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.

• 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.

• 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.