Number of DNA Strands After Four Cycles of PCR

A duplex DNA will give rise to how many strands after four cycles of PCR.
(a) 16 double-stranded DNA
(b) 16 single strands of DNA
(c) 32 single strands of DNA
(d) 32 double-stranded DNA

✅ Correct Answer: (d) 32 double-stranded DNA

Explanation

PCR (Polymerase Chain Reaction) is an exponential amplification technique used to replicate a target DNA sequence through multiple cycles. After each cycle, the number of double-stranded DNA molecules doubles, leading to exponential amplification.

🔬 1. PCR Amplification Formula

The number of double-stranded DNA molecules after n cycles of PCR can be calculated using the formula:

  Number of DNA strands=2ⁿ

Where:

• n = Number of PCR cycles
• 2ⁿ = Total number of double-stranded DNA molecules after n cycles

📌 2. Calculation After 4 Cycles

• Initial number of DNA molecules = 1 double-stranded DNA
• After 4 cycles:

24=16 double-stranded DNA molecules2^4 = 16 \text{ double-stranded DNA molecules}24=16 double-stranded DNA molecules

Since each double-stranded DNA consists of two single strands, the total number of single strands after 4 cycles = 16 × 2 = 32 single strands

✅ Total Number of Products:

• 16 double-stranded DNA
• 32 single strands of DNA

However, the question asks about the total number of double-stranded DNA molecules after four cycles — which equals 16 double-stranded molecules.

Why Other Options Are Incorrect

How PCR Works Step-by-Step

🔎 1. Denaturation

• The reaction is heated to 94–98°C to break the hydrogen bonds between DNA strands, resulting in single-stranded DNA templates.

🧪 2. Annealing

• The reaction is cooled to 50–65°C to allow primers to bind to complementary sequences on the single-stranded DNA.

🌟 3. Extension

• DNA polymerase extends the primers using dNTPs at 72°C, synthesizing the complementary strand.

Cycle-wise Amplification of DNA

Applications of PCR Amplification

✅ 1. Genetic Diagnosis

• Detecting mutations, genetic diseases, and infections.

✅ 2. Cloning and Sequencing

• Amplification of genes for molecular cloning.

✅ 3. Forensic Analysis

• DNA fingerprinting and criminal investigations.

✅ 4. Evolutionary Studies

• Amplifying ancient DNA for phylogenetic analysis.

Challenges in PCR Amplification

🔴 1. Primer-Dimer Formation:

• Leads to non-specific products → Can be reduced by using a hot-start polymerase.

🔴 2. Low Yield:

• Caused by low Mg²⁺ concentration or suboptimal annealing temperature.

🔴 3. Non-Specific Amplification:

• Caused by low specificity of primers → Increase annealing temperature.

How to Optimize PCR Amplification

✅ 1. Use High-Fidelity Polymerase

• Ensures accurate and specific amplification.

✅ 2. Optimize Mg²⁺ Concentration

• Typically between 1.5 – 3.0 mM.

✅ 3. Use Touchdown PCR

• Gradually reduce the annealing temperature to increase specificity.

Summary

• PCR amplifies DNA exponentially using the formula 2ⁿ.
• After 4 cycles, the total number of double-stranded DNA molecules = 16.
• Since each molecule is double-stranded, total single strands = 32.
• The correct answer is (d) 32 double-stranded DNA.