Types of Chromosomal Abnormalities

by Sam Malone

Chromosomes are the building blocks of the cells. They make up the DNA that is present in the nucleus of every single cell in the body. In fact, it is the chromosomes that govern the functioning of the cell and you could call them the operator of each cell. Before we go into the abnormalities in chromosomes, let’s take a closer look at chromosomes.

What are Chromosomes?

Chromosomes are basically protein molecules arranged in a certain order. Each molecule in the chromosomes is called a nucleotide. There is nothing special about each individual nucleotide, but each chromosome is very important because of the arrangement of the nucleotide in each chromosome. There are 23 pairs of chromosomes in each cell, making 46 in total. The 22 pairs provide all the genetic material, while one pair solely deals with the sex, giving it specific genotype.

Since each chromosome has so many nucleotides, they are compacted and arranged into an X shape giving them the traditional double helix shape that we are all familiar with. Each time a cell replicates itself, it has to ensure that the chromosomes replicate correctly. Of course, this is a complicated process and there are errors sometimes; however, these aberrations are taken care of by a process called apoptosis. Apoptosis is a process of programmed cell death, where any cell that is found to have failed its ‘quality check’ is destroyed so that it cannot pass on its aberration to others. Every day about 50 to 70 billion cells die due to apoptosis. Apoptosis is very important because whenever cells don’t die, they replicate uncontrollably and turn malignant, causing cancer.

What are the different kinds of abnormalities that can occur?

Abnormalities in chromosomes are generally due to some defect in the structure of the chromosome. This results in a change in the genetic material when compared to the original, resulting in an aberration in the cell. Generally there are seven ways in which this can happen, and these have been described below.

Deletions: A deletion is when a portion of the chromosome gets deleted during cell division. This results in a loss of genetic material, resulting in an aberration.
Duplication: Duplication is when a certain portion of the chromosome gets duplicated, resulting in a net addition of genetic material. This too is an aberration.
Translocation: A translocation is when a portion of one chromosome gets added to another chromosome. This can happen in two ways. A reciprocal translocation results in an exchange of genetic material between two chromosomes, while a Robertsonian translocation results in one chromosome getting added on to another.
Inversion: An inversion happens when one portion of a chromosome breaks off, gets inverted, and then reattaches itself to the same chromosome. Although all the genetic material is there, this portion will have it inverted, changing the order of the nucleotide. Hence, this is also an aberration.
Insertion: An insertion happens when one portion of a chromosome breaks off and reattaches to another chromosome.
Rings: A ring is a unique aberration where a portion of a chromosome breaks off and instead of reattaching forms another ring within the nucleus. Once again, there is no net increase or decrease in the nucleotides, but since it is the order of the nucleotides that is important rather than the nucleotides themselves, even these form aberrations in the cells.
Isochromosome: An isochromosome is one that has formed a mirror image copy of itself. Cells with isochromosomes typically show unstable chromosomes, which result in the cells becoming prone to certain forms of malignancy.

Each living body may have a number of aberrations during cell division, but as we mentioned, these are all taken care of by apoptosis. When these malignancies are not removed by apoptosis, but allowed to grow, they cause cancer. But what happens when it happens during reproduction?

Genetic Disorders

Sometimes, an aberration forms in the chromosomes of the sperm or the egg, which means that this aberration gets replicated in each cell of the offspring. This results in genetic diseases that there is no treatment for as of now. Also, a person who has a genetic disorder will be able to pass it on to his or her offspring.

Some of the most common genetic disorders are as follows:

Hemophilia, caused by a deletion of a part of a chromosome.
Down’s syndrome, where a whole piece of a chromosome is missing, is added, or both.
Cystic fibrosis, where a deletion or insertion occurs within one chromosome.
Color blindness, where a deletion or insertion occurs within one chromosome.

There are a number of other diseases too that are caused by an aberration in the chromosomes, but they are rare. In fact, the list of possibilities is very long, and there is almost nothing that cannot go wrong when an aberration occurs in the chromosomes. Thankfully, this does not happen frequently, and most genetic defects occur in very small percentages of the population.

References

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