Nondisjunction is an error that occurs during cell division, representing a failure in the precise distribution of genetic material. This event causes chromosomes to separate incorrectly, resulting in the unequal partitioning of genetic material into the resulting daughter cells. This chromosomal mis-segregation is a primary source of genetic disorders and a significant factor in reproductive health.
What Nondisjunction Is
Nondisjunction describes the event where paired chromosomes or chromatids fail to separate during cell division. Instead of receiving a complete set of genetic information, one cell gains an extra chromosome, while the other ends up with a missing one. This mishap disrupts the normal count of 46 chromosomes found in human somatic cells.
The outcome of nondisjunction is aneuploidy, meaning the cell has an abnormal number of chromosomes. If aneuploidy occurs in a reproductive cell (gamete), the resulting embryo, when fertilized by a normal gamete, will have an incorrect chromosome count in every cell. This state is categorized into two main types based on the imbalance.
A cell that possesses an extra copy of a specific chromosome is called a trisomy. This means the cell contains three copies of that chromosome instead of the normal pair, leading to a total count of 47 chromosomes in humans. Conversely, a cell that is missing one chromosome from a pair is defined as a monosomy, resulting in only one copy of that chromosome and a total count of 45 chromosomes in humans.
How Errors Occur in Cell Division
Nondisjunction errors occur during two main types of cell division: meiosis (for sex cells) and mitosis (for body cells). Errors during meiosis are the most common cause of genetic syndromes, as they affect the gametes that form the entire organism. Meiotic errors are distinguished by whether they happen during the first or second phase of the process.
Nondisjunction in Meiosis I
Nondisjunction in Meiosis I occurs when homologous chromosomes fail to separate. Normally, these pairs move to opposite poles of the cell, but in this error, both pairs move together into one secondary cell. This failure means that all four resulting gametes will have an abnormal chromosome number: two gametes with an extra chromosome and two gametes missing a chromosome.
Nondisjunction in Meiosis II
Nondisjunction in Meiosis II occurs when the sister chromatids fail to separate. Sister chromatids are the identical halves of a replicated chromosome that should split during the second phase of meiotic division. If this separation fails, the resulting gametes are less affected than in Meiosis I errors, resulting in two normal gametes, one gamete with an extra chromosome, and one gamete with a missing chromosome.
Mitotic Nondisjunction
Mitotic Nondisjunction is a less common form that happens in a body cell after fertilization. Here, sister chromatids fail to separate during the division of a somatic cell. This results in two daughter cells with an abnormal number of chromosomes, but only in the cell line descended from the initial error. This leads to mosaicism, where an individual has some cells with a normal chromosome number and some cells with an aneuploid number.
Genetic Syndromes Caused by Nondisjunction
The consequences of aneuploidy are varied, depending heavily on which chromosome is involved. Most aneuploidies are incompatible with life, often resulting in miscarriage. However, some imbalances, particularly those involving smaller chromosomes or the sex chromosomes, can result in live births with specific genetic syndromes.
One of the most recognized examples is Down syndrome, a trisomy of chromosome 21. Individuals with this condition have three copies of the 21st chromosome instead of the usual two. The majority of cases are caused by nondisjunction during the mother’s Meiosis I.
Nondisjunction can also affect the sex chromosomes (X and Y), leading to conditions that alter sexual development and fertility. Turner syndrome is a sex chromosome monosomy, where a female is born with only one X chromosome (XO) instead of the typical two (XX). This is the only monosomy of a complete chromosome commonly compatible with life.
Klinefelter syndrome is a sex chromosome trisomy characterized by the presence of an extra X chromosome in a male (XXY). The extra genetic material can lead to reduced fertility and specific physical characteristics. Both Turner and Klinefelter syndromes illustrate how an imbalance in the number of sex chromosomes significantly impacts development.