Karyotype is the science of sorting and arranging metaphase chromosomes according to their size, shape, and structure. The chromosomes are classified based on the position of centromeres as metacentric, submetacentric, acrocentric, and telocentric chromosomes. In metacentric chromosomes, the centromere is situated in the exact or near centre of the chromosomes dividing the chromosomes into two equal arms. In submetacentric chromosomes, the centromere is placed not exactly in the centre dividing the chromosomes resulting in long arm and short arm. In acrocentric chromosomes, the centromere is placed near the end of chromosome resulting in one large and one short arm. In telocentric chromosomes, the centromere is located in the end of chromosomes resulting in only one arm. The arms are designated as "p" and "q", the letter "p" denotes short arm and "q" for long arm.
Karyotyping can be done from any somatic cells which undergo cell division. From adults, blood is the good source as the white blood cells in blood can be stimulated to cell division. From the fetus, the amniotic fluid or placenta is used for cell culture. The cells are stimulated with mitogens which induces cell divisions in the somatic cells. The cells are treated with colchicines after few days of cell growth and division to arrest the dividing cells in metaphase stage of mitosis. The chromosomes in the metaphase are highly condensed and can be easily visualized in light microscopy for it shape and structure, which is useful for karyotyping procedure. The cells are harvested in metaphase stage and slides are prepared from staining.
The chromosomes are stained using different dyes according to the requirements. The most common staining of chromosomes is with Giemsa stain, which results in a particular banding pattern called G-banding. To localize the centromere alone C-banding is done. The chromosomes are later photographed and arranged into different groups according to the size, shape, and position of centromere.
Somatic cells of human species contains 46 chromosomes as diploid number among them 22 pairs represents autosomes and one pair represents sex chromosomes or allosomes. In humans, the 46 chromosomes are arranged in 7 groups called A, B, C, D, E, F, and G groups. A group consists of chromosomes 1, 2, and 3 which are longest chromosomes in human cells. Chromosomes 1 and 3 are metacentric, while the chromosome 2 is submetacentric. B group consists of two chromosomes, chromosome 4 and 5, which are large and submetacentric chromosomes. Group C consists of medium sized sub metacentric chromosomes, which are 6, 7, 8, 9, 10, 11, 12, and one of the sex chromosome which is X chromosome. D group consists of three medium sized acrocentric chromosomes designated as 13, 14, and 15. Group E consists of three small chromosomes as 16, 17, and 18. In Group E, chromosomes 16 is metacentric, while chromosomes 17 and 18 are sub metacentric chromosomes. F group contains 2 small metacentric chromosomes 19 and 20. Group G consists of small acrocentric chromosomes 21, 22 and one sex chromosome Y. In human Karyotype, chromosome 1 is the largest and chromosome 21 is the shortest chromosomes. Chromosomes 13, 14, 15, 21, 22, and Y chromosomes are called satellite chromosomes which has bulge structure beyond telomeric end. Human Karyotype lacks telocentric chromosomes.
The main applications of Karyotyping are in detection of chromosomal aberrations such as duplications, deletions, and translocations and finding ploidy of chromosomes. The loss of chromosomes or addition chromosomes are identified with karyotyping enables to determine the risk of the individuals and to future generations with such chromosomal variations. The aneuploidy is one of the reasons for miscarriages in the aged mothers which occur due to the unequal cell division during egg (ovum) development. As the age of the mother increases, the risk of eggs with aneuploidy also increases. The fertilization of eggs having aneuploidy chromosomes results in fetus with syndromes such as Trisomy 18 (47, XX or XY + 18), Trisomy 21 (47, XX or XY + 21), Down syndrome (45, XO), and klinefelter syndrome (47, XXY). Further karyotyping of the fetus helps to detect the birth defects, which can be used for treatment of such cases. Karyotyping is a simple cytological technique, but has many applications in the field of medicine in this age of nanotechnology.
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