Lecture Notes on Meiosis

Introduction

• Lecture by Amoeba Sisters
• Topic: Meiosis and genetic variety
• Meiosis vs. Mitosis:
  • Mitosis: produces identical body cells (e.g., skin cells), important for growth, repair, and cell replacement.
  • Meiosis: contributes to genetic variety, produces gametes (sperm and egg cells).

Chromosome Numbers

• Humans have 46 chromosomes in most body cells.
• Gametes (sperm and egg cells) have 23 chromosomes.
• Fertilization combines 23 chromosomes from sperm and egg to form 46 chromosomes in a zygote.

Meiosis Overview

• Known as a reduction division:
  • Starts with a cell with 46 chromosomes.
  • Ends with gametes having 23 chromosomes.

Pre-Meiosis: Interphase

• Occurs before meiosis (and mitosis).
• Key activities:
  • Cell growth
  • DNA replication
  • Carrying out cell functions
• Duplicates chromosomes (46 chromosomes become 92 chromatids, but still counted as 46 chromosomes due to centromeres).

Stages of Meiosis

• Meiosis involves two rounds of division (meiosis I and meiosis II) with stages named PMAT (prophase, metaphase, anaphase, telophase).

Meiosis I

1. Prophase I
   • Chromosomes condense and pair with homologous chromosomes.
   • Crossing over occurs: Exchange of genetic material between homologous chromosomes, leading to recombinant chromosomes.
2. Metaphase I
   • Homologous chromosomes line up in the middle of the cell in pairs.
3. Anaphase I
   • Spindle fibers pull homologous chromosomes away from each other.
4. Telophase I
   • Two nuclei form, resulting in two cells.
   • Cytokinesis follows, dividing the cytoplasm.

Meiosis II

1. Prophase II
   • Chromosomes and spindles form.
   • No crossing over occurs.
2. Metaphase II
   • Chromosomes line up in a single file in the middle.
3. Anaphase II
   • Spindle fibers pull chromatids apart to opposite sides.
4. Telophase II
   • Four nuclei form, resulting in four cells.
   • Followed by cytokinesis to split cytoplasm.

Outcomes of Meiosis

• Produces four genetically unique gametes.
• In males: four different sperm cells.
• In females: four different egg cells.
• Genetic variety due to independent assortment and crossing over.

Nondisjunction

• Occurs when chromosomes do not separate correctly.
• Can lead to genetic disorders, an area of ongoing research.

Conclusion

• Meiosis is crucial for genetic diversity.
• Important for understanding genetic variability in siblings.

• Reminder from Amoeba Sisters: Stay curious!