Mitosis Calculators

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Mitosis is the process of nuclear division in eukaryotic cells that produces two genetically identical daughter nuclei, each with the same number of chromosomes as the parent cell. It is part of the cell cycle: interphase (G1 → S → G2) is followed by mitosis (M phase) and cytokinesis. The four phases of mitosis are: prophase (chromosomes condense; spindle forms), metaphase (chromosomes align at the metaphase plate), anaphase (sister chromatids separate; pulled to poles), and telophase (nuclear envelopes reform; chromosomes decondense). Mitosis is essential for growth, tissue repair, and asexual reproduction. It is distinct from meiosis, which produces haploid gametes.

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Phases of Mitosis

  1. Prophase: Chromosomes condense and become visible; centrosomes migrate to poles; mitotic spindle begins to form; nuclear envelope begins to break down
  2. Prometaphase: Nuclear envelope fully breaks down; kinetochore microtubules attach to kinetochores on centromeres; chromosomes begin moving
  3. Metaphase: Chromosomes align at the metaphase plate (cell equator); spindle assembly checkpoint verifies all kinetochores attached before proceeding
  4. Anaphase: Cohesins cleaved by separase → sister chromatids separate; kinetochore microtubules shorten → chromatids pulled to opposite poles; cell elongates
  5. Telophase: Chromosomes arrive at poles; nuclear envelopes reform; chromosomes decondense; cytokinesis begins (cleavage furrow in animal cells; cell plate in plant cells)

Spindle Assembly Checkpoint (SAC)

Ensures all kinetochores are properly attached to spindle microtubules before anaphase begins. Mad1/Mad2 proteins: unattached kinetochore → inhibits APC/C → delays anaphase. SAC failure → chromosome missegregation → aneuploidy → cancer.

Glossary

Mitosis
Nuclear division producing two genetically identical daughter cells; phases: prophase, prometaphase, metaphase, anaphase, telophase; followed by cytokinesis; maintains ploidy (2n → 2n).
Spindle Assembly Checkpoint (SAC)
A quality control mechanism that delays anaphase until all kinetochores are properly attached to spindle microtubules; Mad1/Mad2 inhibit APC/C; failure causes aneuploidy.
Anaphase
Mitosis phase when cohesin is cleaved by separase, sister chromatids separate, and kinetochore microtubules shorten to pull chromatids to opposite poles.

Frequently Asked Questions

Mitosis is the nuclear division process that produces two genetically identical daughter cells. Phases: Prophase: chromosomes condense; centrosomes move apart; spindle begins forming. Prometaphase: nuclear envelope breaks down; spindle microtubules capture chromosomes at kinetochores. Metaphase: chromosomes align at the metaphase plate; spindle assembly checkpoint (SAC) verifies all kinetochores are attached. Anaphase: cohesin cleaved by separase → sister chromatids separate → pulled to opposite poles by shortening kinetochore microtubules. Telophase: chromosomes at poles; nuclear envelopes reform; chromosomes decondense. Cytokinesis: cytoplasm divides (animal: cleavage furrow; plant: cell plate formation by vesicle fusion at the phragmoplast).

The spindle is a bipolar structure made of microtubules (α/β-tubulin polymers) emanating from two spindle pole bodies (centrosomes in animal cells; without centrosomes in plant cells). Kinetochore microtubules: attach to the kinetochore protein complex at the centromere of each sister chromatid. Sister chromatids: held together by cohesin complexes (ring proteins encircling the two chromatid arms). Metaphase: tension from both poles' microtubules aligns chromosomes at the equator. Anaphase: separase enzyme cleaves the Scc1/Rad21 subunit of cohesin → cohesins release → sister chromatids separate. Kinetochore microtubules shorten from the plus end → chromatids are reeled to the poles. Cell elongates via polar microtubule sliding (kinesin-5 pushes anti-parallel microtubules apart).

Mitosis vs. meiosis: Mitosis: one cell → two identical daughter cells; diploid → diploid (2n → 2n); no crossing over; sister chromatids separate; used for growth, repair, asexual reproduction. Meiosis: one cell → four genetically unique haploid cells; diploid → haploid (2n → n); crossing over (recombination) in meiosis I; homologous chromosomes separate in meiosis I; sister chromatids separate in meiosis II; produces gametes (sperm, eggs) or spores. Meiosis consists of two rounds of division (meiosis I and II); mitosis has one. Meiosis is only in germ cells (gonads); mitosis is in all somatic cells.

The spindle assembly checkpoint (SAC) is a cell cycle control mechanism ensuring all kinetochores are properly attached to spindle microtubules before sister chromatid separation (anaphase) begins. How it works: unattached or incorrectly attached kinetochore → Mad1 recruits Mad2 → Mad2 inhibits Cdc20 (activator of APC/C) → APC/C cannot trigger separase → cohesin stays intact → anaphase blocked. Once all kinetochores are bioriently attached: checkpoint is satisfied → APC/C activated → securin degraded → separase active → cohesin cleaved → anaphase begins. SAC failure: chromosome missegregation → aneuploidy (wrong chromosome number in daughter cells). Aneuploidy is a hallmark of cancer cells and causes Down syndrome, trisomy 18, and other chromosomal disorders.