Lesson 3 — Meiosis


Comparing Mitosis and Meiosis

Mitosis and meiosis both involve the division of a nucleus, but they have differences that result in very different daughter cells. The major difference between meiosis and mitosis occurs during prophase I. The homologous chromosome pairs synapse and form a tetrad during prophase I of meiosis, but this does not occur in mitosis. When the tetrad is separated and homologous chromosomes (each consisting of two sister chromatids) move to the poles, the number of sets of chromosomes is reduced. No reduction of chromosomes occurs in mitosis.

 Unlike mitosis, genetic recombination occurs during crossing-over in prophase I and independent assortment in Metatphase I of meiosis. Therefore, the daughter cells resulting from meiosis II differ from each other. The daughter cells resulting from mitosis are identical to each other.

Meiosis II is very similar to mitosis in that the sister chromatids separate to different poles.

Consider the similarities and differences of the two processes.

  Process
  Mitosis
 Meiosis
DNA Synthesis
DNA synthesis occurs in the S phase of interphase.
DNA synthesis occurs in the S phase of interphase.
Synapsis
Synapsis does not occur in mitosis.
Synapsis of homologous chromosomes occurs during prophase I.
Crossing Over Crossing over does not occur in mitosis.
Crossing over occurs in prophase I. 
Homologous Chromosomes Line Up in the Middle
This does not occur in mitosis.
Homologous chromosomes line up in the middle of the cell during metaphase I.
Sister Chromatids Line Up in the Middle
Sister chromatids line up in the middle of the cell during metaphase. Sister chromatids line up in the middle of the cell during metaphase II.
Resulting Daughter Cells
Mitosis results in two genetically identical daughter cells. Mitosis in diploid (2n) cells results in diploid (2n) daughter cells. Meiosis results in four genetically different daughter cells.  Meiosis in diploid (2n) cells results in haploid (n) daughter cells.


Watch This

Watch the following video comparing the processes of meiosis and mitosis.







Self-Check

Complete the following questions.

  1. What two main functions does meiosis accomplish?

  2. Where does meiosis occur?

  3. At the end of meiosis II, how many haploid cells have been formed from the original diploid parent cell?

  4. A diploid organism has four pairs of chromosomes in each somatic cell. Assuming that no crossing over occurs, how many genetically distinct gametes can this organism produce?

  5. Gametes are formed from specialized germ cells. A human germ cell in interphase has 23 pairs of chromosomes. If this cell undergoes cell division, how many chromosomes are in each of the following phases? Indicate whether the chromosomes appear as linked sister chromatids, single chromosomes, or homologous pairs.
    1. Metaphase (mitosis)
    2. Metaphase I (meiosis)
    3. Metaphase II (meiosis)

  6. Is it possible for identical twins to be different sexes? Explain in terms of the cellular processes that result in identical twins.

  7. In what stage of meiosis do chromosome tetrads align at the cell equator? What feature of this alignment contributes to genetic diversity?

Self-Check Answers


  1. The process of meiosis produces haploid gametes from diploid parent cells as is necessary for sexual reproduction. Meiosis contributes to genetic variation by producing many genetically different gametes. Variation in gametes results from crossing over, in which non-sister chromatids exchange chromosome sections during prophase I. Also in metaphase I, homologous chromosomes assort independently, which allows for various combinations of parental chromosomes in the gametes.

  2. Meiosis occurs in spermatogonial cells in the testes in males and in the oogonial cells in the ovaries of females.

  3. At the end of meiosis II, four haploid cells have been formed from the original parent cell.

  4. Assuming that no crossing over occurs, a diploid organism with four pairs of chromosomes can produce 24 or 16 genetically distinct gametes. Each gamete will have four chromatids. Each chromatid can be one of two distinct orders. That will be 2 x 2 x 2 x 2.

  5. (a) Metaphase (mitosis): In metaphase of mitosis, 23 pairs of homologous chromosomes occur as linked sister chromatids (92 chromatids in total). (Note: Of these 23 pairs of chromosomes, males actually have two non-homologous sex chromosomes, the X chromosome and the Y chromosome.)
    (b) Metaphase I (meiosis): In metaphase I of meiosis, twenty-three pairs of homologous chromosomes are arranged in pairs. Each homologous chromosome contains 2 linked sister chromatids and is part of a tetrad of four chromatids.
    (c) Metaphase II (meiosis): In metaphase II of meiosis, each cell contains twenty-three chromosomes, one member of the earlier homologous chromosome pairs. Each chromosome consists of a pair of linked sister chromatids.

  6. Identical twins cannot be different sexes. Identical twins are born of a single zygote. During its first two weeks of embryonic development, it divides into two separate bodies. Because the sex of the zygote is determined upon fertilization, each twin must be the same sex.

  7. In metaphase I of meiosis, the chromosome tetrads align at the cell equator. The feature of this alignment that contributes to genetic diversity is independent assortment.


Discussion

 
© Getty Images

In 1996 a sheep named Dolly made history as the first animal to be cloned from an existing adult. In several ways, this was similar to making an identical twin, but much later in life. This was an amazing scientific development, and all kinds of groups began discussing how we could use this technology.

Some advocated cloning people to help infertile couples. Others wanted to clone all kinds of livestock to make vast herds of perfect producers. However, some problems with this procedure became evident about six years after Dolly was born. She developed arthritis and lung cancer and had to be put down. Dolly's breed of sheep normally lives 12 to 15 years. The cell she was created from was about six years old. Some speculate that her chromosomes were already 6 years old and therefore, she actually died of old age.

Consider the ethics of cloning: is it an advantage or a disadvantage?



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