What is the role of spindle during mitosis?
During mitosis, the spindle plays a crucial and central role in organizing and coordinating the separation of replicated chromosomes and ensuring their accurate distribution into the two daughter cells. The spindle apparatus is a highly dynamic structure composed of microtubules, motor proteins, and associated proteins. Here are the key functions of the spindle:
1. Formation and Alignment of Chromosomes:
- During prophase, the spindle fibers begin to organize and capture the condensed chromosomes.
- The chromosomes, each consisting of two sister chromatids, are initially scattered within the cell.
- The spindle fibers, particularly the kinetochore microtubules, attach to the kinetochores, which are protein complexes on the centromere regions of the chromosomes.
- The spindle then aligns the chromosomes along the metaphase plate, an equatorial plane between the spindle poles.
- This precise alignment ensures that each sister chromatid will be distributed to opposite daughter cells.
2. Chromosome Segregation:
- Once the chromosomes are properly aligned at the metaphase plate during metaphase, the spindle plays a critical role in separating the sister chromatids.
- Motor proteins associated with the spindle microtubules, such as dynein and kinesin, utilize the energy from ATP hydrolysis to generate the forces necessary for chromosome segregation.
- These motor proteins move along the microtubules, pulling the sister chromatids apart.
- As a result, each sister chromatid is segregated and moves toward opposite spindle poles.
3. Formation of Daughter Nuclei:
- Following chromosome segregation, the spindle poles continue to move further apart, leading to the elongation of the cell.
- Around each set of separated chromosomes at opposite poles, a nuclear envelope begins to form during telophase.
- The nuclear envelope encloses the genetic material, creating two distinct daughter nuclei.
4. Cytokinesis:
- While not directly part of the spindle apparatus, the spindle plays an indirect role in cytokinesis, the division of the cytoplasm.
- In animal cells, the spindle fibers help to position a specialized structure called the contractile ring, composed of actin and myosin filaments.
- The contractile ring constricts along the equator of the cell, pinching the cell membrane inward and eventually splitting the cytoplasm into two separate compartments, resulting in the formation of two individual daughter cells.
In summary, the spindle is a crucial structure during mitosis. It facilitates the alignment of chromosomes, ensures the accurate segregation of sister chromatids into daughter cells, and indirectly participates in cytokinesis, leading to the successful division of the cell.