1 of 1

Write now Article Tools

Page 2 of 3

changes in the number of copies of a particular chromosome (aneuploidy/polyploidy). Other techniques like FISH can help detect these changes.

Microarray Based CGH: a step ahead
Array based CGH is a step ahead of conventional CGH and tries to combine the features of conventional CGH and FISH to achieve greater resolution, does away with the preparation of chromosome spreads and instead involves using cloned DNA fragments spotted onto slides and importantly is highly specific, sensitive and takes less time due to semi-automation of certain steps. Thus it is a high-throughput technique.

In this method, regions of the DNA are chosen which are not repetitive and these are cloned using various systems, most commonly Bacterial artificial chromosomes (BAC). These BAC systems can hold upto 350kb of DNA. These cloned fragments are then spotted onto a glass slide. The sequence of each spot and its location on the chromosome are known. Next, DNA from the patient and a normal control, labeled with green and red dye respectively are hybridized to the array (usually reactions are done in triplicate). After incubation, the fluorescence is read and averaged digitally to obtain the average relative fluorescence ratio between the patient and normal samples. This ratio is usually expressed as log values (for e.g. 0.5 times logarithmic change corresponds to a 2-fold change (increase/decrease)). A value more than 2-fold is considered significant. By cloning overlapping regions of DNA, we can narrow down the region which is differentially amplified/deleted in the study subject. By using this technique, it has been possible to identify change in copy number of a region as small as 40 kb (by cloning smaller and smaller fragments of DNA).

Advantages of array CGH
The array CGH, like conventional CGH allows genomewide screening for changes in copy number. However, unlike the conventional CGH, the array method has a higher resolution, thus it is possible to pinpoint exactly the region where the change has occurred.

Some applications of CGH
The aberrations that can be detected by CGH can be divided for the sake of convenience into numerical and structural types.
Numerical aberrations include the Down's syndrome which is characterized classically by a trisomy of the 20th chromosome, or in other cases by duplication of only a part of the chromosome, unbalanced rearrangements of the chromosome or mosaicism of the chromosome. This can be detected by CGH even in cases where FISH is negative.
Structural

• << previous page
• 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!