Lecture Notes: Understanding Meiosis

Introduction

• Meiosis explains why siblings with the same parents can look different.
• Meiosis vs Mitosis:
  • Mitosis: Produces identical body cells (e.g., skin, stomach).
  • Meiosis: Contributes to genetic variety; produces gametes (sperm and egg cells).

Chromosome Numbers

• Humans typically have 46 chromosomes in most body cells.
• Sperm and egg cells have 23 chromosomes each.
• Purpose: When combined, they form 46 chromosomes in a fertilized egg.
• Meiosis is termed as "reduction division" due to the reduction in chromosome number.

Pre-Meiosis: Interphase

• Occurs before both mitosis and meiosis.
• Functions:
  • Cell growth
  • DNA replication
  • Cell processes
• Chromosome duplication: 46 chromosomes duplicate into 92 chromatids.

Meiosis Process

• Consists of two divisions (PMAT twice):
  • Meiosis I
  • Meiosis II

Meiosis I

1. Prophase I
   • Chromosomes condense and line up with homologous pairs.
   • Crossing Over: Exchange of genetic information between homologous chromosomes, leading to recombinant chromosomes.
2. Metaphase I
   • Chromosomes align in the middle as pairs, not in a single line.
3. Anaphase I
   • Chromosomes pulled away by spindle fibers.
4. Telophase I & Cytokinesis
   • Two new nuclei form, leading to two new cells.

Meiosis II

1. Prophase II
   • Chromosomes and spindles form; no homologous pairs or crossing over.
2. Metaphase II
   • Chromosomes align in the middle in a single file line.
3. Anaphase II
   • Chromatids are pulled away to opposite sides.
4. Telophase II & Cytokinesis
   • Nuclei reform, resulting in 4 cells.

Outcomes of Meiosis

• Produces sperm in males and eggs in females.
• Results in genetic variety due to independent assortment and crossing over.
• Each of the 4 sperm cells produced is different from each other and from the original cell.

Genetic Disorders

• Scientists study meiosis due to nondisjunction, where chromosomes don't separate correctly.
• Can lead to genetic disorders.

Conclusion

• Meiosis is a key process leading to genetic diversity.
• Reminder to "stay curious" about scientific phenomena.