Chapter 5: Cell Division

Importance of Cell Division

• Essential for growth and reproduction in all life forms.
• All cells originate from pre-existing cells.
• Allows a single-celled fertilized egg to grow into a complex organism with trillions of cells.
• Includes somatic cells (e.g., skin, eyes, lungs) involved in mitosis.

Cell Types and Replication

• Somatic Cells: Body cells that regularly divide, like skin and blood cells.
• Nervous System Cells: Limited division; mature cells don’t divide again, affecting injury recovery.
• Apoptosis: Programmed cell death, essential for removing damaged cells and during development (e.g., tadpole tail loss, human digit formation).

The Cell Cycle

• Stages:
  • Interphase: Includes G1, S, and G2 phases.
    • G1 Phase: Cell growth and organelle duplication.
    • S Phase: DNA replication, forming sister chromatids.
    • G2 Phase: Preparation for mitosis.
  • Mitotic Phase (Mitosis): Active division of the nucleus.
• Cytokinesis: Final division of the cytoplasm, resulting in two identical daughter cells.

Mitosis

• Purpose: Produce two identical 2N cells with the same number of chromosomes.
• Chromosome Structure:
  • Chromatin: DNA-protein complex forming chromosomes.
  • Histones: Proteins aiding DNA structure.
• Phases of Mitosis:
  • Prophase: Chromosome condensation, nuclear membrane breakdown, spindle fiber formation.
  • Metaphase: Chromosomes align in the cell center.
  • Anaphase: Sister chromatids separate and move to opposite poles.
  • Telophase: Reformation of nuclear envelope, followed by cytokinesis.

Differences in Plant and Animal Cells

• Animal Cells: Cleavage furrow forms during cytokinesis.
• Plant Cells: Formation of a cell plate leads to new cell wall development.

Meiosis

• Purpose: Produce haploid cells for sexual reproduction, ensuring genetic diversity.
• Phases:
  • Meiosis I: Reduction division, separating homologous chromosomes.
    • Prophase I: Synapsis and crossing over.
    • Metaphase I: Independent assortment of chromosomes.
  • Meiosis II: Separation of sister chromatids, forming four genetically distinct haploid cells.
• Genetic Variation: Achieved through crossing over and independent assortment.

Comparison: Mitosis vs. Meiosis

• Mitosis: Produces two identical diploid daughter cells; occurs throughout life for growth and repair.
• Meiosis: Produces four genetically varied haploid cells; occurs at maturity for reproduction.
• Replication: DNA replicates once in both processes, but meiosis includes two divisions.
• Chromosome Alignment: Homologous chromosomes align during meiosis I, while sister chromatids align in mitosis.
• Applications:
  • Mitosis: Continuous from conception to death.
  • Meiosis: Specific to reproductive maturity.

Key Definitions

• Diploid (2N): Full set of chromosomes.
• Haploid (N): Half set of chromosomes, found in gametes.
• Sister Chromatids: Identical copies of a chromosome.
• Homologous Chromosomes: Set of one maternal and one paternal chromosome pairing up during meiosis.

These notes summarize the key concepts of cell division, highlighting the processes and their biological significance.