Polymerase chain reaction (PCR) is used for

Options

A. In vivo replication of specific DNA sequence using thermostable DNA polymerase
B. In vitro synthesis of mRNA
C. In vivo synthesis of mRNA
D. Separation of DNA fragments according to their size.

Answer

### Step-by-Step Solution for the Question: "Polymerase chain reaction (PCR) is used for"

1. **Understanding PCR**: Polymerase Chain Reaction (PCR) is a technique used to amplify specific DNA sequences. It allows for the production of millions of copies of a particular DNA segment from a small initial sample.

2. **Process of PCR**:
- **Denaturation**: The first step involves heating the DNA sample to a high temperature (around 94-98°C). This causes the double-stranded DNA to separate into two single strands.
- **Annealing**: The temperature is then lowered (around 50-65°C) to allow primers to bind (anneal) to the specific target sequences on the single-stranded DNA.
- **Extension**: The temperature is raised to about 72°C, which is optimal for the enzyme Taq polymerase. This enzyme synthesizes new DNA strands by adding nucleotides to the primers, effectively extending the DNA strand.

3. **Amplification**: The cycle of denaturation, annealing, and extension is repeated typically 25-35 times. Each cycle doubles the amount of DNA, leading to exponential amplification. After 30 cycles, it is possible to produce approximately 1 billion copies of the target DNA sequence.

4. **Thermostable DNA Polymerase**: Taq polymerase, derived from the bacterium Thermus aquaticus, is used in PCR because it remains stable and active at high temperatures, which is essential for the denaturation step.

5. **Conclusion**: The primary use of PCR is for the in vivo replication of specific DNA sequences using thermostable DNA polymerase, making it a crucial tool in molecular biology for cloning, gene analysis, and forensic science.

### Final Answer:
Polymerase chain reaction (PCR) is used for the in vivo replication of specific DNA sequences using thermostable DNA polymerase.

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