What is the Difference Between Nondisjunction and Translocation Mutations

The main difference between nondisjunction and translocation mutations is that nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division whereas translocation is the exchange of sections of DNA between two, non-homologous chromosomes. Furthermore, nondisjunction results in the abnormal number of chromosomes in the daughter cells while translocation results in chromosomal rearrangements.  

Nondisjunction and translocation are two types of chromosomal mutations which bring changes to the structure or number of chromosomes. 

Key Areas Covered 

1. What is a Nondisjunction
     – Definition, Types, Effect
2. What is a Translocation
     – Definition, Types, Effect
3. What are the Similarities Between Nondisjunction and Translocation Mutations
     – Outline of Common Features
4. What is the Difference Between Nondisjunction and Translocation Mutations
     – Comparison of Key Differences

Key Terms 

Aneuploidy, Chromosomal Mutations, Nondisjunction, Nonreciprocal Translocation, Reciprocal Translocation 

What is a Nondisjunction  

Nondisjunction is a type of chromosomal abnormality which occurs due to the failure of homologous chromosomes or sister chromatids to separate from each other during cell division. Thus, this results in the abnormal chromosome number in the daughter cells. Generally, there are three types of nondisjunction:

• During meiosis I, the failure of the homologous chromosomes to separate from each other.
• In the process of meiosis II, the failure of sister chromatids to separate from each other results in nondisjunction.
• And, the third type of nondisjunction occurs during mitosis due to the failure of sister chromatids to separate from each other. 

Here, the first two types of nondisjunction forms result in abnormal chromosome number in gametes while the third form of nondisjunction results in abnormal chromosome number in the somatic cells.  

Figure 1: Nondisjunction During Meiosis

Furthermore, aneuploidy refers to the presence of abnormal chromosome numbers in a cell. Aneuploidy conditions result in different disease conditions. The two main types of aneuploidy in humans are monosomy and trisomy. Monosomy is the lack of a single chromosome in the cell. Monosomy in sex chromosomes results in Turner syndrome (monosomy of the X chromosome in females). It is the only type of monosomy compatible with life. Trisomy is the presence of three copies of a particular chromosome. In humans, it often occurs in the chromosomes 21, 18, and 13. The most common viable trisomy is the trisomy of chromosome 21 or Down syndrome. In addition, trisomy of chromosome 13 results in Patau syndrome. Also, the trisomy of chromosome 18 results in Edwards syndrome. On the other hand, the two trisomy conditions in sex chromosomes result in the Klinefelter Syndrome (XXY in male) and triple X syndrome (XXX in female).  

What is a Translocation  

Translocation is another type of chromosomal abnormality which occurs due to the exchange of sections of DNA between nonhomologous chromosomes. And, this results in the rearrangements of chromosomes. It drastically alters the size and the position of the centromere of both chromosomes. The two main types of translocations are the reciprocal translocation and non-reciprocal translocation. In reciprocal translocations, exchange of two segments of DNA takes place between two nonhomologous chromosomes. On the other hand, non-reciprocal translocations, a segment of one chromosome, chunks off from one chromosome and moves to the other.  

Figure 2: Reciprocal Translocation

In some cases, some translocations cause symptoms similar to the loss or gain of function of the entire chromosome. For example, though the presence of an extra chromosome 21 causes Down syndrome, the translocation of a large segment of chromosome 21 results in Down syndrome as well. Moreover, translocations result in several forms of cancers, including leukemia and infertility.  

Similarities Between Nondisjunction and Translocation Mutations 

• Nondisjunction and translocation are two types of chromosomal mutations which cause abnormalities in chromosomes. 
• Since their changes are huge, the effect of this type of mutations can be huge or lethal. 

Difference Between Nondisjunction and Translocation Mutations 

Definition 

Nondisjunction refers to the failure of one or more pairs of homologous chromosomes or sister chromatids to separate normally during nuclear division, usually resulting in an abnormal distribution of chromosomes in the daughter nuclei. But, translocation refers to a movement of a chromosomal segment from one position to another; this movement can be within the same chromosome or to a another chromosome. Thus, this is the main difference between nondisjunction and translocation mutations.

Type of Change 

Also, the type of changes they bring is a major difference between nondisjunction and translocation mutations. Nondisjunction brings changes to the number of chromosomes while translocations bring changes to the structure of chromosomes. 

Causes  

Moreover, the failure of homologous chromosomes or sister chromatids to separate properly during cell division results in nondisjunction while the exchange of segments of DNA between two, non-homologous chromosomes results in translocations. Hence, this also accounts to the difference between nondisjunction and translocation mutations.

Effect 

Their effect is another major difference between nondisjunction and translocation mutations. Nondisjunction results in aneuploidy while translocation results in cancer, infertility, and Down syndrome. 

Conclusion 

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. Thus, this results in the abnormal number of chromosomes in the daughter cells. On the other hand, translocation is the exchange of segments of chromosomes between two, nonhomologous chromosomes. And, this results in the alteration of the size and the position of the centromere of chromosomes. Both nondisjunction and translocation mutations bring considerable changes to the genome. Therefore, the main difference between nondisjunction and translocation mutations is the type of change they bring to the chromosomes.

References:

1. Gottlieb SF, Tegay DH. Genetics, Nondisjunction. [Updated 2018 Oct 27]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2018 Jan-. Available Here
2. Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000. Translocations. Available Here 

Image Courtesy:

1. “Nondisjunction Diagrams” By Tweety207 – Own work (CC BY-SA 3.0) via Commons Wikimedia  
2. “Translocation-4-20” By Courtesy: National Human Genome Research Institute – [1] (file) (Public Domain) via Commons Wikimedia