metaphase, dynamic tension from opposing spindle poles congresses all chromosomes to the equatorial plane (metaphase plate).

Driving chromosome segregation into daughter nuclei:

• • At the onset of anaphase, cohesin complexes holding sister chromatids together are cleaved.
• • Depolymerization of kinetochore microtubules at their plus ends generates mechanical pulling forces.
• • These forces pull the split sister chromatids (now individual daughter chromosomes) toward opposite centrosomal poles.
• • This guarantees equal and faithful genetic distribution into both newly forming daughter cells.

The fundamental and primary function of spindle fibers during mitosis is to separate the chromosomes.