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Nondisjunction and Aneuploidy

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  1. 0:06 The Importance of Meiosis
  2. 0:36 Chromosome Segregation Errors
  3. 2:38 Consequences of Nondisjunction
  4. 4:23 Human Chromosomal Disorders
  5. 5:48 Lesson Summary
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Taught by

Greg Chin

Learn why proper chromosome segregation is essential during meiosis. We'll see how having an extra chromosome can lead to Down Syndrome and also examine the consequences of having too few chromosomes. Disruptions to the delicate balance of cell division can have far-reaching effects.

The Importance of Meiosis

So we've seen how meiosis is an important biological process. Without it, gametes wouldn't be possible and sexually reproducing organisms, well, they wouldn't be able to reproduce. So proper segregation of chromosomes during meiosis is vital for propagating the species and ensuring healthy, viable offspring. But cell division is a complex process and nature isn't perfect. So what happens when mistakes are made?

Chromosome Segregation Errors

Recall that normally in meiosis, homologous chromosomes pair and align at the metaphase plate and segregate to opposite poles during meiosis I.

Illustration of normal chromosome segregation during meiosis I
Normal Chromosome Segregation
sister chromatidsnondisjunction

Let's examine the effect nondisjunction of human chromosome 21 at meiosis I would have on the resulting gametes. To make it easier, let's just consider what happens to chromosome 21; however, keep in mind throughout this example that the rest of the chromosomes are behaving normally.

If the homologs in the chromosome 21 tetrad fail to separate during meiosis I, one daughter cell would receive both chromosome 21 homologs and the other one would receive none. If those cells proceed to meiosis II, both homologous chromosomes would line up at the metaphase plate in cell number 1, and segregate into the daughter cells. The result would be gametes with two, rather than one copy of chromosome 21.

Now let's consider what happens to the second daughter cell produced by meiosis I. It received no copies of chromosome 21 during the meiosis I division. That means that the gametes produced by this cell in meiosis II will have no copies of chromosome 21.

Consequences of Nondisjunction

Now let's consider what would happen if these abnormal gametes fuse with a gamete with the proper number of chromosomes.

First, let's consider an egg with two copies of chromosome 21 fusing with a sperm with one copy of chromosome 21. The result of this union will be a cell with three copies of chromosome 21. Recall again that we've only been considering chromosome 21 so far. Chromosome nondisjunction is generally rare, so the new cell probably has two copies of all of the other 22 chromosomes, just as it should.

A cell that has an abnormal number of chromosomes is called aneuploid. More specifically, a human who develops from this cell, will have a normal diploid set of every chromosome except three copies of chromosome 21. An otherwise diploid cell that has three copies of a chromosome is called trisomic.

Before we consider the phenotypic consequences of this abnormal ploidy, let's consider the outcome of the other type of aberrant gamete. If an egg with no copies of chromosome 21 fuses with a sperm with one copy of chromosome 21, the resulting cell would have a normal complement of chromosomes with the exception of only one copy of chromosome 21. This cell would also be considered aneuploid. In this case, the cell would be called monosomic because it only has one copy of chromosome 21.

Human Chromosomal Disorders

The importance of faithfully segregating chromosomes during meiosis becomes more apparent when the consequences of aneuploidy are considered.

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