Nondisjunction in Meiosis — Overview & Impact on Cells

Nondisjunction

During cell division, DNA is duplicated and then divided equally into two daughter cells. This way, each new cell contains all the genetic information it needs to function.

Nondisjunction occurs when chromosomes fail to separate during division, causing chromosome abnormalities. This can occur in meiosis or mitosis. Here's more on nondisjunction.

During mitosis, it can occur during anaphase. This results in two diploid daughter cells. One cell will have an extra chrosmome ($2n+1$). This is called trisomy, which often happens with chromosome 21. The other cell will have a missing chromosome ($2n-1$). This is called monosomy. Nondisjunction during mitosis will be limited to one cell or one type of tissue because it originates in a somatic (non-sex) cell.

When nondisjunction occurs during meiosis, it can happen during anaphase I or anaphase II. When it occurs during anaphase I (as seen in the diagram below, on the right), the homologous chromosomes do not separate. The cells then go through meiosis II normally, resulting in four possible cells. Since this is meiosis, these cells are called gametes and are haploid. All daughter cells are affected. Two of the gametes will have an extra copy of the chromosome and the other two will not get any copies of that chromosome.

The bottom row of the diagram shows what the cells would look like after fertilization. Two of the cells would have an extra chromosome ($2n+1$) and two cells have a missing chromosome ($2n-1$). Notice that the diagrams only include the chromosome which undergoes nondisjunction, meaning that the "empty" cells actually contain other chromosomes which did not undergo nondisjunction.

When nondisjunction occurs in meiosis II, cells divide normally during anaphase I (homologous chromosomes separate correctly), but sister chromatids fail to separate during anaphase II (as seen in the diagram above, on the left). This again results in four possible haploid gametes. Two of the gametes have the normal, single copy of the chromosome, one will have two copies of the chromosome, and one won't have any copies of the chromosome.

The bottom row shows what the cells would look like after fertilization. Here only two of the cells are affected. One cell shows trisomy ($2n+1$) and the other shows monosomy ($2n-1$).

(Video) Visualizing Nondisjunction

by Jack Lewis

This video describes meiotic nondisjunction, a process where chromosomes don't split correctly during anaphase. This results in a sex cell that either has too many or too few chromosomes.

What's Nondisjunction?

Nondisjunction is an error during meiosis that creates eggs or sperm cells that have the wrong number of chromosomes.

Alright, recall that meiosis is the process of making haploid gametes from diploid cells. During fertilization, two of these haploid cells combine to create a new diploid cell with a set of chromosomes from each haploid. This basically means a diploid cell that has a chromosome from each parent.

Well, sometimes meiosis goes a bit wrong and the chromosomes do not separate properly during anaphase. When this happens, you end up with haploids that have a different number of chromosomes–either one too many or one too few. If one of these haploid cells becomes fertilized, the offspring produced will have an abnormal number of chromosomes.

$$\color{#8925AE}{\text{Vocabulary Review:}}$$

$\color{#8925AE}{\text{Interphase}}$ : The longest part of the cell cycle where the cell grows and DNA is replicated to prepare for nuclear division

$\color{#8925AE}{\text{Nuclear Division}}$ : The process of dividing the nucleus

$\color{#8925AE}{\text{Meiosis}}$ : The process of dividing one nucleus into four unique haploid cells. Only occurs in sex cells

$\color{#8925AE}{\text{Cytokinesis}}$ : The final step of cell division where the cytoplasm is divided to form daughter cells

$\color{#8925AE}{\text{Diploid Cell}}$ : Cell that contains two sets of chromosomes

$\color{#8925AE}{\text{Haploid Cell}}$ : Cell that contains only half the number of chromosomes. These are sperm and egg cells