G banding

Technique for staining chromosomes

G banding (or Giemsa banding) is a technique used in cytogenetics to produce a visible karyotype by staining condensed chromosomes. It is one of the most common methods for identifying chromosomal abnormalities and for chromosome mapping.

Principle

G banding involves treating chromosomes with a proteolytic enzyme, such as trypsin, followed by staining with Giemsa stain. The Giemsa stain binds to the phosphate groups of DNA, but it stains regions rich in adenine (A) and thymine (T) more intensely. This results in a pattern of light and dark bands, known as G bands, along the length of each chromosome.

Procedure

The process of G banding involves several steps:

1. Cell Culture: Cells are cultured in vitro to increase their number. Lymphocytes from blood samples are commonly used.
2. Cell Harvesting: Cells are arrested in metaphase using a mitotic inhibitor such as colchicine.
3. Hypotonic Treatment: Cells are treated with a hypotonic solution to swell them, making the chromosomes more spread out.
4. Fixation: Cells are fixed using a mixture of methanol and acetic acid.
5. Slide Preparation: The fixed cells are dropped onto a microscope slide, where they burst open, spreading the chromosomes.
6. Enzyme Treatment: The slide is treated with trypsin to partially digest the proteins associated with the chromosomes.
7. Staining: The slide is stained with Giemsa stain, which binds to the DNA and produces the characteristic banding pattern.

Applications

G banding is used in various applications, including:

• Karyotyping: To identify and evaluate the size, shape, and number of chromosomes in a sample of body cells.
• Diagnosis of Genetic Disorders: To detect chromosomal abnormalities such as Down syndrome, Klinefelter syndrome, and Turner syndrome.
• Cancer Cytogenetics: To identify chromosomal changes in cancer cells, which can help in diagnosis and treatment planning.
• Prenatal Diagnosis: To assess the chromosomal health of a fetus using amniocentesis or chorionic villus sampling.

Advantages and Limitations

Advantages

• High Resolution: G banding provides a high-resolution view of chromosomes, allowing for detailed analysis.
• Reproducibility: The technique is well-established and reproducible across different laboratories.

Limitations

• Resolution Limit: G banding cannot detect very small chromosomal changes, such as microdeletions or microduplications.
• Requires Dividing Cells: The technique requires cells to be in metaphase, which limits its use to dividing cells.

Images

Human karyotype with bands and sub-bands

Human chromosomes

NHGRI human male karyotype

Related pages

• Karyotype
• Chromosome banding
• Cytogenetics
• Giemsa stain