**Question:** In PCR, which of the following is NOT a correct step?

A. Denaturation of DNA
B. Primer annealing
C. DNA synthesis
D. Extension of DNA synthesis

**Correct Answer:** **D.** Extension of DNA synthesis

**Core Concept:** Polymerase Chain Reaction (PCR) is a molecular technique used to amplify specific DNA sequences. PCR consists of three main steps: Denaturation, Primer annealing, and DNA synthesis. In the final step, DNA synthesis occurs via the enzyme DNA polymerase, synthesizing complementary strands of DNA from the single strands produced during denaturation and primer annealing. Extension of DNA synthesis is the process by which the newly synthesized DNA strands are elongated by the polymerase.

**Why the Correct Answer is Right:** Option D is incorrect because extension of DNA synthesis forms the final, continuous DNA product during PCR. This step is crucial for amplifying target DNA sequences. By synthesizing complementary strands, the polymerase extends the partially double-stranded DNA, creating a full double-stranded DNA molecule.

**Why Each Wrong Option is Incorrect:**

A. **Denaturation of DNA:** This step is essential for releasing the target DNA sequence from other molecules and denatures the DNA, allowing primers to bind to the complementary regions. Without denaturation, primers would not be able to bind efficiently, leading to inefficient amplification.

B. **Primer annealing:** This step involves the hybridization of primers to the complementary regions on the target DNA sequence. Primer annealing is necessary for the subsequent steps to occur, as primers help guide the polymerase to the target sequence for amplification.

C. **DNA synthesis:** This step involves the polymerase synthesizing complementary strands of DNA from the single strands produced during denaturation and primer annealing. This step aims to create a full double-stranded DNA molecule, which is essential for amplifying the target DNA sequence.

**Clinical Pearl:** PCR is a fundamental technique used in molecular biology research and diagnosis, allowing for the amplification of specific DNA sequences from a limited amount of DNA template, such as in forensic science and genetic testing. Understanding the correct sequence of steps ensures efficient PCR results and accurate detection of genetic information.