Lecture Notes: Phases of Meiosis

Introduction to Meiosis

• Definition: Meiosis, also known as reduction division, is the type of cell division that produces gametes (sperm in males, eggs in females).
• Stages: Meiosis is divided into two stages:
  • Meiosis I
  • Meiosis II

Meiosis I

• Comprised of four phases: Prophase I, Metaphase I, Anaphase I, Telophase I

Prophase I

• Begins with a diploid cell.
• Chromatin State: Two uncoiled chromosome sets (one from each parent) condense into X-shaped chromosomes after DNA replication.
• Synapsis: Each chromosome pairs with its homologous chromosome, forming a tetrad (4 chromatids).
• Genetic Information: Chromosomes contain genes; different versions are alleles.
• Crossing Over:
  • Chromatids exchange alleles (recombination).
  • Results in genetic variety, explaining differences in gametes and between children and parents/siblings.
• Cell Changes:
  • Nuclear membrane disappears.
  • Centrioles move to opposite ends, spindle fibers form.

Metaphase I

• Homologous chromosomes line up at the cell equator, attaching to spindle fibers from opposite poles.

Anaphase I

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

Telophase I

• One chromosome from each homologous pair is located at opposite poles.
• Chromosomes still have sister chromatids (no longer identical due to crossing over).
• Nuclear membrane reforms, spindle fibers disappear.
• Cytokinesis: Results in two genetically different haploid daughter cells.

Meiosis II

• Similar phases as Meiosis I: Prophase II, Metaphase II, Anaphase II, Telophase II
• Key Difference: No DNA replication before it begins.

Prophase II

• Nuclear membrane disappears, spindle fibers form.

Metaphase II

• Chromosomes line up at the equator in each cell, attaching to spindle fibers from both poles.

Anaphase II

• Sister chromatids separate and move to opposite poles, becoming individual chromosomes.

Telophase II

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

Key Points to Remember

• Meiosis starts with a diploid cell and produces haploid gametes.
• Meiosis consists of two cell division stages.
  • Meiosis I: Homologous chromosomes separate, producing haploid cells with sister chromatids.
  • Meiosis II: Sister chromatids separate, forming individual chromosomes.
• Key processes:
  • Synapsis: Homologous chromosome pairing (tetrad formation).
  • Crossing Over: Exchange of alleles leads to genetic diversity.
• All gametes produced are haploid.