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Nondisjunction

Definition:

The term “Nondisjunction” refers to the type of chromosome in which the homologous chromosomes will fail to divide properly during the process of cell division. The process would result in gametes with an additional or a missing chromosome. The condition that involves the loss or gain of the single chromosome is referred to as “aneuploidy”. The impacts of this phenomenon will be quite severe and they would cause several medical conditions.

Understanding the Nondisjunction Mechanism:

To start with, the homologous chromosomes are the identical ones (sister chromatids), which would occur in pairs. Each member of this pair will be inherited from each parent. Thus, the humans contain 46 chromosomes or in other words, 23 pairs. Now, let’s try to understand the nondisjunction mechanism in different types of cell division:

Mitosis:

As one might be aware already, the somatic cells will follow the mitosis type of cell division. From every parent cell, two daughter cells will be created by mitosis. In the normal cell division process, these homologous chromosomes will pair up and get aligned to the cell’s equator. In case of normal mitosis, these chromatids will separate and each one will go into one daughter cell. In contrast to this, when nondisjunction takes place, the chromatids will not separate. Thus, one cell would take up both the chromatids, while the other will get nothing. As a result, at the end of mitosis, all the daughter cells would have an abnormal number of chromosomes.

Meiosis:

The eggs and sperms will follow the meiosis type of cell division. As such, one cell will be divided to give four daughter cells through two phases of meiosis: meiosis-I and meiosis-II. Here, meiosis-I is more or less like the mitosis (described above). During the anaphase stage, the disjunction would occur in a normal meiosis-I. On the contrary, when there is nondisjunction, homologous chromosomes will not separate. As a result of this, one cell would have both the chromatids, which the other cell will have no chromatid at all. Thus, when these abnormal cells will proceed further with the meiosis-II, all the resulting cells will be abnormal.

It’s to be noted here that the nondisjunction can occur in the second phase of meiosis even if it did not take place in the meiosis-I. During the second phase of meiosis, each cell would split up from diploid to haploid as a preparation for the fertilization process. If these pairs of sister chromatids do not separate during the anaphase, then one of the daughter cells would have both the chromatids, while the other would not have any.

Human Aneuploid Conditions:

Aneuploidy can be called as monosomy when the daughter cells with the error contain one chromosome missing from its pairs and can be called as trisomy when the daughter cells with the error contain one chromosome as additional to its pairs. When an aneuploid gamete is subjected to fertilization, several syndromes might arise. Examples of such aneuploid conditions are as follows:

Human Monosomy Conditions:

Turner Syndrome:

This is found to be the only survivable condition in humans, which results when the affected person in monosomic for the “Chromosome X”. This is one of the conditions of sex chromosome aneuploidy.

Human Trisomy Conditions:

Down syndrome:

This syndrome is the result of trisomy of “Chromosome 21”. This is the most common aneuploidy condition in humans. Most of the cases of Down syndrome are found to be occurring due to the nondisjunction during the maternal meiosis-I.

Edwards syndrome:

This is the result of trisomy of “Chromosome 18”.

Patau syndrome:

This is the result of trisomy of “Chromosome 13”.

Klinefelter syndrome:

This is the most common type of sex chromosome aneuploidy in humans. This is the result of trisomy of “Chromosome 47”, wherein the chromosome will be XXY. This is the most common cause of infertility in males.

XYY Male:

This is the result of trisomy of “Chromosome 47”, wherein the chromosome will be XYY in males. This usually occurs due to the nondisjunction during the paternal meiosis-II.

Trisomy X:

This is the sex chromosome aneuploidy in females, where there will be three X chromosomes in place of two.

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