Cell Biology

Mitosis

Cell division — one cell becomes two genetically identical daughters

Mitosis is the process of cell division producing two genetically identical daughter cells from one parent. Five phases: prophase (chromosomes condense), prometaphase (nuclear envelope breaks; spindle attaches), metaphase (chromosomes line up at center), anaphase (sister chromatids separate to poles), telophase (nuclear envelopes reform; cytokinesis follows). Different from meiosis (reduction division producing gametes; halves chromosome number). Critical for: growth, repair, asexual reproduction. Cell cycle: G1, S (DNA replication), G2, M (mitosis). Strict checkpoints prevent errors. Cancer: dysregulated mitosis.

  • Five phasesProphase, prometaphase, metaphase, anaphase, telophase
  • ResultTwo genetically identical diploid daughters
  • Different fromMeiosis (produces haploid gametes)
  • Cell cycleG1, S, G2, M
  • Mitotic spindleMicrotubules; pulls chromosomes apart
  • CytokinesisCytoplasm divides (after telophase)

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Why mitosis matters

  • Growth. Cell number increases.
  • Tissue repair. Replace damaged cells.
  • Asexual reproduction. Single-celled organisms.
  • Stem cells. Maintain stem cell pool.
  • Cancer. Dysregulated mitosis.
  • Drug targets. Anti-cancer drugs.
  • Development. Building multicellular organism.

Common misconceptions

  • Mitosis = meiosis. Different (meiosis halves chromosome number).
  • Cells always dividing. Most differentiated cells in G0.
  • Phases distinct. Continuous transitions.
  • DNA replicates during mitosis. S phase before mitosis.
  • Cytokinesis is part of mitosis. Separate process; often grouped.
  • Mitosis is fast. ~1-2 hours typically.

Frequently asked questions

What are the phases of mitosis?

Five. (1) Prophase: chromosomes condense; nucleolus disappears. (2) Prometaphase: nuclear envelope fragments; spindle microtubules attach to kinetochores. (3) Metaphase: chromosomes align at metaphase plate (cell center). (4) Anaphase: sister chromatids separate; pulled to opposite poles. (5) Telophase: nuclear envelopes reform; chromosomes decondense. Followed by cytokinesis (cytoplasm divides).

What's the cell cycle?

Sequence of events for cell division. G1 (Gap 1; growth, normal activity), S (Synthesis; DNA replicated), G2 (Gap 2; preparation for division), M (Mitosis; division). G0: non-dividing state (most adult cells). Total: ~24 hours typical mammalian cell. Strict checkpoints: G1/S (DNA damage, growth signals), G2/M (DNA replicated correctly), spindle assembly checkpoint (chromosome attachment).

How is the spindle formed?

Microtubules emanating from centrosomes (microtubule organizing centers). Each centrosome contains pair of centrioles. Centrosomes duplicate during S phase; move to opposite poles in prophase. Microtubules grow from each pole; some attach to chromosomes at kinetochores; some interact with each other (interpolar). Spindle: bipolar structure that aligns and separates chromosomes.

What happens at metaphase?

Chromosomes aligned at center (metaphase plate). Each chromosome has 2 sister chromatids attached at centromere. One kinetochore from each chromatid attached to microtubule from each pole (bipolar attachment). Tension across centromere indicates correct attachment. Spindle assembly checkpoint: prevents anaphase until all chromosomes correctly attached.

What happens at anaphase?

Sister chromatids separate. Cohesin proteins (held chromatids together) cleaved by separase enzyme. Kinetochore microtubules shorten, pulling chromatids to opposite poles. Other microtubules push poles apart. Result: full set of chromosomes at each pole. Each pole receives identical chromosomes (one from each pair).

What's cytokinesis?

Cytoplasm division. Animal cells: cleavage furrow forms (actin-myosin contractile ring); pinches cell in two. Plant cells: cell plate forms in middle; new cell wall built between daughters (rigid wall doesn't allow furrow). Result: two separate cells. Coordinated with mitosis but distinct process.

How does cancer relate to mitosis?

Cancer cells have dysregulated cell cycle. Mutations in: checkpoints (p53, Rb), oncogenes (MYC, RAS), tumor suppressors. Result: uncontrolled division. Many chemotherapeutics target dividing cells: vinca alkaloids, taxol (microtubule-targeting), DNA replication inhibitors, etc. Side effect: damages other rapidly dividing tissues (bone marrow, gut, hair).