Phases of Meiosis

Introduction to Meiosis

• Meiosis: Also known as reduction division.
• Purpose: Produces gametes (sex cells - sperm in males, eggs in females).
• Two stages: Meiosis I and Meiosis II.

Meiosis I

Phases of Meiosis I

1. Prophase I

   • Starts with a diploid cell.
   • Chromatin contains two sets of chromosomes (one from each parent).
   • DNA replication occurs, forming X-shaped chromosomes with identical sister chromatids.
   • Synapsis: Homologous chromosomes pair up, forming a tetrad (4 sister chromatids).
   • Crossing over: Exchange of alleles between chromatids of homologous chromosomes, resulting in genetic recombination.
   • Disappearance of nuclear membrane, movement of centrioles to opposite poles, formation of spindle fibers.

2. Metaphase I

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

3. Anaphase I

   • Spindle fibers pull homologous chromosomes to opposite poles.

4. Telophase I

   • Chromosomes (each with sister chromatids) move to opposite poles.
   • Nuclear membrane reforms, spindle fibers disappear.
   • Followed by cytokinesis, resulting in two genetically different haploid daughter cells.

Meiosis II

• Unlike Meiosis I, DNA does not replicate before Meiosis II.

Phases of Meiosis II

1. Prophase II

   • Nuclear membrane disappears, spindle fibers fan out.

2. Metaphase II

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

3. Anaphase II

   • Sister chromatids separate, moving to opposite poles.
   • Once separated, they are called chromosomes.

4. Telophase II

   • Nuclear membrane reforms.
   • Cytokinesis occurs in both cells, resulting in four genetically different haploid daughter cells.

Key Points

• Meiosis starts with a diploid cell and produces haploid gametes.
• Gametes are genetically different.
• Meiosis I: Separation of homologous chromosomes.
• Meiosis II: Separation of sister chromatids.

Prophase I Highlights

• Synapsis: Pairing of homologous chromosomes.
• Tetrad: Four chromatids of homologous chromosomes.
• Crossing over: Exchange of alleles, resulting in genetic variation.
• All gametes produced are haploid.