Meiosis Overview

Meiosis is a type of cell division that produces gametes (sex cells).

• Gametes: sperm cells in males and egg cells in females
• Called reduction division because it reduces the chromosome number.

Stages of Meiosis

• Meiosis is divided into two stages: Meiosis I and Meiosis II.
• Each stage consists of four phases:
  • Meiosis I:
    1. Prophase I
    2. Metaphase I
    3. Anaphase I
    4. Telophase I
  • Meiosis II:
    1. Prophase II
    2. Metaphase II
    3. Anaphase II
    4. Telophase II

Meiosis I

Prophase I

• Starts with a diploid cell.
• Chromatin contains two sets of chromosomes (one from each parent).
• DNA replication occurs; chromosomes condense into X-shaped forms.
• Synapsis happens: pairing of homologous chromosomes forms a tetrad (four sister chromatids).
• Crossing Over: chromatids exchange segments of alleles, leading to genetic diversity in offspring.
• Nuclear membrane disappears, centrioles move to opposite ends, and spindle fibers fan out.

Metaphase I

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

Anaphase I

• Spindle fibers separate homologous chromosomes in each tetrad.
• Chromosomes are pulled to opposite poles.

Telophase I

• Each pole has one chromosome from each homologous pair (still consists of sister chromatids).
• Nuclear membrane reforms and spindle fibers disappear.
• Cytokinesis occurs.
• Results in two genetically different haploid daughter cells.

Meiosis II

• No DNA replication before starting Meiosis II.

Prophase II

• Nuclear membrane disappears; spindle fibers fan out from centrioles.

Metaphase II

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

Anaphase II

• Sister chromatids separate and move to opposite poles (now individual chromosomes).

Telophase II

• Spindle fibers disappear, nuclear membranes reform, and cytokinesis occurs.
• Results in four genetically different haploid daughter cells.

Key Points to Remember

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

Key Points about Prophase I

• Synapsis: pairing of homologous chromosomes.
• Tetrads consist of four chromatids.
• Crossing Over: leads to genetic differences in gametes.
• All gametes produced are haploid.