Cell Cycle and Mitosis Notes

Cell Replication

• Reasons for cell replication:

  • Reproduction (asexual reproduction in organisms like fungi and protists)

  • Growth and tissue renewal (in multicellular organisms, somatic cells replicate for growth and tissue renewal)

• General process:

  • Replication of DNA.

  • DNA copies separated.

  • Cell splits to produce two genetically identical daughter cells.

  • Cell cycle in eukaryotic cells.

  • Binary fission in prokaryotic cells.

Cell Cycle (Eukaryotic)

• Three stages:

  • Interphase (DNA replication)

  • Mitosis (M-phase) (DNA copies separated)

  • Cytokinesis (M-phase) (Division of cytoplasm – cell splits in two)

Interphase

• Prepares the cell for division.

  • G1 phase (First gap phase): Cell grows physically and makes proteins needed for DNA replication.

  • S phase (synthesis phase): DNA is replicated.

  • G2 phase (Second gap phase): Cell keeps growing, makes more organelles, and makes microtubules.

Mitosis

• Five phases:

  • Prophase:

    • DNA packs tightly into visible chromosomes (condensation of chromatin).

    • Microtubule spindle starts to assemble.

    • Centrosomes migrate to poles.

  • Prometaphase:

    • Nuclear envelope breaks down.

    • Microtubules attach to the chromosomes (at the kinetochores).

  • Metaphase:

    • Chromosomes are arranged along the metaphase plate at the cell’s equator.

    • All chromosomes are attached to microtubules via kinetochores.

  • Anaphase:

    • Sister chromatids break apart and begin moving towards the poles.

    • Each sister chromatid now becomes a daughter chromosome.

  • Telophase:

    • Chromosomes loosen into chromatin.

    • A nuclear envelope forms around each set of chromosomes.

    • Cleavage furrow forms.

Cytokinesis

• Division of cytoplasm, resulting in two daughter cells.

• Microfilament ring contracts and pinches cell in half.

Cell Division Regulation

• Checkpoints: Cell must pass the checkpoints to continue the cell cycle properly. Failures result in delays or cell death.

Cancer Cells

• Cells that have lost control over cell division and divide uncontrollably.

• Ignore checkpoints.

• Replicate in the absence of a signal.

• Proliferate in an uncontrolled manner.

• Accumulation of mutations in genes that control the cell cycle (tumor suppressor genes (slow down/stop cell cycle) and proto-oncogenes (continue division).

Benign vs. Malignant Tumors

• Benign tumor: Abnormal cells remain only at the original site and stop growing; not cancerous.

• Malignant tumor: Cancerous; grows and spreads throughout the body. Can invade surrounding tissues and metastasize (spread to other tissues/organs)