Which of the following steps are related to polymerase chain reaction (PCR)?

(A) Extension

(B) Annealing

(C) Propagation

(D) Denaturation

The correct option is (A), (B), (C) and (D).

Polymerase Chain Reaction (PCR) is a fundamental molecular biology technique used to amplify a specific segment of DNA. The process involves and relates to the following key steps and concepts:

1. Denaturation (D): The reaction mixture is heated (typically to 94–98 °C) to separate the double-stranded DNA template into two single strands by breaking the hydrogen bonds between complementary bases.

2. Annealing (B): The temperature is lowered (typically to 50–65 °C) to allow synthetic DNA primers to bind specifically to their complementary target sequences on the single-stranded DNA templates.

3. Extension (A): The temperature is adjusted to the optimum activity level of the DNA polymerase enzyme (typically 72 °C), which synthesizes a new DNA strand complementary to the template by adding free deoxynucleoside triphosphates (dNTPs) in the 5' to 3' direction.

4. Propagation (C): Through the sequential repetition of denaturation, annealing, and extension, the target DNA segment is exponentially propagated or amplified across multiple thermal cycles, leading to millions or billions of copies of the specific DNA sequence.

Since all the listed processes are essential parts of or directly related to the execution and objective of PCR, all four options are correct.