Meiosis is a specialized form of cell division that occurs in sexually reproducing organisms. It involves two sequential divisions, resulting in the formation of four non-identical haploid cells. The main purpose of meiosis is the production of gametes (sperm and egg cells) with genetic diversity.

Phases of Meiosis:

Meiosis I:

  1. Prophase I:

  • Chromosome condensation: The chromatin, a combination of DNA and proteins, of the parent cell, is condensed during Prophase I. The condensed chromatins are referred to as chromosomes.
  • Formation of bivalents: In Prophase I, homologous chromosomes pair up, and their DNA exchanges fragments in the process of crossing over. This results in the formation of bivalents (or tetrads).
  • Five substages: Prophase I consists of five distinct stages, also known as leptotene, zygotene, pachytene, diplotene, and diakinesis.
  1. Leptotene:

    • Chromosomes begin to condense, becoming visible under a microscope.
    • Homologous chromosomes (chromosomes with the same genes but possibly different alleles) are present as unpaired individual chromatids.
  2. Zygotene:

    • Homologous chromosomes start pairing up through a process called synapsis.
    • The protein structure called the synaptonemal complex forms between homologous chromosomes, facilitating the physical connection.
  3. Pachytene:

    • Chromosomes continue to condense further.
    • Crossing over occurs, where homologous chromosomes exchange genetic material. This results in the formation of chiasmata, visible points of crossover.
  4. Diplotene:

    • Synaptonemal complex begins to disintegrate.
    • Homologous chromosomes remain attached at chiasmata, and the crossing-over points become visible.
  5. Diakinesis:

    • Chromosomes continue to condense and become maximally visible.
    • The nuclear envelope breaks down, and spindle fibers begin to form.
    • Chromosomes are fully paired and ready for separation.

Stages of Prophase | BioNinja

  1. Metaphase I:

    • Tetrads Align: Tetrads align at the metaphase plate.
    • Independent Assortment: Random alignment of homologous pairs contributes to genetic variability.
    • Spindle Fibers Attach: Spindle fibers attach to each chromosome within the tetrad.
  2. Anaphase I:

    • Homologous Chromosomes Separate: Homologous chromosomes move to opposite poles.
    • Sister Chromatids Remain Attached: Sister chromatids do not separate at this stage.
  3. Telophase I:

    • Chromosomes Reach Poles: Homologous chromosomes arrive at opposite poles.
    • Nuclear Envelopes Reform: Nuclear envelopes begin to reform.
    • Cytokinesis: The cell divides, resulting in two haploid daughter cells, each with half the chromosome number.

Meiosis II:

  1. Prophase II:

    • Chromosome Condensation: Chromosomes condense again.
    • Nuclear Envelope Breakdown: The nuclear envelope breaks down.
    • Spindle Formation: Spindle fibers begin to form.
  2. Metaphase II:

    • Chromosomes Align: Chromosomes align at the metaphase plate.
    • Spindle Fibers Attach: Spindle fibers attach to centromeres.
  3. Anaphase II:

    • Sister Chromatids Separate: Sister chromatids of each chromosome separate and move to opposite poles.
  4. Telophase II:

    • Chromatids Reach Poles: Chromatids reach opposite poles.
    • Nuclear Envelopes Reform: Nuclear envelopes reform around the chromatids.
    • Cytokinesis: The cell divides, resulting in four non-identical haploid daughter cells.

Significance of Meiosis

  1. Genetic Diversity:

    • Meiosis is a key process in introducing genetic diversity in sexually reproducing organisms.
    • Crossing over during prophase I and independent assortment during metaphase I lead to the formation of genetically distinct gametes.
  2. Formation of Haploid Gametes:

    • Meiosis reduces the chromosome number from diploid to haploid.
    • Ensures that the fusion of gametes during fertilization restores the diploid chromosome number in the zygote.
  3. Crossing Over:

    • Occurs during prophase I.
    • Exchange of genetic material between homologous chromosomes.
    • Results in the creation of unique combinations of alleles on a chromosome.
  4. Independent Assortment:

    • Occurs during metaphase I.
    • Random alignment of homologous chromosomes at the metaphase plate.
    • Leads to a variety of possible combinations of chromosomes in gametes.
  5. Adaptation and Evolution:

    • Genetic diversity introduced by meiosis is a driving force for adaptation and evolution.
    • Populations with a broader range of genetic variations are better equipped to adapt to changing environments.
  6. Prevention of Chromosome Doubling:

    • Meiosis ensures that the chromosome number is halved in gametes.
    • Prevents a doubling of chromosome number with each generation, maintaining stability.
  7. Sexual Reproduction:

    • Meiosis is essential for the formation of gametes (sperm and egg cells) in sexually reproducing organisms.
    • It enables the combination of genetic material from two parents during fertilization.
  8. Genetic Recombination:

    • The shuffling of genetic material during meiosis leads to the formation of new combinations of alleles.
    • Provides the basis for genetic recombination and variation in populations.
  9. Maintenance of Species Integrity:

    • Meiosis helps maintain the species-specific chromosome number by ensuring the reduction of chromosome number in gametes.
  10. Production of Non-Identical Haploid Cells:

    • Meiosis results in the formation of four non-identical haploid cells (gametes), each with a unique genetic composition.
    • Increases the diversity of offspring in sexually reproducing organisms.


Key Concepts:

  1. Genetic Diversity:

    • Meiosis introduces genetic diversity through the processes of crossing over and independent assortment.
    • It results in gametes with unique combinations of genetic material.
  2. Reduction in Chromosome Number:

    • Meiosis reduces the chromosome number from diploid to haploid, ensuring the restoration of diploidy during fertilization.
  3. Gamete Formation:

    • Meiosis produces gametes (sperm and egg cells) necessary for sexual reproduction.
  4. Significance in Evolution:

    • Genetic variability introduced by meiosis contributes to the adaptability and evolution of species.