Ch+5+pt+1+FP-module 4

Chapter 5: Cell Division

Part 1: Cell Cycle and Mitosis

Cellular Basis of Reproduction

• Cell Theory:

  • All living things are composed of cells.

  • All cells arise from preexisting cells.

• Single-celled Organisms:

  • Reproduce by simply dividing.

• Multi-celled Organisms:

  • Generate gametes (sperm and egg) through division.

  • Originate from single cells (zygotes), which divide into somatic cells forming the adult organism.

• Importance of Cell Division in Adults:

  • Continues to repair and replace old or damaged cells.

The Cell Cycle

• Typical Cell Division:

  • Most body cells undergo regular division, while some do not.

• Main Phases:

  • Mitotic Phase: Division of cells.

  • Interphase: Normal cell activity, growth, and DNA replication.

Interphase

• Cycle Composition: 90% of the cell cycle.

• Activities During Interphase:

  • Cells carry out normal functions, grow, and prepare for division.

• Stages of Interphase:

  • G1 Phase:

    • Growth.

    • Organelles double.

  • S Phase:

    • DNA replication occurs.

  • G2 Phase:

    • Additional growth.

    • Production of proteins necessary for division.

• Transition: Ends with the start of the mitotic phase.

Mitotic Phase

• Cycle Composition: 10% of the cell cycle.

• Stages:

  • Mitosis: Involves the division and separation of nuclei and chromosomes.

  • Cytokinesis: Division of cytoplasm and organelles.

• Outcome: Two daughter cells, identical to the parent cell.

• Characteristic: Unique to eukaryotic cells.

Apoptosis: Programmed Cell Death

• A natural cycle that removes unwanted or damaged cells.

• Process:

  • Orderly disassembly of the cell.

  • Loss of contact with neighboring cells.

  • Fragmentation of the nucleus.

  • Final breakdown by white blood cells digesting fragments.

Control of Cell Cycle

• Regulation:

  • Influenced by both internal and external signals.

• Key Checkpoints:

  • G1 Checkpoint: Determines if the cell should divide; needs growth factor to proceed; otherwise enters G0 (non-dividing state).

  • G2 and M Checkpoints: Ensure DNA integrity before proceeding.

• Functions of Checkpoints:

  • Prevents replication if DNA is damaged or missing, can initiate apoptosis if necessary.

Cancer and Cell Cycle Control

• Characteristics of Cancer:

  • Unregulated cell division (runaway mitosis).

  • Growth conditions are density independent.

  • Cells bypass checkpoints and can produce their own growth factors.

  • Cells can intermittently stop division and are "immortal" (continue to divide without stopping).

• Treatments:

  • Surgical removal of cancer cells.

  • Radiation therapy to damage cancer cell DNA.

  • Chemotherapy to inhibit the division process.

Chromosome Structure

• Each human cell has approximately 2 meters of DNA in a 5mm nucleus.

• Chromatin:

  • DNA double helices wound around proteins.

  • Condenses to become visible before mitosis.

Chromosome Number

• Diploid Cells: Two sets of chromosomes (2n).

• Haploid Cells: One set of chromosomes (n), typically seen in gametes (sperm and egg).

• Fertilization: The fusion of sperm and egg restores diploid state leading to zygote formation, which then undergoes multiple mitotic divisions.

Copying Chromosomes for Division

• Before duplication, chromosomes exist as single chromatids.

• After S phase, duplicated chromosomes consist of two sister chromatids held together at the centromere.

• During division, sister chromatids separate to form daughter chromosomes.

Mitosis

• Most common form of cell division.

• Produces two nuclei with an identical chromosome number as the parent nucleus (2n → 2n).

• Continuous process divided into four phases:

  • Prophase

  • Metaphase

  • Anaphase

  • Telophase

• Telophase and Cytokinesis: Often occur simultaneously.

Before Mitosis Starts

• Centrosome, microtubule organizing center, duplicates, including centrioles in animals.

• Centrosomes arrange microtubules to form the spindle, which distributes chromosomes.

• Chromosomes are duplicated but not yet condensed (invisible).

Prophase

• Nucleolus disappears, nuclear envelope fragments.

• Chromatin condenses, making chromosomes visible.

• Centrosomes move to opposite ends of the cell.

Metaphase

• Fully formed mitotic spindle.

• Chromosomes align at the metaphase plate (center of the cell).

Anaphase

• Division of centromeres occurs.

• Daughter chromosomes (formerly sister chromatids) move to opposite ends of the cell.

• Cell elongation due to persistent pushing of polar spindle fibers.

Telophase

• Disappearance of the spindle apparatus.

• Formation of nuclear envelopes around chromosomes.

• Chromosomes begin to decondense into diffuse chromatin.

• Nucleolus reappears.

• If Cytokinesis occurs, cleavage furrow or cell plate begins to form.

Cytokinesis

• Division of cytoplasm and organelles into new cells.

• In Animals:

  • Actin filaments form a contractile ring around the cleavage furrow.

• In Plants:

  • Vesicles from the Golgi apparatus create new membrane and cell wall at the cell plate.