Meiosis Lecture Notes

Introduction to Meiosis

• Meiosis, also known as reduction division, is a type of cell division that produces gametes (sex cells).
  • Gametes: Sperm cells (males) and egg cells (females).
• Meiosis consists of two stages: Meiosis I and Meiosis II.

Meiosis I

• Prophase I

  • Starts with a diploid cell.
  • DNA Replication: Chromatin replicates and condense into x-shaped chromosomes with identical sister chromatids.
  • Synapsis: Chromosomes pair up with homologous chromosomes, forming a tetrad (four sister chromatids).
  • Genes and Alleles: Chromosomes contain genetic information (genes), with different alleles on each chromosome.
  • Crossing Over: Chromatids exchange segments of alleles, resulting in genetic recombination and variation.
  • Cell Changes: Nuclear membrane disappears, centrioles move to opposite ends, spindle fibers form.

• Metaphase I

  • Homologous chromosomes line up at the equator and attach to spindle fibers from opposite poles.

• Anaphase I

  • Spindle fibers separate homologous chromosomes in each tetrad, pulling them to opposite poles.

• Telophase I

  • Each pole has one chromosome from each homologous pair (consisting of sister chromatids).
  • Spindle fibers disappear, and nuclear membrane reforms.
  • Cytokinesis: Two genetically different haploid daughter cells are formed.

Meiosis II

• Prophase II

  • Nuclear membrane disappears and spindle fibers form from paired centrioles.

• Metaphase II

  • Chromosomes line up at the equator and attach to spindle fibers from both poles.

• Anaphase II

  • Sister chromatids separate and move to opposite poles, each becoming a chromosome.

• Telophase II

  • Spindle fibers disappear, nuclear membranes reform.
  • Cytokinesis: Ends with four genetically different haploid daughter cells.

Key Points

• Meiosis starts with a diploid cell and produces haploid gametes.
• Meiosis I separates homologous chromosomes, forming two haploid cells with paired sister chromatids.
• Meiosis II separates sister chromatids, resulting in four genetically different haploid gametes.
• Prophase I Highlights:
  • Synapsis and tetrad formation.
  • Crossing over leads to genetic diversity in gametes.