meiosis II

(noun)

the second part of the meiotic process; the end result is production of four haploid cells from the two haploid cells produced in meiosis I

Examples of meiosis II in the following topics:

  • Meiosis II

    • Meiosis II initiates immediately after cytokinesis, usually before the chromosomes have fully decondensed.
    • In contrast to meiosis I, meiosis II resembles a normal mitosis.
    • In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II.
    • The two cells produced in meiosis I go through the events of meiosis II together.
    • The process of chromosome alignment differs between meiosis I and meiosis II.

  • Comparing Meiosis and Mitosis

    • The main differences between mitosis and meiosis occur in meiosis I.
    • Meiosis II is much more similar to a mitotic division.
    • During anaphase II and mitotic anaphase, the kinetochores divide and sister chromatids, now referred to as chromosomes, are pulled to opposite poles.
    • Meiosis II is not a reduction division because, although there are fewer copies of the genome in the resulting cells, there is still one set of chromosomes, as there was at the end of meiosis I.
    • Meiosis II is, therefore, referred to as equatorial division.

  • Disorders in Chromosome Number

    • Aneuploidy, an abnormal number of chromosomes in a cell, is caused by nondisjunction, or the failure of chromosomes to separate at meiosis.
    • Nondisjunction can occur during either meiosis I or II, with differing results.
    • If sister chromatids fail to separate during meiosis II, the result is one gamete that lacks that chromosome, two normal gametes with one copy of the chromosome, and one gamete with two copies of the chromosome .
    • Nondisjunction occurs when homologous chromosomes or sister chromatids fail to separate during meiosis, resulting in an abnormal chromosome number.
    • Nondisjunction may occur during meiosis I or meiosis II.

  • Introduction to Meiosis

    • Meiosis is the nuclear division of diploid cells into haploid cells, which is a necessary step in sexual reproduction.
    • Meiosis employs many of the same mechanisms as mitosis.
    • However, because there are two rounds of division, the major process and the stages are designated with a "I" or a "II."
    • Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and so on.
    • Meiosis II, the second round of meiotic division, includes prophase II, prometaphase II, and so on.

  • Gametogenesis (Spermatogenesis and Oogenesis)

    • The cell starting meiosis is called a primary oocyte.
    • A secondary meiotic arrest occurs, this time at the metaphase II stage.
    • If the secondary oocyte is fertilized, the cell continues through the meiosis II, completing meiosis, producing a second polar body and a fertilized egg containing all 46 chromosomes of a human being, half of them coming from the sperm.
    • Meiosis begins with a cell called a primary spermatocyte.
    • The cell produced at the end of meiosis is called a spermatid.

  • Meiosis I

    • In meiosis I, the first round of meiosis, homologous chromosomes exchange DNA and the diploid cell is divided into two haploid cells.
    • Meiosis is preceded by an interphase consisting of three stages.
    • Finally, during the G2 phase (also called the second gap phase), the cell undergoes the final preparations for meiosis.
    • The crossover events are the first source of genetic variation produced by meiosis.
    • In each cell that undergoes meiosis, the arrangement of the tetrads is different.

  • Gene rearrangement within genomes

    • During meiosis in eukaryotes, genetic recombination involves the pairing of homologous chromosomes.
    • During meiosis, as chromosomes condense and pair with their homologs (prophase I), they interact at distinct points.
    • In meiosis and mitosis, recombination occurs between similar molecules (homologs) of DNA.
    • Gene conversion occurs at high frequency at the actual site of the recombination event during meiosis.
    • Crossing over is essential for the normal segregation of chromosomes during meiosis.

  • Life Cycles of Sexually Reproducing Organisms

    • Fertilization and meiosis alternate in sexual life cycles.
    • The zygote immediately undergoes meiosis to form four haploid cells called spores.
    • Specialized cells of the sporophyte will undergo meiosis and produce haploid spores.
    • The zygote undergoes meiosis to produce haploid spores.
    • The diploid plant is called a sporophyte because it produces haploid spores by meiosis.

  • Life Cycle of a Conifer

    • In the male cones (staminate cones), the microsporocytes give rise to pollen grains by meiosis.
    • One megaspore mother cell (megasporocyte) undergoes meiosis in each ovule.
    • This phase may take more than one year between pollination and fertilization while the pollen tube grows towards the megasporocyte (2n), which undergoes meiosis into megaspores.

  • Chromosomal Theory of Inheritance

    • The speculation that chromosomes might be the key to understanding heredity led several scientists to examine Mendel's publications and re-evaluate his model in terms of the behavior of chromosomes during mitosis and meiosis.
    • That same year, Walter Sutton observed the separation of chromosomes into daughter cells during meiosis .
    • During meiosis, homologous chromosome pairs migrate as discrete structures that are independent of other chromosome pairs.
    • Despite compelling correlations between the behavior of chromosomes during meiosis and Mendel's abstract laws, the Chromosomal Theory of Inheritance was proposed long before there was any direct evidence that traits were carried on chromosomes.