Lecture on Mitosis and Meiosis

Introduction to DNA

• DNA Structure: Double helix, composed of nucleotides.
• Histones: Proteins that DNA wraps around to conserve space and regulate gene access.
  • Tightly wrapped DNA around histones prevents gene access.
  • Example: In females, one X chromosome is wrapped to avoid gene dosing issues.

Cell Cycle

• Interphase: Majority of the cell's life; includes normal activities and preparation for division.
  • G1 Phase: Cell growth.
  • S Phase: DNA replication.
  • G2 Phase: Final preparations for mitosis.

Mitosis

• Purpose: Division of the nucleus, followed by cytokinesis (actual cell division).
• Stages of Mitosis: PMAT mnemonic.
  • Prophase: DNA condenses, nucleus dissolves.
  • Metaphase: Chromosomes line up in the middle.
  • Anaphase: Chromatids are pulled apart.
  • Telophase: Nuclei reform.
• Cytokinesis: Division of the cell into two.

Different Cell Types

• Somatic Cells: Most body cells, some divide (e.g., skin cells).
• Sex Cells: Produced via meiosis, contain half the number of chromosomes.

Meiosis

• Two Divisions: Meiosis I and Meiosis II.
  • Meiosis I: Homologous chromosomes separate.
  • Meiosis II: Sister chromatids separate, similar to mitosis.
• Outcome: Four haploid cells (23 chromosomes each).

Chromosomes

• Humans: 23 pairs of homologous chromosomes.
• Sex Chromosomes: Determine biological sex (XX or XY).
• Gene Dosing Issues: Too many copies can be problematic (e.g., Trisomy 21).

Meiosis Process

• Crossover: Exchange of genetic material between homologous chromosomes.
• Importance: Ensures genetic diversity, crucial for reducing chromosome number in sex cells.

Specifics on Sperm and Egg Formation

• Sperm Cells: All four cells from meiosis develop into sperm.
• Egg Cells: Only one of the four develops into an egg; others become polar bodies.

Conclusion

• Mitosis and meiosis are critical for cellular and genetic continuity.
• Errors in these processes can lead to conditions like Down syndrome.