Nondisjunction describes an error in cell division where chromosomes or sister chromatids fail to separate properly. This cellular malfunction occurs during meiosis, a specialized type of cell division that produces reproductive cells, or gametes, such as sperm and egg cells. These errors fundamentally disrupt the precise distribution of genetic material.
Understanding Meiosis
Meiosis is a two-stage cell division process that reduces the chromosome number by half, creating haploid cells from a diploid parent cell. In Meiosis I, homologous chromosomes, which are pairs of chromosomes inherited one from each parent, separate from each other. Each daughter cell receives one chromosome from each homologous pair.
Following Meiosis I, cells proceed into Meiosis II. During this stage, sister chromatids, identical copies of a single chromosome joined at the centromere, separate. This results in four haploid cells, each containing a single set of chromosomes, ready for sexual reproduction. Precise segregation of chromosomes during both stages is essential for producing genetically balanced gametes.
Nondisjunction in Meiosis I
Nondisjunction can occur during Meiosis I when homologous chromosomes fail to separate and move to opposite poles. Instead of each daughter cell receiving one chromosome from the homologous pair, one cell receives both, while the other receives none.
When this error occurs, the resulting secondary spermatocytes or oocytes carry an abnormal number of chromosomes. After Meiosis II, all four gametes produced will be aneuploid. Two gametes contain an extra chromosome (n+1), meaning they have a duplicated chromosome. The other two are missing a chromosome (n-1), lacking a particular chromosome entirely.
Nondisjunction in Meiosis II
Nondisjunction can also occur during Meiosis II when sister chromatids fail to separate. This error occurs in one of the two cells produced after a normal Meiosis I. In this scenario, homologous chromosomes separate correctly in Meiosis I, but sister chromatids of a single chromosome do not pull apart in Meiosis II.
If sister chromatids fail to separate in a secondary meiotic cell, the outcome differs from Meiosis I nondisjunction. Two of the four resulting gametes will be normal (n), each containing a single set of chromosomes. One gamete will have an extra chromosome (n+1), while the other will be missing a chromosome (n-1). This outcome contrasts with Meiosis I nondisjunction, where all four gametes produced are abnormal.
Outcomes of Nondisjunction
Nondisjunction results in gametes with an abnormal number of chromosomes, a condition known as aneuploidy. Aneuploidy means a cell has either too many or too few chromosomes compared to the normal haploid set.
For example, if an (n+1) gamete fertilizes a normal (n) gamete, the resulting zygote will have three copies of a particular chromosome (2n+1), a condition called trisomy. Conversely, if an (n-1) gamete is fertilized by a normal (n) gamete, the zygote will have only one copy of a specific chromosome (2n-1), known as monosomy. These chromosomal imbalances can impact the development and viability of the organism.