G banding

G banding, also known as Giemsa banding, is a technique used in cytogenetics to produce a visible karyotype by staining condensed chromosomes. It is a critical tool for identifying chromosomal abnormalities and for genetic and medical research. The technique involves treating chromosomes with the Giemsa stain, which binds to the phosphate groups of DNA, resulting in a series of dark and light bands across the chromosomes.

Overview

G banding is the most common method used to visualize chromosomes and their features. The process involves arresting cells in mitosis, a stage of cell division where chromosomes are most condensed and visible under a microscope. The cells are then treated to spread and flatten the chromosomes on a slide, followed by staining with Giemsa. The staining process preferentially binds to regions of DNA that are rich in adenine-thymine (A-T) base pairs, producing a distinct pattern of bands. Each chromosome can be identified by its unique banding pattern, which is useful for diagnosing genetic diseases and abnormalities.

Procedure

The G banding procedure includes several steps: 1. Cell Culture: Cells, often from blood or bone marrow, are cultured to encourage division. 2. Colcemid Treatment: Colcemid, a chemical that disrupts the spindle fibers, is added to the culture to arrest cells in metaphase. 3. Hypotonic Treatment: Cells are exposed to a hypotonic solution, causing them to swell and making the chromosomes more spread out. 4. Fixation: Cells are fixed with a mixture of acetic acid and methanol to preserve their structure. 5. Staining: The slide is stained with Giemsa stain, which highlights the chromosomes in a banding pattern. 6. Analysis: Chromosomes are examined under a microscope, and images are captured for analysis.

Applications

G banding is used extensively in clinical and research settings:

• Diagnosis of Genetic Disorders: It helps in identifying chromosomal abnormalities such as deletions, duplications, inversions, and translocations, which are associated with genetic disorders like Down syndrome, Klinefelter syndrome, and chronic myelogenous leukemia.
• Cancer Research: G banding is used to identify chromosomal abnormalities in cancer cells, which can provide information on prognosis and treatment options.
• Evolutionary Biology: Comparative G banding analysis across different species can provide insights into evolutionary relationships and chromosomal evolution.

Limitations

While G banding is a powerful tool, it has limitations:

• Resolution: G banding can only detect changes larger than 5-10 million base pairs, meaning smaller genetic alterations may go unnoticed.
• Quality of Specimens: The quality of the chromosome preparation can affect the clarity and detail of the banding pattern, potentially obscuring important details.

See Also

• Cytogenetics
• Karyotype
• Chromosome
• FISH (Fluorescence In Situ Hybridization)
• Comparative Genomic Hybridization