Consequences of Mistakes in Mitosis and Meiosis
Mistakes in Mitosis
Mitosis produces two identical daughter cells to the parental cell. During mitosis, once an incorrect DNA is formed, DNA mutations can occur. These mutations (or mistakes) will be multiplied further by mitosis. These mutations can disrupt the cell cycle and form tumors which do not stop dividing. This DNA copying error can be a causative agent for cancer.
Chromosome abnormalities can occur also. Chromosomes are attached to the spindle in the equator of metaphase. If one chromosome is not able to attach to the spindle during this period, a daughter cell will receive an extra or missing copy of chromosomes after cell division. Disorders may occur due to these changes in chromosome number. A common disorder that you should know about is Down Syndrome, which is a trisomy of chromosome 21 (there are three chromosomes 21 instead of just two).
Organelle and Cell Damage
As you know, there are cellular organelles with specific functions. Once organelles are damaged due to unregulated cell division during mitosis, they don’t have a chance to repair and recover between cell cycles. This can lead to cell death.
Somatic mosaicism is when some body cells have a mutated version of a gene while other cells have a normal version of the same gene. This is due to the mitosis amplifying the errors in DNA. Therefore, those mutations will be passed to some cells only. If the mutant gene is harmful and widespread, diseases can occur. Marfan syndrome (long limb) and hemophilia (blood clotting disorder) are the most common examples.
Mistakes in Meiosis
Usually, meiosis produces four haploid daughter cells from a diploid parent cell. However, sometimes there will be mistakes during meiosis and daughter cells will be produced with too few or too many chromosomes. This condition is known as aneuploidy.
Sometimes, homologous chromosomes don’t separate from each other during cell division and sometimes even sister chromatids don’t separate with each other. This is known as nondisjunction and it occurs during Meiosis I, Meiosis II and Mitosis. If nondisjunction occurs in meiosis I, both chromosomes are pulled towards the same pole and that results in a daughter cell without the chromosome. So, in meiosis II, 4 gametes are produced with an unusual number of chromosomes. Two gametes will be produced with a complete set of chromosomes plus an additional chromosome, labelled as (n+1). The other two gametes will lack a chromosome and are labelled as (n-1).
If meiosis I proceeds normally, each chromosome pair will migrate to opposite poles and nondisjunction occurs in meiosis II, where both chromatids will migrate to the same pole. One chromosome separates abnormally and that results in 2 abnormal daughter cells. Two gametes are normal with a haploid number of chromosomes. The other two gametes are abnormal and one gamete bears (n+1) number of chromosomes while the other gamete contains (n-1) number of chromosomes.
These abnormal gametes have a major sequence on the next generation. If a (n+1) gamete fuses with a normal gamete (n), it will produce a diploid zygote with a third copy of the chromosome. That is shown as (2n+1) and it is known as trisomy. Down syndrome is a classic example of this.
Similarly, if the (n-1) gamete fuses with a normal gamete, the zygote is missing a chromosome. Therefore, the cell has only one copy of a particular chromosome. This is known as monosomy. Most monosmic conditions are lethal (causing death) to humans but, females can survive with monosomy. Turner Syndrome is a classic example of this.
If a diploid gamete fuses with haploid gametes, a triploid zygote results with three chromosomes. This triploidy is lethal to humans. However, plants can tolerate these changes in chromosome number.