Q.The given schematic illustration shows three steps 'P', 'Q' and 'R' of the polymerase chain reaction. [Figure: schematic of PCR steps P, Q, R showing DNA strands with 5' and 3' ends] Which of the following statements are correct with reference to the illustration given above ?

The key idea is the Polymerase Chain Reaction (PCR), which involves three main steps: denaturation, annealing, and extension.

1. Identify the PCR steps:

   • Step 'P' shows the separation of double-stranded DNA into single strands, which is denaturation and occurs at high temperatures (94-98°C).
   • Step 'Q' shows primers binding to the single-stranded DNA, which is annealing and occurs at lower temperatures (50-65°C).
   • Step 'R' shows the synthesis of new DNA strands from the primers, which is extension and occurs at an intermediate temperature (70-72°C) in the presence of DNA polymerase.

2. Evaluate each statement:

   • (i) Step 'P' is showing denaturation at low temperature. This is incorrect. Denaturation occurs at high temperature. …

Concept and Intuition

The Polymerase Chain Reaction (PCR) is a molecular biology technique used to amplify a specific segment of DNA across several orders of magnitude, generating millions or billions of copies of a particular DNA sequence. It's like a molecular photocopier for DNA. The process relies on rapid temperature changes to drive three core steps, which are repeated in cycles:

1. Denaturation: The double-stranded DNA template is heated to a high temperature (typically 94-98°C). This breaks the hydrogen bonds between complementary base pairs, separating the DNA into two single strands. This is crucial because DNA polymerase can only synthesize new DNA on single-stranded templates.
2. Annealing: The temperature is then lowered (typically 50-65°C). This allows short, synthetic DNA sequences called primers to bind (anneal) to their complementary sequences on the single-stranded DNA templates. Two primers are used: a forward primer and a reverse primer, which define the start and end points of the DNA segment to be amplified.
3. Extension: The temperature is raised again to an optimal level for the DNA polymerase (typically 72°C for Taq polymerase). The DNA polymerase then synthesizes new DNA strands by adding deoxyribonucleotides (dNTPs) to the 3' end of the annealed primers, using the single-stranded DNA as a template. Since the polymerase must withstand repeated high-temperature denaturation steps, a thermostable DNA polymerase (like Taq polymerase, isolated from Thermus aquaticus) is essential.

These three steps constitute one PCR cycle, and the cycle is typically repeated 25-35 times, leading to an exponential amplification of the target DNA sequence.

Step-by-step Analysis

Let's analyze each statement with reference to the typical schematic representation of PCR steps P, Q, and R.

1. Analyze Step 'P' and Statement (i):

   • The schematic for step 'P' typically shows a double-stranded DNA molecule separating into two single strands. This process is known as denaturation.
   • Statement (i) says: "Step 'P' is showing denaturation at low temperature."
   • While 'P' correctly represents denaturation, this process occurs at a high temperature (e.g., 94-98°C) to break the strong hydrogen bonds holding the DNA strands together. It does not occur at a low temperature.
   • Therefore, statement (i) is incorrect.

2. Analyze Step 'Q' and Statements (ii) and (iv):

   • The schematic for step 'Q' typically shows the single-stranded DNA molecules from step 'P' having short primers bound to their complementary regions. This process is known as annealing.
   • Statement (ii) says: "Step 'Q' is a denaturation of DNA strand at high temperature, followed by annealing." …