Meiosis: Reduction Division of Cells

Overview

• Meiosis is a type of cell division producing gametes (sex cells).
  • Gametes: Sperm cells (males) and egg cells (females).
• Also known as reduction division.
• Involves two stages: Meiosis I and Meiosis II.

Meiosis I

• Four Phases: Prophase I, Metaphase I, Anaphase I, and Telophase I.

Prophase I

• Starts with a diploid cell (two sets of chromosomes).
• Chromatin condenses into X-shaped chromosomes after DNA replication.
• Synapsis: Chromosomes pair with homologous chromosomes forming a tetrad (four chromatids).
• Crossing Over (Recombination):
  • Exchange of alleles between chromatids from homologous chromosomes.
  • Results in genetic variety among gametes.
• Nuclear membrane disappears.
• Centrioles move to opposite ends; spindle fibers form.

Metaphase I

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

Anaphase I

• Spindle fibers separate homologous chromosomes and pull them to opposite poles.

Telophase I

• Chromosomes at opposite poles; consist of sister chromatids.
• Spindle fibers disappear; nuclear membrane reforms.
• Cytokinesis: Results in two genetically different haploid daughter cells.

Meiosis II

• Four Phases: Prophase II, Metaphase II, Anaphase II, and Telophase II.
• No DNA replication before Meiosis II.

Prophase II

• Nuclear membrane disappears again.
• Spindle fibers form from paired centrioles.

Metaphase II

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

Anaphase II

• Sister chromatids separate and move to opposite poles.
• Once separated, they're called chromosomes.

Telophase II

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

Key Points

• Meiosis begins with a diploid cell.
• Produces genetically different haploid gametes.
• Two stages: Meiosis I (homologous chromosomes separate) and Meiosis II (sister chromatids separate).
• Cytokinesis in Meiosis II produces four genetically different haploid gametes.