Meiosis Lecture Notes

Introduction

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• Topic: Meiosis, not to be confused with Mitosis
• Mitosis creates identical body cells (e.g., skin, stomach cells)
  • Important for growth and repair
• Meiosis contributes to genetic diversity
  • Produces sperm and egg cells (gametes)

Chromosome Numbers

• Humans have 46 chromosomes in most body cells
• Sperm and egg cells have 23 chromosomes each
  • Upon fertilization, they combine to form 46 chromosomes
• Meiosis is called reduction division
  • Start cell: 46 chromosomes
  • End cells (sperm and egg): 23 chromosomes

Interphase

• Occurs before meiosis (and mitosis)
• Cell grows, replicates DNA, and performs cell processes
• 46 chromosomes duplicate but remain counted as 46 because sister chromatids are attached
  • Results in 92 chromatids

Meiosis Stages

• Similar to Mitosis stages PMAT (Prophase, Metaphase, Anaphase, Telophase), but occurs twice

Meiosis I

1. Prophase I

• Chromosomes condense and pair with homologous chromosomes
• Crossing over occurs (exchange of genetic information)

2. Metaphase I

• Chromosomes align in the center in homologous pairs

3. Anaphase I

• Chromosomes are pulled away by spindle fibers

4. Telophase I

• Formation of two new nuclei
• Cytokinesis follows, splitting the cytoplasm

Meiosis II

1. Prophase II

• Chromosomes and spindle fibers form (no crossing over)

2. Metaphase II

• Chromosomes align in a single file line in the center

3. Anaphase II

• Chromatids are pulled away by spindle fibers

4. Telophase II

• Formation of new nuclei in 4 new cells
• Cytokinesis follows, splitting the cytoplasm

Key Points

• Meiosis results in sperm cells in males and egg cells in females
• Independent assortment and crossing over contribute to genetic variety
  • Four sperm cells in males are different from the original cell and each other
  • Leads to genetic diversity among siblings

Errors in Meiosis

• Non-disjunction: improper separation of chromosomes
  • Can result in genetic disorders

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