What is the difference between meiosis I and meiosis II?

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In meiosis I, chromosomes in a diploid cell resegregate, producing four haploid daughter cells. It is this step in meiosis that generates genetic diversity.
The phases of meiosis I & II Prophase I

DNA replication precedes the start of meiosis I. During prophase I, homologous chromosomes pair and form synapses, a step unique to meiosis. The paired chromosomes are called bivalents, and the formation of chiasmata caused by genetic recombination becomes apparent. Chromosomal condensation allows these to be viewed in the microscope. Note that the bivalent has two chromosomes and four chromatids, with one chromosome coming from each parent.

The nuclear membrane disappears. One kinetochore forms per chromosome rather than one per chromatid, and the chromosomes attached to spindle fibers begin to move.

Bivalents, each composed of two chromosomes (four chromatids) align at the metaphase plate. The orientation is random, with either parental homologue on a side. This means that there is a 50-50 chance for the daughter cells to get either the mother's or father's homologue for each chromosome.

Chiasmata separate. Chromosomes, each with two chromatids, move to separate poles. Each of the daughter cells is now haploid (23 chromosomes), but each chromosome has two chromatids.

Nuclear envelopes may reform, or the cell may quickly start meiosis II.

Cytokinesis :

Analogous to mitosis where two complete daughter cells form.
Meiosis II :
Meiosis II is similar to mitosis. However, there is no "S" phase. The chromatids of each chromosome are no longer identical because of recombination. Meiosis II separates the chromatids producing two daughter cells each with 23 chromosomes (haploid), and each chromosome has only one chromatid.
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  • rfedrocks answered 7 years ago
    key:
    'n' refers to the number of sets of chromosomes in the cell at that particular phase. e.g. n would indicate haploidy
    'x' is an arbitrary constant referring to the amount of DNA in the cell.

    1. Prophase I vs Prophas II
    PI: 2n, 8x
    PII: n, 4x
    - in PI, homologous chromosomes pair to form a bivalent/tetrad via synapsis, there is chiasmata formation and the exchange of genetic material
    - in PII, none of the above occur. PII is very similar to prophase in mitosis

    2. Metaphase I vs Metaphase II
    MI: 2n, 8x
    MII: n, 4x
    - in MI, there is random arrangement of the bivalents along the equator of the cell
    - in MII, the chromosomes line up along the equator of the cell, just as it is in mitosis.

    3. Anaphase I vs Anaphase II
    AI: 2n, 8x
    AII: 2n, 4x
    - in AI, the homologous chromosomes separate to the polar ends of the cell due to contraction of the spindle fibres
    - in AII, the sister chromatids separate to polar ends of the cell

    4. Telophase I vs Telophase II
    TI: n, 4x
    TII:, n, 2x
    - there isn't much differences between the 2. both processes result in 2 daughter cells
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  • Dastardly answered 7 years ago
    In meiosis I chromosome pairs line up on the metaphase plate and are separated to opposite poles producing haploid daughter nuclei. In meiosis II replicated chromosomes line up on the metaphase plate to be separated into unreplicated chromatids resulting in haploid cells.
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  • margueritasr answered 7 years ago
    In meiosis I, chromosomes in a diploid cell resegregate, producing four haploid daughter cells

    Meiosis II separates the chromatids producing two daughter cells each with 23 chromosomes (haploid), and each chromosome has only one chromatid.
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