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.

  • Oogenesis

    • Immediately after meiosis I, the haploid secondary oocyte initiates meiosis II.
    • However, this process is also halted at the metaphase II stage until fertilization, if such should ever occur.
    • When meiosis II has completed, an ootid and another polar body is created.
    • Both polar bodies disintegrate at the end of meiosis II, leaving only the ootid, which then eventually undergoes maturation into a mature ovum.
    • The function of forming polar bodies is to discard the extra haploid sets of chromosomes that have resulted as a consequence of meiosis.

  • 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.

  • Fertilization

    • Through fertilization, the egg is activated to begin its developmental process (progressing through Meiosis II), and the haploid nuclei of the two gametes come together to form the genome of a new diploid organism.
    • Nondisjunction during the completion of meiosis or problems with early cell division in the zygote to blastula stages can lead to problems with implantation and pregnancy failure.

  • 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.

  • Spermatogenesis

    • Spermatogenesis is the process by which male primary sperm cells undergo meiosis and produce a number of cells termed spermatogonia, from which the primary spermatocytes are derived.
    • *Each primary spermatocyte then moves into the adluminal compartment of the seminiferous tubules, duplicates its DNA, and subsequently undergoes meiosis I to produce two haploid secondary spermatocytes
    • Secondary spermatocytes produced earlier rapidly enter meiosis II and divide to produce haploid spermatids.

  • 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.