Meiosis Lecture Notes

Introduction

• Meiosis: process leading to genetic variety among siblings
• Differences from mitosis:
  • Mitosis produces identical body cells (e.g., skin, stomach cells)
  • Meiosis produces gametes (sperm and egg cells)

Chromosome Basics

• Humans typically have 46 chromosomes in body cells
• Sperm and egg cells have 23 chromosomes each
• Combination of sperm and egg (23 + 23) results in 46 chromosomes in a fertilized egg

Meiosis Overview

• Referred to as 'reduction division'
• Starts with cells having 46 chromosomes; results in gametes with 23 chromosomes
• Occurs after interphase (cell growth, DNA replication)

Interphase

• Precedes meiosis (and mitosis)
• Chromosomes duplicate during this phase
• Despite duplication, counted as 46 chromosomes because of centromere count; results in 92 chromatids

Meiosis Stages

• Involves two divisions, hence PMAT occurs twice

Meiosis I

1. Prophase I
   • Chromosomes condense and pair with homologous chromosomes
   • Crossing over occurs, exchanging genetic material for recombinant chromosomes
2. Metaphase I
   • Chromosomes align in the middle in homologous pairs
3. Anaphase I
   • Homologous chromosomes pulled apart
4. Telophase I and Cytokinesis
   • Formation of two new cells, each with a nucleus

Meiosis II

1. Prophase II
   • Spindles form; no crossing over
2. Metaphase II
   • Chromosomes align single-file in the middle
3. Anaphase II
   • Chromatids pulled apart
4. Telophase II and Cytokinesis
   • Formation of four genetically unique cells

Outcomes of Meiosis

• Male meiosis results in 4 distinct sperm cells
• Female meiosis results in distinct egg cells
• Contribution to genetic diversity due to independent assortment and crossing over

Nondisjunction

• Error in chromosome separation leading to genetic disorders
• Area of ongoing study in genetics

Conclusion

• Meiosis explains genetic variety among siblings with the same parents
• Encouragement to remain curious about biological processes