**Core Concept:** Polymerase Chain Reaction (PCR) is a molecular technique used to amplify specific DNA sequences in a sample. PCR involves three main steps: denaturation, annealing, and extension. The correct components of PCR are: DNA polymerase, primers, and a thermal cycler.

**Why the Correct Answer is Right:** Polymerase Chain Reaction is a highly sensitive and specific method for amplifying DNA. The correct answer, **DN A polymerase**, is crucial for the PCR process as it catalyzes the synthesis of new DNA strands, using the original DNA strands as templates.

**Why Each Wrong Option is Incorrect:**

A. **Primers:** These are short, single-stranded DNA sequences that anneal to the complementary template DNA strand, acting as a starting point for polymerase to synthesize new complementary strands. Primers are important for specific amplification of the target DNA sequence but are not a component of the polymerase chain reaction itself.

B. **Thermal cycler:** This is the device used to control the temperature changes required for denaturation, annealing, and extension steps in PCR. A thermal cycler is a tool for performing PCR, not a component of the PCR reaction itself.

C. **Primers and thermal cycler:** Although primers are important for specific amplification and a thermal cycler is essential for proper temperature control, neither of them is a component of the PCR reaction mixture.

**Clinical Pearl:** Understanding PCR components helps in performing accurate and reliable molecular diagnostics, such as identifying infectious agents or genetic mutations.

**Correct Answer:** **DN A polymerase**