meiosis

Examples of meiosis in the following topics:

• Comparing Meiosis and Mitosis

  • Mitosis and meiosis share some similarities, but also some differences, most of which are observed during meiosis I.
  • The main differences between mitosis and meiosis occur in meiosis I.
  • All of these events occur only in meiosis I.
  • Meiosis II is much more similar to a mitotic division.
  • Meiosis and mitosis are both preceded by one round of DNA replication; however, meiosis includes two nuclear divisions.

• 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.

• Introduction to Meiosis

  • Meiosis is the nuclear division of diploid cells into haploid cells, which is a necessary step in sexual reproduction.
  • Haploid cells that are part of the sexual reproductive cycle are produced by a type of cell division called meiosis.
  • Meiosis employs many of the same mechanisms as mitosis.
  • 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.

• 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 homologous chromosomes fail to separate during meiosis I, the result is two gametes that lack that particular chromosome and two gametes 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.

• 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.

• Oogenesis

  • Immediately after meiosis I, the haploid secondary oocyte initiates meiosis II.
  • When meiosis II has completed, an ootid and another polar body would have been 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.
  • Oogenesis is the process of meiosis specific to the production of oocytes, particularly the uneven distribution of cellular components, including organelles and matrix, during divisions.

• 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.

• Gametogenesis (Spermatogenesis and Oogenesis)

  • Gametogenesis, the production of sperm and eggs, takes place through the process of meiosis.
  • The cell starting meiosis is called a primary oocyte.
  • Meiosis begins with a cell called a primary spermatocyte.
  • The cell produced at the end of meiosis is called a spermatid.
  • Four sperm result from each primary spermatocyte that goes through 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.