Chapter 21: Cell Cycle

Definition: The sequence of changes in cell involving period of growth, replication of DNA, followed by cell division is called cell cycle.

Phases

• Interphase: Period between two cell divisions (non-apparent division). Divided into:
  • G1 Phase (Gap 1): Cell grows, protein synthesis, DNA base units accumulated. Post-mitotic cells can exit cell cycle during G1 entering G0 phase (resting phase).
  • S Phase (Synthesis): DNA is synthesized and chromosome number get doubled.
  • G2 Phase (Gap 2): Energy stored for chromosome movement, mitosis specific proteins (tubulin) are synthesized.
• M Phase (Mitosis): Period of division.

Definition: Process of nuclear and cell division in which number of chromosomes in daughter cells are same as that of parent cell.

Karyokinesis (Nuclear Division)

• Prophase: Chromatin condenses into chromosomes. Nuclear envelope disappears. Centrioles migrate to poles. Mitotic apparatus forms.
• Metaphase: Chromosomes align at the equator forming metaphase plate. Kinetochore fibers attach to kinetochores.
• Anaphase: Kinetochore fibers contract, separating sister chromatids to opposite poles.
• Telophase: Chromosomes unfold back to chromatin. Nuclear membrane reforms.

Cytokinesis (Cell Division)

• Animal Cells: Contractile ring (actin/myosin) forms cleavage furrow which deepens to divide cell.
• Plant Cells: No centrioles. Phragmoplast (from Golgi vesicles) forms cell plate which becomes cell wall.

Importance

• Equal distribution of hereditary material.
• No genetic variation (cloning).
• Regeneration, healing, and growth.

Definition: Unregulated division of cells forming tumors.

Types of Tumors

• Benign: Small, localized, non-destructive.
• Malignant: Divide rapidly, spread to other tissues (Metastasis). Cause cancer.

Causes

• Mutations in somatic cells.
• Mutations in genes regulating cell division (accumulation of 3-20 mutations).

Characteristics of Cancer Cells

• High nucleus to cytoplasm ratio.
• Prominent nucleoli.
• Rapid mitosis.
• Break contact with other cells (Metastasis).

Definition: Special type of division where chromosome number is reduced to half.

Phases

• Meiosis I (Reduction Division):
  • Prophase I: Longest phase.
    • Leptotene: Chromosomes condense.
    • Zygotene: Homologous pairing (Synapsis) forms Bivalents/Tetrads.
    • Pachytene: Crossing over (exchange of segments between non-sister chromatids).
    • Diplotene: Paired chromosomes repel but remain attached at Chiasmata.
    • Diakinesis: Maximum condensation. Nuclear membrane disappears.
  • Metaphase I, Anaphase I, Telophase I: Homologous pairs separate.
• Meiosis II: Like mitosis. Sister chromatids separate.

Importance

• Produces genetic variation (crossing over, random assortment).
• Maintains constant chromosome number across generations.

Definition: Failure of chromosomes to segregate during anaphase.

Types of Syndromes

• Down's Syndrome (Mongolism): Trisomy 21 (2n+1 = 47). Flat face, mental retardation. Risk increases with maternal age (1/100 at age 40).
• Klinefelter's Syndrome: XXY (47). Phenotypically male but enlarged breasts, small testes, sterile.
• Turner's Syndrome: XO (45). Phenotypically female, short stature, webbed neck, no ovaries.

Apoptosis (Programmed Cell Death)

• Internal program of events where cell commits suicide.
• Cell shrinks, condenses, splits into apoptotic bodies which are phagocytosed.
• Importance: Removes unwanted structures (e.g. tail in embryo, digits formation), controls neuron number.

Necrosis (Accidental Cell Death)

• Due to tissue damage/injury.
• Cell swells and bursts releasing intracellular contents.
• Causes inflammation.