What Is a Nondisjunction? Nondisjunction and Chromosomal Anomalies

Understanding nondisjunction and chromosomal anomalies require us to back up all the way and start at the beginning—the very beginning of it all, namely the beginning of life. Fertilization is the combination of a human egg and sperm in order to form what is known as a zygote or a fertilized egg. The fancy term for the beginning of life is also the onset of prenatal development.

Fertilization is also marked by the fusion of a sperm, the male reproductive cell, and the egg, the female reproductive egg. This is also when a combination of chromosomes and, therefore, genetic material takes place. This is also called mitosis, which involves several steps during which male and female reproductive cells combine to double the genetic material (genes) and then separate again as the cells multiply. It is this very step that has to be understood and defined a bit further in order to understand both nondisjunctions as well as chromosomal abnormalities.

One of the most common causes of an abnormal number of chromosomes, or aneuploidies, is nondisjunction. An estimated 25 percent of aneuploidies that may affect human oocytes are a result of nondisjunction. There are three different types of nondisjunction, depending on when they have occurred. These are nondisjunction in meiosis I, II and in mitosis.

Mitosis: A Critical Part of the Cell Cycle

So, let us talk about mitosis, one of the most critical parts of the cell cycle during which replicated chromosomes are split up into two new nuclei. While the number of chromosomes remains the same, cell division yields genetically identical cells. Mitosis is, in fact, a process that consists of several steps, each of which is marked by the completion of a set of activities and the beginning of the next. It is specific to eukaryotic cells, meaning in cells that have a nucleus that is enclosed by a double membrane. (This is contrary to prokaryotes whose cell compartments, or organelles, are not enclosed by a membrane, such as bacteria or archaea) The most fundamental outcome of mitosis is the transfer of the mother’s and the father’s genetic material into two daughter cells.

The beginning of mitosis is marked by a condensation of chromosomes, which also makes them a lot more visible. It is actually a very short process compared to the beginning, or interphase, during which each cell prepares for what is about to take place. The several stages of mitosis are prophase, metaphase, anaphase, and telophase. Each is a step in the sequence that leads to a creation of two daughter cells that have the same genetic components. Nondisjunction during mitosis happens during anaphase when two sister chromatids do not separate from one another when they were supposed to. In this case, one of the two cells that are created has less genetic material and the other has more.

Meiosis: More Cell Division

Meiosis cell divisions

So, let us briefly talk about meiosis as well, as it is another process where cells divide. However, unlike mitosis, meiosis has two cell divisions and results in twice the number of cells, each with just one copy of the genetic material. The first round, meiosis I, is marked by the same stages as mitosis, namely prophase I, metaphase I, anaphase I, and telophase I. The second stage of meiosis has the same steps, prophase II, metaphase II, anaphase II, and telophase II. Similar to mitosis, it is during anaphase stages of meiosis that genetic material does not separate as it should, which leads to chromosomal anomalies. The only difference is that the nondisjunction that takes place is meiosis I has much larger consequences as all four of the cells that are produced have an altered number of chromosomes, and this carries over into meiosis II.

The consequences are the loss of a chromosome that results in a condition called monosomy. It can also lead to an addition of a chromosome, in which case it is referred to as a trisomy. Either scenario leads to a number of conditions.

One is termed Turner syndrome or monosomy of the sex-linked chromosome whereby the zygote has only one X chromosome. This zygote is always a female, characterized by sterility as well as underdeveloped gender traits. Furthermore, those who have Turner syndrome are also short in height and have a much higher risk of suffering from cardiovascular issues, diabetes, and hypothyroidism.

One of the more commonly known conditions is Down syndrome, whereby the zygote ends up having an extra chromosome. This is also why Down syndrome is also referred to as a trisomy. Specifically, it is termed trisomy 21 due to an extra chromosome 21. Down syndrome happens in one in every 1000 births. Those who have the condition are characterized by lower intelligence as well as lower immune system function. Their eyes are slanted, they have a small mouth with a protruding tongue, and their growth is also affected. They are also at a higher risk of cardiovascular defects as well as irregular thyroid functions.

Another condition that is caused by nondisjunction is Klinefelter syndrome that is caused by a sex-linked chromosome trisomy. More specifically, it is caused by nondisjunction in the chromosomes on the father’s side during meiosis I. Those who suffer from the condition have traits related to delayed sexual development. Those males can also develop breasts as well as suffer from higher risk of disorders that typically affect females, namely autoimmune disorders, breast cancer, and osteoporosis.

Retinoblastoma, a rare form of cancer that affects the retina,is also a result of nondisjunction. It causes loss of vision in the affected eye, and it typically affects young children. Specifically, the retinoblastoma protein that stops cancer, also called a tumor suppressor, is coded for by the RB1 gene that is located on chromosome 13. Mutations in the RB1 gene could lead to a loss of the protein, in which case there is no protein to suppress cancer that is marked by an unchecked cell division.

Final Thoughts

Now that you know a little bit about nondisjunction and chromosomal anomalies, if you think about it, we are all just a small step away from a syndrome, a condition, or a disease, so appreciate your health!

Laura Day