1 of 1

Write now Article Tools

COMPARATIVE GENOMIC HYBRIDIZATION (CGH)

Comparative genomic hybridization is a technique to study changes in copy number of DNA. Classical CGH procedure consists in labeling DNA from the "test" patient" with a particular fluorescent dye that emits green light, while DNA from a normal individual serves as the control and is labeled with a different dye that emits red light when excited by fluorescence. These two DNA samples are mixed in equal amounts and hybridized to chromosome spreads prepared from normal cells arrested in metaphase on a glass slide. After washing off the excess DNA and incubating for adequate time, the fluorescence in different areas of each chromosome is read. The color of a particular region of the chromosome depends upon the ratio of the red to green fluorescence. If the number of copies of a particular DNA sequence is increased in the patient sample compared to the test, then that particular region will appear green, while, conversely, there is a decrease in the number of copies of a particular DNA sequence in the patient DNA sample compared to the test, then, that region of the chromosome will appear red. If there is no change in the number of copies of a particular DNA sequence between the patient and the test, then that region appears yellow. Here we assume that both the test and the patient DNA samples have an equal probability of hybridizing to the DNA on the metaphase chromosomes.

This classical CGH method, though highly effective in being able to detect large areas with copy number change, suffers from certain shortcomings. Perhaps one of the most significant is that it still has a low resolution, usually upto 5-10 M. this means that a change in copy number cannot be detected if this involves a DNA sequence less than 5-10 Mb (5-10 million base pairs) long. These regions can be mapped with other techniques like spectral karyotyping and fluorescent in-situ hybridization, but in case of FISH, prior knowledge of the chromosomal region involved is essential to design a probe specific for it. Another shortcoming of this technique is that it cannot identify balanced translocations, inversions and mosaicism. In other words, this technique is good to identify only changes in number of copies of a particular DNA sequence larger than 5-10 Mb. As is usual, the technique is more sensitive for detecting amplification than deletions. Further, though it can detect changes in number of copies of a particular subregion of the DNA, it cannot detect

• 1
• 2
• 3
• next page >>

Add your voice

Know something about Molecular biology techniques: Comparative genomic hybridization?
We want to hear your view. Write now!