Notes on Meiosis Lecture

Introduction to Meiosis

• Definition: Meiosis is a process that contributes to genetic variety.
• Distinction from Mitosis:
  • Mitosis: Creates identical body cells (e.g., skin, stomach cells) for growth and repair.
  • Meiosis: Creates gametes (sperm and egg cells) with genetic variation.

Chromosome Count

• Human Chromosomes:
  • Humans have 46 chromosomes in most body cells.
  • Sperm and egg cells have 23 chromosomes each.
  • Fertilization: 23 (sperm) + 23 (egg) = 46 (fertilized egg).
• Reduction Division: Meiosis reduces the chromosome number from 46 to 23.

Interphase

• Definition: Preparation phase before meiosis (also occurs before mitosis).
  • Cell grows, replicates DNA, and carries out processes.
• Chromosome Duplication:
  • Starting cell: 46 chromosomes → duplicates to 92 chromatids while remaining counted as 46 chromosomes (due to sister chromatids being attached).

Stages of Meiosis

General Structure

• Meiosis involves two rounds of division: Meiosis I and Meiosis II.
• Use of PMAT (Prophase, Metaphase, Anaphase, Telophase) in both divisions.

Meiosis I

1. Prophase I:

   • Chromosomes condense and thicken.
   • Homologous chromosomes pair up.
   • Crossing Over: Exchange of genetic material between chromosomes, leading to recombinant chromosomes.

2. Metaphase I:

   • Homologous pairs line up in the middle of the cell (not single file).

3. Anaphase I:

   • Homologous chromosomes are pulled apart by spindle fibers (A for away).

4. Telophase I:

   • Two newly formed nuclei appear; cytokinesis follows, resulting in two cells.

Meiosis II

1. Prophase II:

   • Chromosomes and spindles form, but no crossing over occurs.

2. Metaphase II:

   • Chromosomes align in a single file in the middle of the cell.

3. Anaphase II:

   • Sister chromatids are pulled apart by spindle fibers (A for away).

4. Telophase II:

   • Nuclei reform, resulting in four cells after cytokinesis.

Outcomes of Meiosis

• Male Meiosis: Produces four unique sperm cells.
• Female Meiosis: Produces one egg cell and three polar bodies (non-functional).
• Genetic Variation: Due to independent assortment and crossing over, each gamete is unique.

Implications of Meiosis

• Reason for Sibling Variation: Unique combinations of genes from each parent.
• Nondisjunction: Incorrect separation of chromosomes can lead to genetic disorders; a topic of ongoing research.

Conclusion

• Stay curious and explore more about meiosis and genetics!