‘Contamination can be challenge to DNA tests’

DURGADAS P KASBEKAR, HEAD, LAB OF NEUROSPORA GENETICS, CDFD

NEWDELHI :Blood samples are a dependable source of DNA. After the DNA molecule is isolated from the blood cells during testing , millions of copies are made using polymerase chain reaction (PCR). The stretches of the DNA are then split to generate a DNA fingerprint, which is unique to each individual. These are then matched with available samples.

In a telephonic conversation with HT’s Malavika

Vyawahare, the head of the Laboratory of Neurospora Genetics, at Centre for DNA Fingerprinting and Diagnostics (CDFD) , Dr Durgadas P Kasbekar explained the nuances of DNA testing.

What is DNA testing used for in India?

DNA testing is finding increasing use in India, though it’s largely for checking parent-child relationships. It’s also been used to analyse DNA in semen stains and vaginal swabs in sexual assault cases, for analysing DNA in bones or teeth recovered from remains at disaster sites to aid identification, and in sweat or spit from cloth retrieved from crime scenes.

Courts, however, only admit the results of DNA tests done in government labs.

How reliable is the method?

DNA extracted from an undegraded and plentiful source, such as a drop of blood taken on a filter paper in a clinical setting, is very reliable.

The same sources of blood give the same results every time, so the results are very reproducible. Tooth and bone samples also give plentiful DNA. But unlike blood drops, one cannot repeatedly take teeth and bones (even from postmortem remains) to demonstrate reproducibility.

What are the main challenges to this method of identification?

Maintaining a reliable “chain of custody” for the source of DNA is very important.

The source of DNA being minuscule and possible contamination with cells of other people who might have handled the source pose big challenges.

For instance, an abortus might be put into formalin, presumably for tissue preservation, but the formalin makes the DNA unextractable; or if the DNA is from soft tissues that suffer rapid decomposition by bacteria and other microbes.