§ B-03 Mitotic Cell Division: Spindle Microtubule Mechanics & Ploidy Conservation (Grade 9–10 · Mitosis & Cell Cycle)
Prophase, Metaphase Plate, Anaphase Sister Chromatid Separation & Telophase (2n → 2n). “Equal partitioning of the replicated genome into daughter cells is orchestrated by the bipolar mitotic spindle: kinetochore microtubules align chromosomes at the equator and synchronously pull sister chromatids to opposite poles.”
Theory Summary & Scientific Mechanism
Mitosis is the division of a eukaryotic somatic cell that partitions a replicated genome equally into two genetically identical daughter cells (2n → 2n). Because DNA replication completes during interphase S-phase (2n2c → 2n4c), chromosomes enter mitosis composed of two sister chromatids joined at the centromere by cohesin.
During prophase, chromatin condenses and the bipolar mitotic spindle assembles from dynamic tubulin microtubules. In metaphase, amphitelic attachment of kinetochore microtubules balances opposing pulling forces (Fₖin ≈ 10–50 pN), aligning all 2n4c chromosomes on the equatorial metaphase plate.
Once the spindle assembly checkpoint (SAC) is satisfied, separase cleaves cohesin to initiate anaphase: sister chromatids separate as independent chromosomes (transiently 4n4c in the cell) and move poleward via microtubule depolymerization (Anaphase A) and spindle elongation (Anaphase B), followed by nuclear envelope reformation in telophase and cytokinesis (2 × 2n2c).
Everyday Analogy
Before mitosis, the cell photocopies every chromosome book during S-phase and staples the twin copies together into an X-shape (2n4c). In metaphase they line up along the equator, and in anaphase the staple is cut so spindle fibers pull one complete set (2n2c) to each daughter pole!
Interactive Question & Derivation
A diploid somatic cell (2n = 4 in a model organism) completes DNA replication during S-phase and enters mitosis. What are the chromosome number (n) and DNA content (c) at the metaphase plate, and at each spindle pole immediately after centromere splitting in anaphase?
- A: In metaphase 1n2c, and at each pole in anaphase 1n1c (haploid)
- B: In metaphase 2n4c (each of the 2n chromosomes consists of two sister chromatids), and in anaphase after cohesin cleavage each pole receives a complete diploid set 2n2c
- C: In metaphase 4n4c, producing four 1n1c cells after telophase
- undefined: DNA molecules replicate directly during anaphase via microtubule tension
Study Hint: At metaphase the cell is 2n4c (two sister chromatids per chromosome); in anaphase, separated chromatids form a 2n2c diploid set at each pole.
Proof & Explanation: DNA replication occurs during interphase S-phase, so the cell enters prophase and metaphase with 2n4c: there are 2n chromosomes, each composed of two identical sister chromatids (4c DNA) held together by cohesin at the centromere. At anaphase onset, separase cleaves cohesin and each separated chromatid becomes an independent daughter chromosome. Temporarily the dividing cell contains 4n4c, meaning each spindle pole receives a genetically identical diploid complement of 2n2c.
Key Equations & 3D Simulation Parameters
2n4c xrightarrowAnaphase: cohesin cleavage (2n2c)ₚole 1 + (2n2c)ₚole 2
- Mitosis stage (1=Prophase, 2=Metaphase, 3=Anaphase, 4=Telophase): 1 … 4 (default: 2)
- Number of homologous chromosome pairs n (diploid 2n): 1 … 3 pairs (default: 2 pairs)
- Kinetochore microtubule pulling tension Fₖin: 0.5 … 2.5 rel. (default: 1.4 rel.)
- Spindle elongation speed (Anaphase B pole separation): 0.5 … 2 rel. (default: 1 rel.)