Nondisjunction is a biological phenomenon that occurs when chromosomes or chromatids fail to separate properly during cell division, resulting in daughter cells with abnormal numbers of chromosomes, known as aneuploidy. Here are some key points about nondisjunction:

• During anaphase of the cell cycle, chromosomes are separated to opposite ends of the cell to create two daughter cells. Nondisjunction is the failure of the chromosomes to separate, which produces daughter cells with abnormal numbers of chromosomes.

• Nondisjunction can occur during anaphase of mitosis, meiosis I, or meiosis II. During anaphase, sister chromatids (or homologous chromosomes for meiosis I), will separate and move to opposite poles of the cell, pulled by microtubules. In nondisjunction, the separation fails to occur, causing both sister chromatids or homologous chromosomes to be pulled to one pole of the cell.

• Mitotic nondisjunction can occur due to the inactivation of either topoisomerase II, condensin, or separase. This will result in two aneuploid daughter cells, one with 47 chromosomes (2n+1) and the other with 45 chromosomes (2n-1) .

• Nondisjunction in meiosis I occurs when the tetrads fail to separate during anaphase I. At the end of meiosis I, there will be two haploid daughter cells, one with n+1 and the other with n-1. Both of these daughter cells will then go on to divide once more in meiosis II, producing four daughter cells, two with n+1 and two with n-1.

• Nondisjunction in meiosis II results from the failure of the sister chromatids to separate during anaphase II. This results in two daughter cells with abnormal chromosomes.

• Nondisjunction can cause chromosomal abnormalities such as Down syndrome, Patau’s Syndrome, Edwards Syndrome, Klinefelter Syndrome, and Turner’s Syndrome.

Nondisjunction can be diagnosed by karyotyping, and amniocentesis is carried out to take out amniotic fluid, which is analyzed to diagnose any chromosomal abnormalities in the fetus.