Which of the following is not true about PCR -a) Thermostable enzyme is usedb) Annealing is done after DNA denaturationc) Specific primers re-requiredd) Required atleast 1 week time for synthesise) DNA polymerase has to be added on each cycle

## **Core Concept**
The Polymerase Chain Reaction (PCR) is a laboratory technique used to amplify specific segments of DNA. This method relies on thermal cycling, which involves repeated heating and cooling steps to denature DNA, anneal primers, and extend the annealed primers. A thermostable DNA polymerase is crucial for this process.

## **Why the Correct Answer is Right**
The correct answer can be deduced by analyzing each statement in relation to how PCR works.
- a) Thermostable enzyme is used: This is true. The most commonly used thermostable DNA polymerase in PCR is **Taq polymerase**, derived from the bacterium *Thermus aquaticus*.
- b) Annealing is done after DNA denaturation: This is true. The process of PCR involves denaturation of DNA (separating the double strands), followed by annealing (where primers bind to their complementary sequences on the DNA strands), and then extension (where the DNA polymerase synthesizes new DNA strands).
- c) Specific primers are required: This is true. Specific primers are necessary to define the segment of DNA that will be amplified.
- d) Required at least 1 week time for synthesis: This is not true. PCR is known for its rapidity, allowing for the amplification of DNA sequences in a matter of hours, not days or weeks.
- e) DNA polymerase has to be added on each cycle: This is not true for PCR using thermostable DNA polymerases like **Taq polymerase**. The enzyme's thermostability allows it to withstand the high temperatures of the denaturation step without losing activity, eliminating the need for adding fresh enzyme for each cycle.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Not provided as it is the correct answer.
- **Option B:** Not provided as it is not the correct answer based on the question's request.
- **Option D:** This option (d) states that PCR requires at least 1 week for synthesis, which is incorrect because PCR can amplify DNA in a few hours.
- **Option E:** This option (e) suggests that DNA polymerase needs to be added on each cycle, which is incorrect for PCR that utilizes thermostable DNA polymerases.

## **Clinical Pearl / High-Yield Fact**
A key point to remember about PCR is its ability to rapidly amplify specific DNA sequences. This technique is foundational in molecular biology and has numerous applications in diagnostics, research, and forensic science. The use of **Taq polymerase** and the cycling through denaturation, annealing, and extension phases are critical for its success.

## **Correct Answer: D. Required atleast 1 week time for synthesis**