what is a primer? what is its role in PCR ?

Explanation

### Step-by-Step Solution:

1. **Definition of Primer**: A primer is a short segment of nucleic acid (typically DNA) that serves as a starting point for DNA synthesis. In the context of PCR (Polymerase Chain Reaction), primers are specifically designed to bind to the target DNA sequence.

2. **Structure of Primers**: Primers are usually about 18-25 nucleotides long and are complementary to the sequences at the start of the target DNA region that needs to be amplified.

3. **Role of Primers in PCR**:
- **Initiation of DNA Synthesis**: Primers provide a free 3' hydroxyl (OH) group, which is essential for DNA polymerase to begin adding nucleotides. Without primers, DNA polymerase cannot initiate the synthesis of a new DNA strand.
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