Mitosis vs. Meiosis
Figure 1. Mitosis or Meiosis?This represents a tiny embryo just beginning to form. Once an egg is fertilized, the resulting single cell must divide many, many times to develop a fetus. Both mitosis and meiosis involve cell division; is this type of cell division an example of mitosis or meiosis? The answer is mitosis. With each division you are making a genetically exact copy of the parent cell, which only happens through mitosis.
Mitosis, meiosis, and sexual reproduction are discussed at https://www.youtube.com/watch?v=2aVnN4RePyI.
Both mitosis and meiosis result in eukaryotic cells dividing. So what is the difference between mitosis and meiosis? The primary difference is the differing goals of each process. The goal of mitosis is to produce two daughter cells that are genetically identical to the parent cell, meaning the new cells have exactly the same DNA as the parent cell. Mitosis happens when you want to grow, for example. You want all your new cells to have the same DNA as the previous cells. The goal of meiosis, however, is to produce sperm or eggs, also known as gametes. The resulting gametes are not genetically identical to the parent cell. Gametes are haploid cells, with only half the DNA present in the diploid parent cell. This is necessary so that when a sperm and an egg combine at fertilization, the resulting zygote has the correct amount of DNA—not twice as much as the parents. The zygote then begins to divide through mitosis.
Pictured below is a comparison between binary fission (Figure below), which is cell division of prokaryotic organisms, mitosis, and meiosis. Mitosis and meiosis are also compared in the table that follows (Table below).
Figure 2. A comparison between binary fission, mitosis, and meiosis.
|Purpose||To produce new cells||To produce gametes|
|Number of Cells Produced||2||4|
|Rounds of Cell Division||1||2|
|Haploid or Diploid||Diploid||Haploid|
|Daughter cells identical to parent cells?||Yes||No|
|Daughter cells identical to each other?||Yes||No|
Changes in Chromosome Number
What would happen if an entire chromosome were missing or duplicated? What if a human had only 45 chromosomes? Or 47? This real possibility is usually due to mistakes during meiosis; the chromosomes do not fully separate from each other during sperm or egg formation. Specifically, nondisjunction is the failure of replicated chromosomes to separate during anaphase II. If a zygote forms from a gamete lacking a chromosome, a viable embryo cannot be produced. Most human abnormal chromosome numbers result in the death of the developing embryo, often before a woman even realizes she is pregnant. Occasionally, a zygote with an extra chromosome can become a viable embryo and develop.
Figure 3 . Nondisjunction in Meiosis: 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.
Trisomy is a state where humans have an extra autosome. That is, they have three of a particular chromosome instead of two. For example, trisomy 18 results from an extra chromosome 18, resulting in 47 total chromosomes. To identify the chromosome number (including an abnormal number), a sample of cells is removed from an individual or developing fetus. Metaphase chromosomes are photographed and a karyotype is produced. A karyotype will display any abnormalities in chromosome number or large chromosomal rearrangements. Trisomy 8, 9, 12, 13, 16, 18, and 21 have been identified in humans. Trisomy 16 is the most common trisomy in humans, occurring in more than 1% of pregnancies. This condition, however, usually results in spontaneous miscarriage in the first trimester. The most common trisomy in viable births is Trisomy 21.
Trisomy 21: Down Syndrome
One of the most common chromosome abnormalities is Down syndrome, due to nondisjunction of chromosome 21 resulting in an extra complete chromosome 21, or part of chromosome 21 (Figure below). Down syndrome is the only autosomal trisomy where an affected individual may survive to adulthood. Individuals with Down syndrome often have some degree of mental retardation, some impairment of physical growth, and a specific facial appearance. With proper assistance, individuals with Down syndrome can become successful, contributing members of society. The incidence of Down syndrome increases with maternal age. The risk of having a child with Down syndrome is significantly higher among women age 35 and older.
See http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001992/ for additional information.
Trisomy 21 (Down Syndrome) Karyotype. A karyotype is a picture of a cell's chromosomes. Note the extra chromosome 21. Child with Down syndrome, exhibiting characteristic facial appearance.
Abnormal Numbers of Sex Chromosomes
What about when a person has more than two Y chromosomes, or more than two X chromosomes? Or a female with only one X chromosome? Sex-chromosome abnormalities may be caused by nondisjunction of one or more sex chromosomes. Many conditions are known in which there are an abnormal number of sex chromosomes. An X chromosome may be missing (XO), or there may be an extra one (XXX or XXY). There may also be an extra Y chromosome (XYY). Any combination of X and Y chromosomes, as long as there is a Y chromosome, will produce a male (up to XXXXY). These individuals can lead relatively normal lives, but they cannot have children. They may also have some degree of mental retardation. These syndromes include Klinefelter’s syndrome, Turner syndrome and trisomy X.
Klinefelter's syndrome is caused by the presence of one or more extra copies of the X chromosome in a male's cells. Extra genetic material from the X chromosome interferes with male sexual development, preventing the testicles from functioning normally and reducing the levels of testosterone. Triple X syndrome (trisomy X) results from an extra copy of the X chromosome in each of a female's cells. Females with trisomy X have a lower IQ than their siblings. Turner syndrome results when each of a female's cells has one normal X chromosome and the other sex chromosome is missing or altered. The missing genetic material affects development and causes the characteristic features of the condition, including short stature and infertility.
extra copy (complete or partial) of chromosome 21 (see Figure above)
developmental delays, distinctive facial appearance, and other abnormalities (see Figure above)
one X chromosome but no other sex chromosome (XO)
female with short height and infertility (inability to reproduce)
Triple X syndrome
three X chromosomes (XXX)
female with mild developmental delays and menstrual irregularities
one Y chromosome and two or more X chromosomes (XXY, XXXY)
male with problems in sexual development and reduced levels of the male hormone testosterone
- The goal of mitosis is to produce a new cell that is identical to the parent cell.
- The goal of meiosis is to produce gametes that have half the DNA of the parent cell.
- When chromosomes do not divide equally among gametes, the damaged gametes produce. This process is called nondisjunction.
- Trisomy is a state where humans have an extra autosome; they have three of a particular chromosome instead of two.
- The most common trisomy in viable births is Trisomy 21 (Down Syndrome).
- Gametes may have more or less sex chromosomes due to nondisjunction.
Use the resources below to answer the questions that follow.
Explore More I
- Mitosis and Meiosis Simulation at http://www.youtube.com/watch?v=zGVBAHAsjJM (11:53)
- What are homologous chromosomes?
- How do the location of specific genes compare between homologous chromosomes?
- What is the outcome from mitosis?
- What is a tetrad? Why are they an important feature of meiosis?
- How does meiosis differ between females and males?
Explore More II
- How Cells Divide at http://www.pbs.org/wgbh/nova/body/how-cells-divide.html
- How many daughter cells arise from mitosis? How many daughter cells are produced in meiosis?
- How does the attachment of spindle fibers differ between mitosis and meiosis I?
- Is anaphase I or anaphase II in meiosis more analogous to anaphase in mitosis? Explain your reasoning.
- How many steps are there in mitosis? How many steps are there in meiosis?
- How does interphase I of meiosis differ from interphase II of meiosis?
- What is the goal of mitosis? Of meiosis?
- How many cells are created from cytokinesis following mitosis? Following meiosis?
- Which process, mitosis to meiosis, creates genetically identical cells?
- "Gametes are haploid cells." What does this sentence mean? Define polygenic traits.
- What is meant by trisomy?
- How can trisomy phenotypes be detected?
- What is the most common viable trisomy disorder?
- List conditions involving an abnormal number of sex chromosomes.
Credit: By Nina Sesina (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons;
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Credit: Zachary Wilson;
Source: CK-12 Foundation
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Credit: OpenStax College, Chromosomal Basis of Inherited Disorders. October 16, 2013;
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Credit: By Courtesy: National Human Genome Research Institute [Public domain], via Wikimedia Commons;
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Credit: By Erin Ryan (Own work) [Public domain], via Wikimedia Commons;
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