PCR Protocol for Beginners: Step-by-Step Guide

Before You Start: What You Need

Before setting up a PCR reaction, ensure you have these materials ready:

• Template DNA: Purified genomic DNA (1–100 ng), plasmid DNA (1–10 pg), or cDNA (1–100 ng)
• Primers: Forward and reverse primers, HPLC or desalted purified, resuspended to 100 µM stock. See primer design rules for design guidelines.
• DNA polymerase: Taq polymerase (standard) or high-fidelity polymerase (for cloning)
• dNTPs: 10 mM each dATP, dCTP, dGTP, dTTP
• PCR buffer: Typically 10X concentration with MgCl₂
• Nuclease-free water: DEPC-treated or molecular-grade water
• Thermal cycler: Programmable PCR machine
• Gel electrophoresis setup: Agarose, TAE/TBE buffer, DNA stain, DNA ladder, gel documentation system

Step 1: Design Your Primers

Primer design is the most critical step. Follow established PCR primer design rules:

• Primer length: 18–24 nucleotides
• GC content: 40–60%
• Tm: 52–65°C (within 2–5°C between forward and reverse)
• Avoid 3' complementarity (prevents primer dimers)
• Avoid hairpins (delta-G > -5.0 kcal/mol)
• BLAST primers against target genome for specificity

Recommended: Use VigyanLLM Primer to automatically check all design parameters before ordering.

Step 2: Prepare Master Mix

Prepare the master mix on ice. The following table shows volumes for a single 25 µL reaction. Multiply each volume by the number of reactions plus 10% excess for pipetting loss.

Step 3: Program the Thermal Cycler

Enter the following program into the thermal cycler. Adjust annealing temperature based on your primer Tm.

Step 4: Run the PCR

1. Place tubes in the thermal cycler and close the lid
2. Start the program (typically takes 1–2 hours for 35 cycles)
3. Monitor the block temperature to ensure it reaches denaturation temperature
4. When the program finishes, remove tubes and store at 4°C or proceed directly to analysis

Step 5: Analyse Products by Gel Electrophoresis

1. Prepare a 1–2% agarose gel (1% for 500–1,000 bp products; 2% for 100–500 bp products) in 1X TAE buffer with 0.5 µg/mL ethidium bromide or SYBR Safe
2. Mix 5 µL of PCR product with 1 µL of 6X loading dye
3. Load samples and a 100 bp or 1 kb DNA ladder
4. Run at 5–10 V/cm for 30–45 minutes
5. Visualise under UV light (for ethidium bromide) or blue light (for SYBR Safe)
6. Compare band size to the ladder to confirm correct amplicon

Step 6: Troubleshooting Common PCR Problems

PCR Tips for Reproducibility

Reproducibility is a major challenge in PCR. To ensure consistent results across experiments: always prepare master mixes as a single batch for all reactions including controls; vortex master mix components gently before use and centrifuge briefly to collect contents; use the same thermal cycler and program for all comparative experiments; record the exact ramp rate and lid temperature settings; and store PCR products at 4°C for short-term or -20°C for long-term storage. For quantitative PCR, always run samples in technical triplicates and report the mean Cq with standard deviation. Following standardised protocols and using validated reagents significantly improves inter-assay reproducibility.

Template Controls

Always include essential controls:

• Positive control: A template known to amplify with these primers
• No-template control (NTC): Replace template with nuclease-free water to detect contamination
• No-reverse-transcriptase control (for RT-PCR): Sample without reverse transcriptase to detect genomic DNA contamination

For a more detailed explanation of the three PCR steps and the science behind each temperature, see our dedicated guide.

Primer Resuspension and Storage

Primers arrive from synthesis as lyophilised pellets. Resuspend to 100 µM stock concentration using nuclease-free TE buffer (10 mM Tris, 0.1 mM EDTA, pH 8.0) or molecular-grade water. Calculate the required volume based on the nanomoles of primer delivered: volume (µL) = nmoles × 10 for 100 µM stock. Vortex thoroughly and centrifuge briefly. Prepare a working dilution of 10 µM (10 µL stock + 90 µL water) to avoid repeated freeze-thaw cycles of the stock. Store stock solutions at -20°C where they remain stable for 2–3 years. Working dilutions can be stored at 4°C for up to 3 months or at -20°C for longer periods. Always verify the concentration by spectrophotometry if accurate quantification is critical for quantitative applications.