Karyotyping

 Karyotyping is a laboratory technique used to visualize and analyze the number, size, and shape  of chromosomes in a cell. This process is essential for detecting chromosomal abnormalities that  can lead to genetic disorders. Karyotyping is typically performed on cells in the metaphase stage  of mitosis when chromosomes are most condensed and easily visible under a microscope. 

Steps in Karyotyping 

1. Sample Collection: 

o Sources: Common sources of cells for karyotyping include blood (white blood  cells), bone marrow, amniotic fluid (for prenatal testing), or tissue samples.

o Cell Culture: The collected cells are cultured in a laboratory to increase their  number and ensure that a sufficient number of cells are in the metaphase stage.

2. Cell Harvesting: 

o Arresting Mitosis: Cells are treated with a mitotic inhibitor like colchicine,  which stops cell division at metaphase, where chromosomes are most visible. o Hypotonic Treatment: Cells are then placed in a hypotonic solution, causing  them to swell. This spreads the chromosomes within the cell, making them easier  to observe. 

3. Chromosome Staining: 

o Giemsa Stain (G-banding): The most common staining technique is G-banding,  which uses Giemsa stain to produce a distinct banding pattern on chromosomes.  These bands represent areas of different densities of chromatin, allowing for the  identification of each chromosome. 

4. Microscopic Examination: 

o Slide Preparation: The stained cells are placed on a microscope slide, and the  chromosomes are observed under a light microscope. 

o Photomicrography: High-resolution images of the chromosomes are taken. 5. Karyotype Analysis:

o Arrangement: Chromosomes are arranged in pairs according to their size,  banding pattern, and centromere position, from largest to smallest. The sex  chromosomes (X and Y) are usually placed at the end. 

o Karyogram: The resulting image, known as a karyogram, shows the complete set  of chromosomes in an individual’s cells. A typical human karyotype consists of  46 chromosomes, arranged into 23 pairs. 

Applications of Karyotyping 

1. Diagnosis of Genetic Disorders: 

o Aneuploidy: Karyotyping can detect abnormalities in chromosome number, such as trisomy (an extra chromosome) or monosomy (a missing chromosome). For example: 

▪ Down Syndrome: Trisomy 21, where there is an extra copy of  chromosome 21. 

▪ Turner Syndrome: Monosomy X, where there is only one X  chromosome. 

o Structural Abnormalities: Karyotyping can also identify structural changes in  chromosomes, such as: 

▪ Translocations: When a segment of one chromosome is transferred to  another chromosome. 

▪ Deletions: Loss of a chromosome segment. 

▪ Duplications: Extra copies of a chromosome segment. 

▪ Inversions: A segment of a chromosome is reversed end to end. 

2. Prenatal Testing: 

o Amniocentesis: Karyotyping of fetal cells obtained from amniotic fluid can  detect chromosomal abnormalities before birth. 

o Chorionic Villus Sampling (CVS): Another prenatal test where cells from the  placenta are analyzed. 

3. Cancer Research and Diagnosis: 

o Chromosomal Aberrations: Certain cancers are associated with specific  chromosomal changes. For example, the Philadelphia chromosome is a 

translocation between chromosomes 9 and 22, found in chronic myeloid leukemia  (CML). 

4. Infertility Investigations: 

o Chromosome Analysis: Karyotyping can help identify chromosomal  abnormalities that might be causing infertility or recurrent miscarriages. 

5. Species Identification and Evolutionary Studies: 

o Comparative Karyotyping: Karyotypes can be compared between species to  study evolutionary relationships. 

Limitations of Karyotyping 

∙ Resolution: Karyotyping cannot detect very small chromosomal changes, such as  microdeletions or duplications. More advanced techniques like fluorescence in situ  hybridization (FISH) or comparative genomic hybridization (CGH) are required for  higher-resolution analysis. 

∙ Time-Consuming: The process of culturing cells and preparing karyotypes can be time consuming, often taking several days. 

Conclusion 

Karyotyping remains a fundamental tool in genetics, providing valuable insights into  chromosomal structure and abnormalities. It plays a crucial role in diagnosing genetic disorders,  understanding cancer, and conducting prenatal screening. Despite its limitations, karyotyping  continues to be widely used due to its ability to visualize the entire chromosome set in a single  analysis.