How does PCR work?

PCR (polymerase chain reaction) replicates DNA sequences through thermal cycling, a process that involves cyclical heating and cooling. 

The PCR process involves three main stages – denaturing, annealing, and extending. 

Stage 1 - Denaturing

In this first stage, a specialized machine known as a thermocycler is used to heat the sample. The heat causes the double-stranded DNA in the sample to separate into two pieces of single-stranded DNA. 

Stage 2 - Annealing

In the second stage, the temperature is lowered so that the reaction cools down. This enables the DNA primers to attach to the template DNA sequences.  

Stage 3 - Extending

The sample is heated again in the third stage. The higher temperature allows the enzyme Taq polymerase to add DNA bases to the template, duplicating the original DNA sample and resulting in the creation of two DNA strands. The thermocycling machine is capable of automating the process and repeating the three stages as many times as required, doubling the number of DNA copies each time. A complete PCR reaction can be performed in a few hours or even less. On completing PCR, the quantity and size of DNA fragments products is analyzed using a technique known as electrophoresis.