Lecture Notes: Meiosis, Genetic Variety, and Chromosomal Division

Introduction

• Topic: Meiosis and how it contributes to genetic variety.
• Key Difference: Not to be confused with mitosis.
  • Mitosis: Produces identical body cells (e.g., skin, stomach) for growth, repair, and replacement.
  • Meiosis: Produces gametes (sperm and egg cells) contributing to genetic diversity.

Chromosome Overview

• Humans have 46 chromosomes in most body cells.
• Gametes (Sperm and Egg Cells): Each has 23 chromosomes.
  • Fertilization results in 46 chromosomes.

Meiosis: A Reduction Division

• Reduction Division: Starting with 46 chromosomes, ending with 23 in each gamete.
• Interphase: Preceding phase where:
  • Cell grows.
  • DNA replication occurs, doubling the chromatids (46 chromosomes to 92 chromatids).

Stages of Meiosis

• Two Divisions: PMAT occurs twice (Meiosis I and II).

Meiosis I

• Prophase I:
  • Chromosomes condense and pair with homologous chromosomes.
  • Crossing Over: Exchange of genetic material between homologous chromosomes creating recombinant chromosomes.
• Metaphase I:
  • Chromosomes align in the middle in pairs (not single file).
• Anaphase I:
  • Homologous chromosomes are pulled away by spindle fibers.
• Telophase I and Cytokinesis:
  • Two new cells formed, each with a nucleus.

Meiosis II

• Prophase II:
  • Chromosomes and spindles form, no homologous pairs or crossing over.
• Metaphase II:
  • Chromosomes line up in the middle in single file.
• Anaphase II:
  • Chromatids (not chromosomes) are pulled apart.
• Telophase II and Cytokinesis:
  • Four new cells formed, each different due to genetic recombination.

Genetic Variety

• Independent Assortment and Crossing Over contribute to variation.
• Outcome: Each sperm or egg cell is unique, leading to genetic diversity among siblings.

Nondisjunction

• Definition: Incorrect separation of chromosomes during meiosis leading to genetic disorders.
• Research Area: Scientists study meiosis to understand and address genetic disorders.

• Conclusion: Meiosis is key to understanding genetic diversity and its implications for human biology.