Introduction to PCR
Polymerase Chain Reaction (PCR) is a powerful molecular biology technique widely used to amplify specific fragments of DNA. Typically, PCR can amplify DNA stretches up to 10 kb (kilobase pairs), although advanced variations allow amplification of fragments up to 40 kb.
At the heart of PCR lies the process of thermal cycling, a sequence of rapid heating and cooling performed in a machine called a PCR thermocycler. Each cycle consists of three essential steps denaturation, annealing, and extension that are repeated multiple times to exponentially increase the number of DNA copies.
Step 1: Denaturation
In the denaturation step (94–98 °C for 20–30 seconds), the hydrogen bonds between complementary DNA bases break, separating the double-stranded DNA into two single strands.
Most PCR thermocyclers use small plastic tubes placed in metal heating blocks that rely on the Peltier effect (reversing electrical currents) for heating and cooling. While effective, this process can be relatively slow and sometimes uneven.
Modern systems, such as the XXpress thermocycler, employ innovative methods like resistive heating elements and forced air cooling to achieve faster, more accurate, and uniform temperature changes, improving PCR efficiency.
Step 2: Annealing
During the annealing step (54–65 °C for 20–40 seconds), DNA primers (short synthetic DNA sequences) attach to the target region on the single-stranded DNA. These primers bind specifically to the 3′ ends of both the sense and antisense DNA strands, defining the start and end points of amplification.
Next, a DNA polymerase enzyme binds to the primers. Commonly used enzymes include:
Taq polymerase (from Thermus aquaticus)
Pfu polymerase (from Pyrococcus furiosus)
Both enzymes are heat-stable, meaning they can withstand repeated temperature shifts without losing activity. This eliminates the need to add fresh enzyme at each cycle.
Step 3: Extension (Elongation)
At 72 °C, the DNA polymerase synthesizes new DNA strands by adding nucleotides (the building blocks of DNA) to the bound primers. This process creates complementary strands, effectively doubling the DNA with each cycle.
Each new DNA fragment consists of one original strand and one newly synthesized strand, which can serve as templates in the next round of amplification. After 20–40 cycles, the reaction can yield over one billion copies of the original DNA segment, known as amplicons.
A final elongation step (70–74 °C for 5–15 minutes) ensures that any remaining single-stranded DNA fragments are fully extended, producing complete double-stranded DNA products.