Polymerase Chain Reaction or PCR and its Use in DNA Testing

DNA testing requires a special method of analysis known as polymerase chain reaction or PCR. DNA testing or DNA profiling, is no more an unheard term now. It has been integrated in educational systems and everyone seems to have a basic knowledge about it. In fact, this is the tool of forensic scientists as it provides accurate results and is used for the vast bulk of tests, including paternity testing. DNA determines the uniqueness of every person and thus, has DNA profiling has helped solve countless crime cases.

What is PCR?

Polymerase Chain Reaction (PCR) works by amplifying the DNA sample so that it can further be effectively used in scientific research. Continuous cycles of repeated heating and cooling, using primers and DNA polymerase generate thousands of copies of a specific DNA sequence. This generated DNA can further be exposed to a chain reaction for a number of replications. PCR testing can thus lead to varied genetic manipulations through extensive modification testing.

Why PCR testing?

Forensic and crime-scene investigation usually involves PCR. PCR testing is used in cases when the forensic evidence has become old (even up to a few decades), has degraded due to warm moist conditions or is available in small amount. The entire PCR genetic testing procedure is also fast and comparatively cheap. More important, it is a highly reliable means providing accurate DNA test results.

Basic process of PCR

PCR is an in vitro DNA replication process that uses DNA polymerases and artificial DNA primers to exponentially increase the available DNA. The thermal cycling process for PCR initially requires splitting of the DNA double helix structure. This requires a suitable DNA polymerase which is able to endure temperatures greater than 90 degrees centigrade. This process is called DNA Melting. Each strand is then cooled at a lower temperature, which is further used as a template for the production of more number of strands.

DNA structure and PCR

DNA (deoxyribonucleic acid) appears as a long narrow string- shaped segment of genes in human body. The DNA structure is referred to as a double helix, which forms through binding of two strands of nucleic acid molecules. Though the bonds are weak, the two strands are intertwined into long segments, which make them difficult to separate. However, DNA melting enzymes, called helicases, help in unwinding the strands so that sequence reading enzymes, such as DNA polymerase, can function on them.

PCR testing machines

PCR testing uses various machines and instruments during the entire process. Several components, like a data interface, detector, carrier gas line, gas flow controller and column, constitutes such a machine. For instance, Gas chromatography instrument is utilized in splitting and examining compounds, while a thermal recycler amplifies the segment size of the DNA.

Scientists involved in PCR DNA testing process have to remain cautious so that any impurities in the laboratory do not contaminate the sample, as each amplification phase will increase this contamination significantly. Laminar flow machines are useful in such cases for avoiding the impurities to come into contact with the DNA sample.

Uses of PCR

Apart from the recognized uses of genetic identification, PCR testing is widely used in various fields of medical and biological research. DNA cloning, analysis of genes and genetic fingerprints and paternity testing are some of its applications. Medical applications of this testing includes diagnosing many hereditary and infectious diseases, such as leukemia and lymphomas, in their early stages.


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