What is a Karyotype?

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A karyotype is a picture of all the chromosomes from an individual’s cells. A karyotype is a test used to check for chromosome abnormalities. A picture of a person’s chromosomes is created by staining the chromosomes with a special dye, photographing them through a microscope and arranging them in pairs. A karyotype gives information about the number of chromosomes a person has, the structure of their chromosomes and the sex of the individual. Down syndrome is diagnosed by a karyotype test. Down syndrome is a congenital condition caused by an extra chromosome. For example, if a human karyotype depicts that a patient has three copies of the 21st chromosome, instead of two, the patient is diagnosed with Trisomy 21, also known as Down's syndrome.

 

Karyotypes describe the number of chromosomes, and what they look like under a light microscope. Attention is paid to their length, the position of the centromeres (centromere is a region of DNA typically found near the middle of a chromosome), banding pattern, any differences between the sex chromosomes, and any other physical characteristics. The study of whole sets of chromosomes is sometimes known as “karyology”. The chromosomes are depicted (by rearranging a microphotograph) in a standard format known as a “karyogram or idiogram”: in pairs, ordered by size and position of centromere for chromosomes of the same size.

 

What is the History of Karyotype?

Chromosomes were first observed in plant cells by Karl Wilhelm von Nägeli, a Swiss botanist in 1842. The name was coined by another German anatomist, Von Waldeyer, a German anatomist, in 1888. The next stage took place after the development of genetics in the early 20th century, when it was appreciated that the set of chromosomes (the Karyotype) was the carrier of the genes. Investigation into the human Karyotype took many years. It was not till mid 1950s, that it became generally accepted that the karyotype of humans included only 46 chromosomes.

 

What are the Observations on karyotypes?

  • Staining: The study of karyotypes is made possible by staining. Staining is an auxiliary technique used in microscopy to enhance contrast in the microscopic image. Basically, a particular dye, such as Giemsa is applied after cells have been arrested during cell division by a solution of colchicine. Colchicine is a toxic natural product originally extracted from plants. For humans, white blood cells are used most often since they are easily induced to divide and grow in tissue culture. Tissue culture is the growth of tissues and/or cells separate from the organism. At times, observations may be made on non-dividing (interphase) cells. The sex of an unborn fetus can be determined by observation of interphase You do not have access to view this node.
  • Human Karyoptype: Most (but not all) species have a standard karyotype. The normal human karyotypes contain 22 pairs of autosomal chromosomes and one pair of sex chromosomes. Normal karyotypes for females contain two X chromosomes and are denoted 46,XX; males have both an X and a Y chromosome denoted 46,XY. Any variation from the standard karyotype may lead to developmental abnormalities.

 

What are the characteristics of Karyotypes?

Six different characteristics of karyotypes are usually observed

  • Differences in the absolute sizes of chromosomes. Chromosomes can vary in absolute size by as much as twenty-fold between genera of the same family.
  • Differences in the position of centromeres. Centromere is a region of DNA typically found near the middle of a chromosome.
  • Differences in relative size of chromosomes can only be caused by segmental interchange of unequal lengths.
  • Differences in the basic number of chromosomes. Humans have one pair fewer chromosomes than the great apes.
  • Differences in number and position of satellites, which (when they occur) are small bodies attached to a chromosome by a thin thread.
  • Differences in degree and distribution of heterochromatic regions. Heterochromatin is a tightly packed form of DNA, which comes in different varieties.

 

How to depict Karyotypes?

Cytogenetics employs several techniques to visualize different aspects of Karyotypes. Cytogenetics is a branch of genetics that is concerned with the study of the structure and function of the You do not have access to view this node, especially the chromosomes

  • Classic karyotype cytogenetics: In the "classic" (depicted) karyotype, a dye, often Giemsa (G-banding), less frequently Quinacrine, is used to stain bands on the chromosomes. Giemsa is specific for the phosphate groups of DNA. Quinacrine is a drug with a number of different medical applications. Quinacrine binds to the adenine-thymine-rich regions. Adenine and thymine are nucleobases, parts of DNA and RNA. Each chromosome has a characteristic banding pattern that helps to identify them; both chromosomes in a pair will have the same banding pattern. Karyotypes are arranged with the short arm of the chromosome on top, and the long arm on the bottom. Some karyotypes call the short and long arms p and q, respectively.
  • Spectral karyotype (SKY technique): Spectral karyotyping is a molecular cytogenetic technique used to visualize all the pairs of chromosomes in an organism in different colors at the same time. Fluorescently labeled probes for each chromosome are made by labeling chromosome-specific DNA with different fluorophores. Fluorescence is the emission of light by a substance that has absorbed light. A fluorophore, in analogy to a chromophore, is a component of a molecule which causes a molecule to be fluorescent. Since there are a limited number of spectrally-distinct fluorophores, a combinatorial labeling method is used to generate many different colors. Spectral differences generated by combinatorial labeling are captured and analyzed by using an interferometer attached to a fluorescence microscope. Image processing software then assigns a pseudo color to each spectrally different combination, allowing the visualization of the individually colored chromosomes. This technique is used to identify structural chromosome aberrations in cancer cells and other disease conditions when Giemsa banding or other techniques are not accurate enough.
  • Digital karyotyping: Digital karyotyping is a technique used to quantify the DNA copy number on a genomic scale. Short sequences of DNA from specific loci all over the genome are isolated and enumerated. This is method is also known as virtual karyotyping.
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