28.8 The Polymerase Chain Reaction

The Polymerase Chain Reaction (PCR)

PCR amplifies a specific segment of DNA by cycling through three temperature-dependent steps: denaturing the double helix, annealing short primers to each strand, and extending new strands with a heat-stable polymerase. Starting from even a single target molecule, PCR can generate millions of copies in a few hours, which is why it's central to molecular biology, forensics, and clinical diagnostics.

Steps of the PCR Process

Each PCR cycle has three stages. A typical reaction runs 25–35 cycles, and the whole process takes place in a thermal cycler, an instrument that rapidly and precisely shifts the temperature of the reaction mixture.

1. Denaturation (94–96 °C)

   • The reaction is heated above the melting temperature ($T_m$) of the target DNA.
   • Heat disrupts the hydrogen bonds between complementary base pairs (A–T and G–C), separating the double-stranded DNA into two single strands.
   • These single strands now serve as accessible templates for the next step.

2. Annealing (50–65 °C)

   • The temperature drops so that short, synthetic oligonucleotide primers can hydrogen-bond to their complementary sequences on each template strand.
   • Two primers are used: a forward primer and a reverse primer, each flanking opposite ends of the target region. Together they define exactly which stretch of DNA gets copied.
   • The annealing temperature is chosen to be just below the $T_m$ of the primers. Too low and primers bind nonspecifically; too high and they won't bind at all.

3. Extension (72 °C)

   • The temperature is raised to 72 °C, the optimum for Taq DNA polymerase.
   • Taq polymerase recognizes each primer–template junction and synthesizes a new complementary strand by adding deoxynucleotide triphosphates (dNTPs) to the 3ʹ-OH end of the primer, reading the template 3ʹ → 5ʹ and building the new strand 5ʹ → 3ʹ.
   • By the end of this step, each original strand has been copied, so the amount of target DNA roughly doubles per cycle.

After the final cycle, a longer extension hold (often 5–10 min at 72 °C) ensures all partially synthesized strands are completed.

Role of Taq DNA Polymerase

Taq polymerase is isolated from Thermus aquaticus, a bacterium that thrives in hot springs. Its thermostability is what makes modern PCR practical:

• It remains active at temperatures up to ~95 °C, so it survives the denaturation step instead of being destroyed.
• Before Taq was introduced, researchers had to add fresh polymerase after every denaturation step. Taq eliminated that requirement and allowed the entire process to be automated in a thermal cycler.
• Its optimal activity at 72 °C gives it high processivity, meaning it can add nucleotides rapidly and stay bound to the template over long stretches of DNA.

One trade-off: Taq lacks 3ʹ → 5ʹ proofreading exonuclease activity, so it has a relatively high error rate (~1 in 10⁴ bases). When high fidelity matters (e.g., cloning or sequencing), proofreading polymerases like Pfu are used instead.

Amplification Factors in PCR

Theoretical amplification

If every target molecule is perfectly copied each cycle, the number of copies after $n$ cycles is:

$2^n$

For example, 10 cycles would give $2^{10} = 1024$ copies of the original target, and 30 cycles would give $2^{30} \approx 1.07 \times 10^9$.

Practical amplification

Real reactions never hit 100 % efficiency. Primers may mis-anneal, dNTPs get consumed, polymerase activity declines, and accumulated product can re-anneal to itself instead of to primers. A more realistic formula accounts for this:

$(1 + E)^n$

where $E$ is the per-cycle efficiency (typically 0.7–0.9, with 1.0 being perfect). With $E = 0.75$ and $n = 10$:

$(1.75)^{10} \approx 180$

That's roughly 6-fold less than the theoretical 1024. Efficiency tends to drop further in later cycles as reagents are depleted, which is why PCR amplification eventually plateaus rather than increasing forever.

PCR Process and Equipment

• Thermal cycler: The instrument that automates PCR by cycling the reaction tube through programmed denaturation, annealing, and extension temperatures. Modern cyclers can process 96 or 384 samples at once.
• Thermal cycling: The repeated heating and cooling that drives strand separation and enzymatic replication each cycle.
• Amplification: The exponential increase in copies of the target DNA sequence over successive cycles.