What is PCR? Describe in detail.
Polymerase Chain Reaction (PCR) is a technique developed by Kary B. Mullis to amplify a very small amount of DNA into thousands to millions of copies. Often called ‘molecular photocopying’, it is a significant scientific advance that earned Mullis the Nobel Prize for Chemistry in 1993.
The technique is based on the principle of DNA replication. It is a chain reaction carried out in repeated cycles of heating and cooling, known as thermal cycling, inside a machine called a thermocycler. It requires a heat stable DNA polymerase called Taq polymerase, which functions at high temperatures (72-78°C), along with primers and nucleotides.
PCR involves three main steps: denaturation, annealing, and extension. In denaturation, the DNA is heated to 94–95°C to break hydrogen bonds and separate the strands. Each strand serves as a template for new synthesis.
In annealing, the temperature is lowered to 50–55°C. This allows the specific primers to anneal to their complementary sites on the template strands. In extension, the temperature is raised to 72°C, and Taq polymerase adds deoxyribonucleotides (dNTPs) to the 3′ ends of the primers to create new strands.
The cycles repeat, exponentially increasing the DNA copies. Variations of this technique include reverse transcription PCR (RT-PCR) and real-time quantitative PCR (real-time qPCR).
Explanation
The answer strictly follows the NCERT textbook content provided in the context. It defines PCR, names the inventor (Kary B. Mullis), and explains the core principle involving thermal cycling and the use of Taq polymerase. It details the three specific steps—Denaturation (94-95°C), Annealing (50-55°C), and Extension (72°C)—with their respective temperature ranges and biological functions as described in the source text. Finally, it mentions the types of PCR (RT-PCR and real-time qPCR) to ensure the 'describe in detail' requirement is met comprehensively for a 5-mark question.