Lecture on Meiosis

Introduction

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• Topic: Why siblings with the same parents can look different.
• Main concept: Meiosis, not to be confused with mitosis.

Mitosis vs. Meiosis

• Mitosis:

  • Creates identical body cells (e.g., skin, stomach cells).
  • Important for growth, repair, and replacing worn out cells.

• Meiosis:

  • Contributes to genetic variety.
  • Produces gametes (sperm and egg cells).
  • Human body cells have 46 chromosomes; sperm and egg cells have 23.
  • Reduction division: starting cell (46 chromosomes) -> ending cells (23 chromosomes each).

Pre-Meiosis: Interphase

• Occurs before meiosis (and mitosis).
• Cell growth, DNA replication, carries out cell processes.
• Chromosomes duplicate (46 chromosomes -> 92 chromatids).
• Chromosome count based on centromeres remains 46.

Meiosis Process

• Divides twice (PMAT phases performed twice):
  • PMAT: Prophase, Metaphase, Anaphase, Telophase.

Meiosis I

• Prophase I:

  • Chromosomes condense and line up in homologous pairs.
  • Crossing over occurs, exchanging genetic information.
  • Result: recombinant chromosomes, contributing to genetic variety.

• Metaphase I:

  • Chromosomes align in the middle, in homologous pairs.

• Anaphase I:

  • Homologous chromosomes pulled away by spindle fibers.

• Telophase I:

  • Two new nuclei form, cytokinesis follows.
  • Results in two new cells.

Meiosis II

• Prophase II:

  • Chromosomes, not in homologous pairs.
  • No crossing over.

• Metaphase II:

  • Chromosomes align in a single file line in the middle.

• Anaphase II:

  • Chromatids pulled away by spindle fibers to opposite sides.

• Telophase II:

  • Nuclei reform, results in four new cells after cytokinesis.

Outcomes of Meiosis

• Produces gametes:
  • Males: 4 diverse sperm cells.
  • Females: 4 diverse egg cells.
• Each gamete is unique due to independent assortment and crossing over.
• Explains genetic diversity among siblings.

Scientific Study

• Nondisjunction:
  • Failure of chromosomes to separate correctly.
  • Can lead to genetic disorders.

Conclusion

• Encouragement to "stay curious."