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Polymerase Chain Reaction (PCR)

In Vitro Amplification of Specific Target DNA Sequences

1 Aim

To amplify a specific DNA fragment in vitro using the Polymerase Chain Reaction (PCR) technique with Taq DNA Polymerase in a Thermal Cycler.

2 Principle

The Polymerase Chain Reaction is a revolutionary molecular biology technique used to exponentially amplify a specific region of DNA. It relies on thermal cycling, consisting of repeated cycles of heating and cooling of the reaction for DNA melting and enzymatic replication.

The 3 Major Steps of PCR:

1. Denaturation (94–95°C): The intense heat breaks the hydrogen bonds between the double-stranded DNA, yielding two single strands.
2. Annealing (50–65°C): The temperature is lowered to allow short, target-specific DNA primers to bind (anneal) to their complementary sequences on the single-stranded template.
3. Extension (72°C): The temperature is raised to the optimal working temperature of Taq DNA Polymerase, which synthesizes a new DNA strand by adding dNTPs (nucleotides).

Fig 1: The three fundamental stages of a single PCR cycle.

Each cycle doubles the DNA amount, resulting in exponential amplification (2ⁿ copies, where n is the number of cycles). After 25–35 cycles, millions of copies of the specific DNA fragment are produced.

3 Materials Required

Chemicals and Reagents

• Template DNA
• Forward & Reverse Primers
• dNTP mixture (dATP, dTTP, dGTP, dCTP)
• 10X PCR buffer
• MgCl₂ (Cofactor for Taq)
• Taq DNA Polymerase
• Nuclease-free water

Equipment

• Thermal Cycler (PCR Machine)
• Micropipettes and sterile tips
• 0.2 ml PCR tubes
• Microcentrifuge
• Vortex mixer
• Gel electrophoresis apparatus

4 Preparation of PCR Reaction Mixture

All reagents should be kept on ice while preparing the master mix. Typical PCR reaction volume is 25 µl.

Component	Volume (per 25 µl rxn)
Template DNA	1–2 µl
Forward Primer (10 µM)	1 µl
Reverse Primer (10 µM)	1 µl
dNTP Mix (10 mM)	1 µl
10X PCR Buffer	2.5 µl
MgCl₂ (25 mM)	1.5 µl
Taq DNA Polymerase	0.5 µl
Nuclease Free Water	Up to 25 µl

5 PCR Amplification Program

Place the sealed PCR tubes into the Thermal Cycler and program the following temperature cycles:

Step	Temperature	Time	Cycles
Initial Denaturation	95°C	3–5 min	30 - 35 Cycles
Denaturation	94°C	30 sec
Annealing	50–65°C*	30 sec
Extension	72°C	1 min/kb
Final Extension	72°C	5–10 min	1
Hold	4°C	∞	-

*Annealing temperature depends on the melting temperature (Tm) of the specific primers used.

6. Procedure

1. Label sterile 0.2 ml PCR tubes properly.
2. Prepare the PCR master mix on ice according to the reaction table (multiply volumes by number of reactions + 1 extra).
3. Aliquot the master mix into individual tubes.
4. Add the template DNA carefully to the respective tubes.
5. Mix the reaction gently using a vortex mixer and briefly centrifuge to bring contents to the bottom.
6. Place the tubes inside the Thermal Cycler block.
7. Program the PCR cycling conditions and start the run.
8. After completion, proceed to analysis or store PCR products at 4°C or −20°C.

7. Troubleshooting PCR

Problem	Likely Cause & Solution
No Bands (No Amplification)	Missing reagent (e.g., Taq or Taq buffer), annealing temp too high, or degraded template DNA.
Non-specific Bands (Multiple Bands)	Annealing temperature is too low (primers binding randomly), or MgCl₂ concentration is too high.
Primer Dimers (Fuzzy band at bottom)	Primers are binding to each other. Use less primer or redesign primers to avoid self-complementarity.

8. Advantages & Applications

Key Advantages

• Highly sensitive: Can amplify from a single DNA molecule.
• Rapid: Yields millions of copies in just 2 hours.
• Specific: Primers ensure only the target region is copied.

Applications

• Gene cloning and sequencing
• Detection of infectious diseases (e.g., COVID-19, HIV)
• Forensic DNA fingerprinting
• Detection of GMOs

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Important Viva Questions

Q1. Why is Taq Polymerase used in PCR instead of normal human DNA polymerase?

Normal polymerases denature and are destroyed at 95°C. Taq polymerase is isolated from the thermophilic bacterium Thermus aquaticus (found in hot springs). It is thermostable and survives the extreme heat of the denaturation step without needing to be replenished every cycle.

Q2. What is the role of MgCl₂ in the PCR reaction?

Magnesium ions (Mg²⁺) act as a crucial cofactor for the Taq DNA polymerase. Without it, the enzyme cannot function. Furthermore, Mg²⁺ helps stabilize the interaction between the primers and the DNA template.

Q3. How do you determine the correct Annealing Temperature?

The annealing temperature is usually set 3°C to 5°C below the lowest Melting Temperature (Tm) of the primers being used. If it is too high, primers won't bind. If it's too low, they will bind randomly, creating non-specific products.

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🧠 Interactive Concept Quiz

Test your knowledge! Click on the questions below to reveal the correct answers.

1. At what temperature does the Denaturation step occur?

✅ Answer: 94°C – 95°C

At this extreme temperature, the hydrogen bonds connecting the double-stranded DNA break apart.

2. What does "dNTP" stand for in the PCR mixture?

✅ Answer: Deoxynucleotide triphosphate

These are the free nucleotides (A, T, C, G) that the polymerase uses as building blocks to create the new DNA strand.

3. Why must we cool the reaction down to 50–65°C during the Annealing step?

✅ Answer: To allow the primers to attach.

Cooling the reaction provides the ideal conditions for the short DNA primers to hydrogen-bond to their complementary target sequences on the template DNA.

4. If you run 30 cycles of PCR starting from a single DNA molecule, approximately how many copies will you have?

✅ Answer: Over 1 Billion (2³⁰)

PCR results in exponential growth. Each cycle doubles the amount of target DNA.