In a polymerase chain reaction after the denaturation step why the mixture needs to cool down to a lower temperature ?
a) To permit specific annealing of the primers
b) To give a halt to the reaction mixture
c) To increase the activity of enzyme Taq polymerase
d) To obtain the multiple copies of the DNA

Options

A. a) To permit specific annealing of the primers
B. b) To give a halt to the reaction mixture
C. c) To increase the activity of enzyme Taq polymerase
D. d) To obtain the multiple copies of the DNA

Answer

To answer the question "In a polymerase chain reaction after the denaturation step why does the mixture need to cool down to a lower temperature?", we can break down the process of PCR and the reasoning behind the cooling step.

### Step-by-Step Solution:

1. **Understanding PCR**: Polymerase Chain Reaction (PCR) is a technique used to amplify specific segments of DNA. It involves three main steps: denaturation, annealing, and extension.

2. **Denaturation Step**: The first step is denaturation, where the double-stranded DNA is heated to about 94°C. This high temperature causes the hydrogen bonds between the DNA strands to break, resulting in two single strands of DNA.

3. **Cooling Down**: After denaturation, the mixture needs to cool down to a lower temperature (typically between 40°C to 60°C). This cooling is crucial for the next step.

4. **Annealing Step**: The purpose of cooling is to allow primers to anneal (bind) to the single-stranded DNA. Primers are short sequences of nucleotides that are complementary to the target DNA sequence. At lower temperatures, the primers can specifically bind to their complementary sequences on the single-stranded DNA.

5. **Conclusion**: The cooling step is essential to ensure that the primers can anneal effectively and specifically to the target DNA sequences, which is necessary for the subsequent extension step where Taq polymerase synthesizes new DNA strands.

### Final Answer:
The correct option is **a) To permit specific annealing of the primers**.