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.
When the cells produced by meiosis I undergo meiosis II, each chromosome aligns at the metaphase plate and sister chromatids are segregated into separate gametes. The homologous chromosomes are said to disjoin or separate at anaphase I.
The sister chromatids are said to disjoin at anaphase II. Thus, failure to separate the linked homologs, or chromatids, during anaphase is called nondisjunction.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.The first example of aneuploidy we identified was trisomy of chromosome 21. Trisomy of chromosome 21 causes the human chromosomal disorder known as Down Syndrome. Roughly 1 in 700 babies born in the United States has Down Syndrome. A few of the many conditions they can be afflicted with are mental retardation, skeletal abnormalities and heart defects.
However, considerable phenotypic variation is observed among individuals with Down Syndrome, with many individuals able to lead relatively long, productive lives.Down Syndrome is one of the few examples of chromosomal aneuploidy that is viable. For the most part, any form of aneuploidy is a lethal event resulting in a miscarriage. Roughly half of miscarriages are initiated due to chromosomal abnormalities.
For instance, consider the other type of aneuploid cell we discussed earlier. A cell that is monosomic for chromosome 21 isn’t viable and would initiate a miscarriage.
In summary, nondisjunction is the failure of linked homologs or chromatids to separate during anaphase of mitosis or meiosis.
Aneuploidy is a state in which a cell has an abnormal number of chromosomes. In a diploid organism, the most common forms of aneuploidy are monosomy and trisomy. Monosomy is a situation in which an otherwise diploid cell has only one copy of a particular chromosome.
Trisomy is a situation in which an otherwise diploid cell has three copies of a particular chromosome.
After you complete this lesson you’ll be able to:
- Explain nondisjunction
- Define aneuploidy
- Differentiate between monosomy and trisomy