Mitosis and Cell Division

• Mitosis is a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth.

Phases of Mitosis

• Prophase: Chromosomes condense and become visible, the nuclear envelope breaks down, and the spindle apparatus forms.
• Metaphase: Chromosomes line up along the metaphase plate (the equator of the cell), attached to spindle fibers.
• Anaphase: Sister chromatids separate and move to opposite poles of the cell.
• Telophase: Chromosomes arrive at the poles, the nuclear envelope reforms, and chromosomes decondense.
• Cytokinesis: The division of the cytoplasm to form two separate cells.

Significance of Mitosis

• Growth: Increases the number of cells in an organism, allowing it to grow.
• Repair: Replaces damaged or dead cells.
• Asexual Reproduction: In some organisms, mitosis is the basis of asexual reproduction.

Meiosis

• Meiosis is a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes and plant spores.

Phases of Meiosis

• Meiosis I:
  • Prophase I: Chromosomes condense, homologous chromosomes pair up (synapsis) to form tetrads, crossing over occurs.
  • Metaphase I: Tetrads line up along the metaphase plate.
  • Anaphase I: Homologous chromosomes separate and move to opposite poles. Sister chromatids remain attached.
  • Telophase I: Chromosomes arrive at the poles, the cell divides (cytokinesis), resulting in two haploid cells.
• Meiosis II:
  • Prophase II: Chromosomes condense.
  • Metaphase II: Chromosomes line up along the metaphase plate.
  • Anaphase II: Sister chromatids separate and move to opposite poles.
  • Telophase II: Chromosomes arrive at the poles, the cell divides (cytokinesis), resulting in four haploid cells.

Significance of Meiosis

• Genetic Diversity: Crossing over and independent assortment during meiosis increase genetic diversity.
• Sexual Reproduction: Meiosis produces haploid gametes, which fuse during fertilization to restore the diploid chromosome number.

Meiosis Disorders

• Nondisjunction: The failure of chromosomes or sister chromatids to separate properly during cell division.
• Can occur during Meiosis I or Meiosis II.
• Results in gametes with an abnormal number of chromosomes (aneuploidy).

Common Meiosis Disorders

• Down Syndrome (Trisomy 21): An extra copy of chromosome 21.
• Turner Syndrome (Monosomy X): Females have only one X chromosome.
• Klinefelter Syndrome (XXY): Males have an extra X chromosome.

Chromosomal Mutation

• Changes in the structure or number of chromosomes.

Types of Chromosomal Mutations

• Deletion: Loss of a portion of a chromosome.
• Duplication: Repetition of a portion of a chromosome.
• Inversion: A segment of a chromosome is reversed end to end.
• Translocation: A segment of one chromosome moves to a non-homologous chromosome.
• Nondisjunction: Results in aneuploidy (abnormal number of chromosomes).
  • Monosomy: Loss of a chromosome (2n-1).
  • Trisomy: Gain of a chromosome (2n+1).
  • Polyploidy: Having more than two complete sets of chromosomes (e.g., 3n, 4n).

Causes of Chromosomal Mutations

• Errors during meiosis (nondisjunction, translocation).
• Exposure to mutagens (radiation, chemicals).

Transport Mechanism

• Processes that move substances across cell membranes.

Types of Transport

• Passive Transport: Does not require energy.
  • Diffusion: Movement of molecules from an area of higher concentration to an area of lower concentration.
  • Facilitated Diffusion: Movement of molecules across a membrane with the help of transport proteins.
    • Channel proteins: Form pores or channels in the membrane.
    • Carrier proteins: Bind to molecules and change shape to shuttle them across the membrane.
  • Osmosis: Movement of water across a semipermeable membrane from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration).
• Active Transport: Requires energy (ATP).
  • Primary Active Transport: Uses ATP directly to move molecules against their concentration gradient.
    • Example: Sodium-potassium pump.
  • Secondary Active Transport: Uses the energy stored in an electrochemical gradient (established by primary active transport) to move other molecules against their concentration gradient.
    • Symport: Two molecules move in the same direction.
    • Antiport: Two molecules move in opposite directions.
• Bulk Transport: For large molecules or large quantities of substances.
  • Endocytosis: Movement of substances into the cell.
    • Phagocytosis: "Cell eating" - engulfing large particles or cells.
    • Pinocytosis: "Cell drinking" - engulfing extracellular fluid.
    • Receptor-mediated endocytosis: Specific molecules bind to receptors on the cell surface, triggering endocytosis.
  • Exocytosis: Movement of substances out of the cell.
    • Vesicles fuse with the plasma membrane, releasing their contents outside the cell.

Tonicity

• The ability of a surrounding solution to cause a cell to gain or lose water.
  • Isotonic: Solute concentration is the same inside and outside the cell; no net water movement.
  • Hypertonic: Solute concentration is higher outside the cell; water moves out, cell shrinks.
  • Hypotonic: Solute concentration is lower outside the cell; water moves in, cell swells and may burst.