PCR (Polymerase Chain Reaction) is a widely-used laboratory technique used to amplify (copy) a specific segment of DNA. It involves a series of steps that are repeated multiple times to generate a large amount of DNA from a small initial sample. The correct sequence of steps in a PCR is as follows:

1. Denaturation:
- The first step of PCR is denaturation, which involves heating the DNA sample to a high temperature (usually around 94-98°C).
- This causes the DNA double helix to separate (denature) into two single strands by breaking the hydrogen bonds between the base pairs.
- Denaturation ensures that the DNA is in a single-stranded state, ready for the next step.

2. Annealing:
- After denaturation, the temperature is lowered to around 50-65°C.
- In this step, short DNA primers (oligonucleotides) that are complementary to the target DNA sequence are added to the reaction mixture.
- The primers attach (anneal) to their complementary sequences on the single-stranded DNA template, providing a starting point for DNA synthesis.

3. Extension:
- The temperature is then raised to around 72°C, the optimal temperature for DNA polymerase activity.
- A heat-stable DNA polymerase enzyme, such as Taq polymerase, extends the primers by adding nucleotides to the 3' end of each primer, synthesizing new DNA strands.
- The polymerase continues to move along the template DNA, synthesizing a new complementary strand for each of the original strands.
- This process is repeated for multiple cycles, resulting in an exponential increase in the amount of DNA.

4. Repeat:
- The three steps (denaturation, annealing, and extension) are repeated for a specific number of cycles, typically 20-40 cycles.
- Each cycle doubles the number of DNA molecules, resulting in a rapid amplification of the target DNA sequence.
- The number of cycles depends on the starting amount of DNA and the desired amount of final product.

By following this sequence of steps, PCR allows for the amplification of a specific DNA segment, making it a powerful tool in various applications such as DNA sequencing, genetic testing, and gene expression analysis.