Q.(a) Identify steps A and B in a cycle of Polymerase Chain Reaction given below.

Here is a crisp and complete answer to your question:

(a)

• Step A is Denaturation, where the double-stranded DNA is heated to a high temperature (typically 94-96°C) to separate it into single strands.
• Step B is Extension, where the DNA polymerase synthesizes new DNA strands by adding nucleotides complementary to the template, starting from the primers.

(b) …

The Polymerase Chain Reaction (PCR) is a powerful technique used to amplify a specific DNA segment, creating millions of copies from a single template molecule. This process occurs in a cyclic manner, with each cycle typically involving three distinct temperature-dependent steps: denaturation, annealing, and extension.

(a) Identifying Steps A and B

• Step A: Denaturation

  This is the first step in each PCR cycle, where the reaction mixture is heated to a high temperature, typically around 94-96°C. At this temperature, the hydrogen bonds holding the two strands of the double-stranded DNA template together break, causing the DNA to unwind and separate into single strands. This separation is essential because DNA polymerase can only synthesise new DNA using a single-stranded template.

  Note

  Between denaturation and extension lies a second step, annealing (not asked here as A or B): the temperature is lowered to about 50-60°C, and short synthetic primers bind (anneal) to their complementary sequences on the separated template strands, providing the free 3'-OH end that DNA polymerase needs to begin synthesis.

• Step B: Extension

  Following annealing, the temperature is raised again, typically to around 72°C — the optimum for the enzyme used. In this step, the enzyme synthesises a new complementary DNA strand by adding nucleotides one by one to the 3'-OH end of each annealed primer, extending it along the template. This duplicates the target DNA segment.

(b) Specific Characteristic Feature of the Enzyme in Carrying Step B

The enzyme that carries out step B (extension) is Taq DNA polymerase, and its defining characteristic feature is its thermostability. …