Introduction
Chromosomal abnormalities and genetic disorders have been a serious concern to humans as children born with these abnormalities suffer all their life, and these disorders do not have any definitive treatments available to date. But, scientific research and advances have helped in the early diagnosis and prevention of these conditions. Chromosomal banding is a diagnostic technique that helps observe and study genetics in humans, plants, and animals.
What Is a Chromosome?
A chromosome is a genetic material present in the nuclei of each cell. These chromosomes are made up of long thread-like material called DNA or RNA, which carries the genetic information in organisms. Every cell in the human body contains 23 pairs of chromosomes, each of which is made up of various proteins. Each chromosome is divided by the centromere into two halves called the P arm and the Q arm. The end of each chromosome is called the telomere. Each chromosome contains binding proteins called adenine, guanine, thymine, and cytosine. Any defect or addition or deletion of any chromosome leads to genetic abnormalities that are difficult to correct and can affect the infant's daily living and normal growth. These abnormalities are difficult to treat; therefore, preventive measures and diagnostic methods are being developed to prevent such disorders.
What Is the Chromosomal Banding Technique?
The chromosomal banding technique, also known as karyotyping, is a procedure where the dividing chromosomes are stained with different dyes to obtain alternate dark and light bands. These stained chromosomes are studied under a microscope. This technique is used to study the components of the cell nucleus and visualize chromosomes. Banding helps detect chromosomal abnormalities, the number of chromosomes, and their shape and size. The bands that occur after staining the chromosome can be classified as follows:
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Heterochromatic bands.
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Euchromatic bands.
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The kinetochore structure is stained distinctly.
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Nucleolar organization region bands.
History of Banding Technique:
The banding technique was first introduced in 1968 by Torbjorn Casperson. This pattern of alternate bands was first observed when they were working on plants. This staining technique was first used in humans in the year 1970. This banding of chromosomes helped differentiate each and region of these chromosomes.
What Are the Types of Banding Techniques?
Chromosomal banding techniques use various dye which stains the proteins in the chromosomes and makes the chromosomes appear dark and light; banding techniques, named based on the dye used, are as follows:
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G-Banding: A most common chromosomal staining technique used is G-banding. The technique was first developed by Maximo Drets and Margery Shaw in 1971. This is a banding technique where the chromosome is stained with Giemsa dye after treating the cells with trypsin to proteolyze excessive proteins. On staining the chromosome with Giemsa stain, the adenine, and thymine-rich regions are stained dark and are called positive G- bands. In contrast, guanine and cytosine-rich regions are lightly stained and are known as negative G-bands.
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R-Banding: Reverse banding technique, called so as this technique, provides results that are opposite to that of the G-banding technique. The positive bands of G-banding are stained negatively and vise-a-versa. During the procedure, a chromosome is denatured to digest adenine and thymine. Then it is stained with Giemsa dye to stain the guanine and cytosine-rich regions dark and the other areas light. This technique is used for chromosome identification, but the more preferred banding technique is the G-banding technique.
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Q-Banding: The chromosome is stained with quinacrine stain, and then the chromosome is exposed to ultraviolet (UV) light which makes the bands appear yellow. When exposed to UV light, quinacrine makes the bands shine brighter, and the other areas remain dark. The banding pattern differs for every chromosome. Hence chromosomal abnormalities can be detected. The results are the same as that of G banding. This is the simplest banding technique and presents more accurate results. This technique being the simplest is still avoided due to the carcinogenic properties of quinacrine.
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C-Banding: This banding technique is called the centromeric heterochromatic or CBG staining technique, as it stains the heterochromatin regions of centromeres. The procedure includes denaturing the chromosome with barium hydroxide and then staining with Giemsa stain. These cells, when viewed under a microscope, show only dark bands. This technique is primarily used to identify the Y chromosome as it is the heterochromatic chromosome in most mammals, including humans.
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T-Banding: A technique used to stain the telomeres.
What Is the Procedure for the Chromosomal Banding Technique?
The chromosomal banding technique is a simple laboratory procedure to visualize the chromosome in the metaphase of the cell cycle.
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Isolate the cell to be studied.
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Add culture media to the cell pellet.
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Let the cells divide to reach the metaphase stage.
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Add colchicine (mitotic poison) to the pellet.
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Stain the chromosomes with desired dye.
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Observe the stained cells under the microscope.
What Are the Uses of Chromosomal Banding?
These chromosomal banding techniques help in visualizing the structure of chromosomes. Therefore the use of these techniques is as follows:
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Determining the size of the chromosomes.
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The shape of the chromosomes.
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The number of chromosomes.
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Telocentric abnormalities.
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Identifying chromosomal abnormalities and genetic disorders.
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Determining the evolutionary link between chimpanzees and humans.
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Diagnosing the bone marrow conditions.
All of the information mentioned above is obtained using banding techniques.
Conclusion
Chromosomal banding is a staining technique where cells are stained with various dyes. Dividing chromosomes are arrested in the metaphase of the cell cycle, as the genetic material is highly condensed in this stage; the chromosomes can be studied distinctly. The cells after stained are observed under the high-resolution microscope, and the stained chromosomes present as alternate dark and light bands. These chromosomal staining techniques help determine chromosomal abnormalities and other genetic disorders. These techniques have gained popularity due to their ease of use and the less invasive procedure. Only a few cells are required to study the chromosome or the defect in certain organs, which are isolated and cultured. The sample for the technique is obtained from various sites like cells from bone marrow, amniotic fluid, and placenta can also be used as a sample. This technique has helped diagnose various genetic disorders like Down syndrome, Klinefelter syndrome, etc.
