Meiosis

What is meiosis?

  • Meiosis occurs during reproduction to create cells with half the chromosome number to create a new phase of the life cycle

  • Meiosis is associated with sexual reproduction

  • It is similar to mitosis, but the process has two successive divisions

    1. Meiosis I: reduction division (reduces the number of chromosomes)

    2. Meiosis II: equatorial division (similar to mitosis)

Why is meiosis important?

  • It is part of the sexual life-cycle of an organism

  • A single, diploid (2N) cell creates four, haploid (1N) cells (e.g. spores or pollen in plants)

Where does meiosis occur?

  • In plants, meiosis occurs in specialized reproductive structures called sporangia, which create spores

    • In seed plants, the male sporangia (microsporangia) are called pollen sacs. The pollen sacs create pollen, also called microspores, which each contain a male gametophyte (known as the pollen tube)

    • The female sporangia are more complicated. Inside each ovule, there is a female sporangium (megasporangium) called the nucellus. The nucellus creates megaspores, which each contain a female gametophyte

  • In animals, meiosis occurs in the gonads to create sperm or eggs.

  • N.B. Please note that meiosis creates spores in plants (sporic meiosis), but meiosis creates gametes in animals (gametic meiosis). In plants, gametes are created by mitosis.

Stages of Meiosis

  • Prophase I

  • Metaphase I

  • Anaphase I

  • Telophase I

  • Prophase II

  • Metaphase II

  • Anaphase II

  • Telophase II

Prophase I

  • Chromosomes continue to condense

  • Homologous chromosomes align

  • Nuclear envelope and nucleolus disassociate

  • Crossing over occurs

Prophase II:

  • Chromosomes become shorter and thicker

Metaphase I

  • Spindle visible

  • Chromosomes aligned in pairs at equator

Metaphase II

  • Spindle visible

  • Chromosomes aligned in pairs at equator

Anaphase I

  • Whole chromosomes migrate to opposite ends of cell

Anaphase II

  • Chromatids migrate to opposite ends of cell

Telophase I

  • Cell plate begins to form

  • Nucleolus forms

Telophase II

  • Cell plate begins to form

  • Nucleolus forms

  • New nuclear envelope forms

Additional Resources

  • Video of Meiosis

  • Pollen and Plant Stress: podcast discussion of how env't factors like UV radiation can disrupt meiosis and make gymnosperms sterile, and indicators of this same mechanism in the fossil record during extinction events (In Defense of Plants 13Feb2022)