Experiment 06In Vitro Amplification • B.Sc. / M.Sc.
Polymerase Chain Reaction (PCR) Amplification of Bacterial 16S rRNA Gene
🎯 Aim: To set up a $25,mutext{L}$ PCR reaction targeting the bacterial $16text{S rRNA}$ gene using universal primers (27F and 1492R), program a 3-step thermal cycler, amplify the $~1,500,text{bp}$ amplicon, and confirm size by agarose gel electrophoresis.
🔬 Theoretical Principle & Enzymatic Kinetics
The Polymerase Chain Reaction (Kary Mullis, 1983) is an in vitro enzymatic method for the exponential geometric amplification of a specific target DNA sequence through automated thermal cycling:
- Thermal Cycling Triad:
- Denaturation ($94–95^circtext{C}, 30,text{sec}$): Thermal energy melts interstrand hydrogen bonds, generating single-stranded DNA templates.
- Primer Annealing ($50–60^circtext{C}, 30,text{sec}$): Synthetic oligonucleotide primers (forward and reverse) hybridize complementarily to flanking sequences. Annealing temperature $T_a approx T_m – 5^circtext{C}$.
- Taq Extension ($72^circtext{C}, 60,text{sec/kb}$): Thermostable Thermus aquaticus DNA polymerase synthesizes nascent DNA in the $5’to 3’$ direction using dNTP substrates and $text{Mg}^{2+}$ cofactor.
- Exponential Yield Equation:
$$N_n = N_0 cdot (1 + E)^n$$
Where $N_0$ is initial template copies, $n$ is cycle count, and $E$ is amplification efficiency ($0 le E le 1.0$). After 30 cycles at $100%$ efficiency, $N_{30} = N_0 cdot 2^{30} approx 10^9text{-fold amplification}$.
Figure 6.1: Polymerase Chain Reaction (PCR) Three-Step Thermal Cycling Profile
📋 PCR Master Mix Setup (25 μL Reaction)
| Reaction Component | Stock Concentration | Final Concentration | Volume per 25 μL Reaction |
|---|---|---|---|
| Sterile Nuclease-Free Water | — | — | 16.8 μL |
| 10× Taq Reaction Buffer | $10times$ | $1times$ | 2.5 μL |
| dNTP Mix (dATP, dCTP, dGTP, dTTP) | 10 mM each | 0.2 mM each | 0.5 μL |
| Forward Primer 27F | 10 μM | 0.4 μM | 1.0 μL |
| Reverse Primer 1492R | 10 μM | 0.4 μM | 1.0 μL |
| Bacterial Genomic DNA Template | 50 ng/μL | ~50 ng | 1.0 μL |
| Taq DNA Polymerase | 5 U/μL | 1.0 U | 0.2 μL |
💬 Viva-Voce Questions & Model Answers
Q1: Why is magnesium chloride (MgCl2) an essential cofactor in PCR?
Magnesium ions ($text{Mg}^{2+}$) coordinate with the alpha-phosphate groups of dNTPs to facilitate nucleophilic attack by the primer $3’text{-OH}$ group during catalysis, and stabilize the negatively charged transition state of the Taq polymerase active site. Insufficient $text{Mg}^{2+}$ causes low yield; excess causes non-specific amplification.
Q2: What is Hot-Start PCR and what artifact does it eliminate?
Hot-Start PCR suppresses polymerase activity at room temperature using an antibody or chemical modification that detaches only after initial heating to $95^circtext{C}$. This prevents non-specific primer annealing and “primer-dimer” formation before thermal cycling begins.
Q3: Why lacks Taq polymerase a 3′ to 5′ proofreading exonuclease activity?
Taq polymerase naturally lacks a $3’to 5’$ exonuclease domain, resulting in an error rate of $approx 1times 10^{-4}$ to $1times 10^{-5}$ errors/bp and terminal nontemplated addition of a single deoxyadenosine ($3’text{-A}$ overhang), which enables TA-cloning into pGEM-T vectors. For high-fidelity cloning, Pfu or Q5 polymerases are used.