General Biology Reviewer

Cell Cycle and Cell Division

Overview of Cell Cycle

  • Cell Cycle: A sequence of events in a eukaryotic cell between one cell division and the next.

Types of Cell Division

  • Mitosis: Process of cell division that results in two identical daughter cells.

  • Meiosis: Type of cell division that reduces the chromosome number by half, producing four haploid cells.

Phases of the Cell Cycle

Interphase (Longest Stage)

  • Non-dividing phase where the cell grows and copies its chromosomes.

    • G1 Phase: Cell increases in size and prepares for DNA synthesis.

    • S Phase: Active DNA synthesis and chromosome replication.

    • G2 Phase: Cell prepares for mitosis; chromatin condenses into chromosomes.

M Phase (Mitotic Phase)

  • Dividing phase; division of the nucleus and cytoplasm to form daughter cells.

Control Points (Checkpoints)

  • G1 Checkpoint: Checks size, nutrient availability, DNA integrity before entering S phase.

  • G2 Checkpoint: Ensures DNA is accurately replicated before cell division.

  • M Checkpoint (Spindle Checkpoint): Assesses if all sister chromatids are attached to spindle fibers.

  • Consequences of Failing Checkpoints: Cells can enter G0 phase (resting), stopping division.

Mitosis

  • Also known as karyokinesis; consists of four phases.

    • Prophase: Chromatin condenses, nuclear envelope breaks down.

    • Metaphase: Chromosomes align at the cell equator.

    • Anaphase: Sister chromatids separate towards opposite poles.

    • Telophase: Nuclear membranes re-form around each set of chromosomes; begins cytokinesis.

Cytokinesis

  • Division of the cytoplasm to form two new cells.

Meiosis

  • Consists of two successive divisions (Meiosis I and Meiosis II), resulting in four haploid cells.

Meiosis I

  • Involves pairing and crossing over of homologous chromosomes.

    • Prophase I: Chromosomes condense, nuclear envelope breaks down.

    • Metaphase I: Tetrads align at metaphase plate.

    • Anaphase I: Homologous chromosomes move to opposite poles.

    • Telophase I: Nuclear membranes reform; cytokinesis occurs.

Meiosis II

  • Similar to mitosis; separates sister chromatids.

    • Prophase II: Chromosomes condense; spindle fibers form.

    • Metaphase II: Chromosomes align at the equator.

    • Anaphase II: Sister chromatids separate.

    • Telophase II: Nuclear membranes reappear; cytokinesis forms four haploid cells.

Biological Significance of Mitosis and Meiosis

  • Mitosis: Responsible for growth and tissue repair.

  • Meiosis: Produces gametes for sexual reproduction, contributes to genetic diversity through crossing over.

Disorders and Diseases from Cell Cycle Malfunction

  • Cancer: Uncontrolled cell division and mutations.

  • Aneuploidy: Incorrect number of chromosomes.

  • Genetic disorders related to structural chromosome abnormalities (deletions, duplications, translocations, inversions).