Cell Division

20.1 The Cell Cycle

• The cell cycle is the series of events that take place in a cell leading to its division and replication.
  • Interphase: The cell grows, replicates DNA, and prepares for division.
    • G1 Phase: Cell growth and normal function.
    • S Phase: DNA replication.
    • G2 Phase: Preparation for mitosis.
  • Mitosis: The process of nuclear division resulting in two identical daughter cells.
  • Cytokinesis: The division of the cytoplasm to form two separate daughter cells.

20.2 Mitosis

• Stages of Mitosis:

  1. Prophase: Chromosomes condense and become visible, the nuclear envelope breaks down, and spindle fibers form.
  2. Metaphase: Chromosomes align at the cell’s equator.
  3. Anaphase: Sister chromatids are pulled apart toward opposite poles of the cell.
  4. Telophase: Nuclear membranes reform around the separated chromatids.
  5. Cytokinesis: The cytoplasm divides, forming two daughter cells.

• Purpose of Mitosis: Growth, repair, and asexual reproduction.

20.3 Meiosis

• Meiosis: A type of cell division that reduces the chromosome number by half, resulting in four non-identical gametes (sperm and egg).
• Stages of Meiosis:
  1. Meiosis I: Homologous chromosomes are separated, reducing chromosome number by half.
  2. Meiosis II: Sister chromatids are separated, similar to mitosis.

20.4 Genetic Variation in Meiosis

• Crossing Over: Exchange of genetic material between homologous chromosomes during Prophase I of meiosis, creating genetic variation.
• Independent Assortment: The random distribution of chromosomes during metaphase I, which leads to genetic variation in gametes.