Question 2 of 6: Polymerase Chain Reaction (PCR)

Question 2: Polymerase Chain Reaction (PCR) (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Primer Design Characteristics

Two characteristics that must be controlled when designing a PCR primer pair are melting temperature (T_m) and length/GC content, and specificity/absence of self- or cross-complementarity. Primers are typically 18–25 nucleotides long with 40–60% GC content, which gives a T_m in the 55–65°C range; critically, the forward and reverse primer T_m values must be closely matched (within a few °C of each other) so that a single annealing temperature lets both primers bind efficiently in the same cycle — a large T_m mismatch causes one primer to anneal poorly, reducing yield or introducing non-specific products. Separately, each primer's sequence must be checked for self-complementarity (hairpins) and complementarity with its partner primer, especially at the 3′ end, since 3′-end primer-dimers are efficiently extended by Taq and can out-compete the intended target amplicon; the sequence should also be checked (e.g. by a BLAST-type search) to ensure it anneals uniquely to the intended template region, since annealing elsewhere in a complex genome produces spurious bands.

(b) The Taq Enzyme

Taq polymerase is a thermostable DNA polymerase originally isolated from the thermophilic bacterium Thermus aquaticus, which lives in hot springs and geothermal vents. What makes it "somewhat" unique among DNA polymerases is precisely this thermostability: because it retains activity after being repeatedly heated to the ~94–98°C denaturation temperature used every PCR cycle, a single aliquot of enzyme added at the start of the reaction survives all 30–40 thermal cycles, whereas earlier PCR protocols using non-thermostable polymerases (e.g. the Klenow fragment) had to have fresh enzyme manually added after every denaturation step. Taq's other notable properties are that it lacks 3′→5′ exonuclease proofreading activity (giving it a relatively modest fidelity, on the order of one misincorporation per 10⁴–10⁵ bases) while retaining 5′→3′ exonuclease activity, and it has a terminal transferase-like activity that adds a single 3′ adenine overhang to blunt-ended products — the basis of TA cloning.

(c) Role of MgCl₂

Mg²⁺ ions are an essential cofactor for Taq polymerase's catalytic activity: the divalent magnesium ion coordinates the enzyme's active site and the phosphate backbone of the incoming dNTPs, stabilizing the phosphodiester-bond-forming reaction and the primer-template duplex. Without free Mg²⁺, Taq cannot catalyse DNA synthesis at all, so MgCl₂ is always included, but its concentration is also a sensitive tuning parameter for the reaction: too little Mg²⁺ lowers yield and can reduce fidelity, while too much Mg²⁺ stabilizes even partially mismatched primer-template pairing, promoting non-specific annealing and off-target amplification and also lowering fidelity. Mg²⁺ concentration (typically 1.5–3 mM MgCl₂) is therefore routinely optimized for a given primer/template combination.

(d) Strand Number After 35 Cycles

Assuming perfect efficiency, the number of double-stranded template molecules doubles every cycle, so after n cycles starting from N₀ copies the count is N = N₀ × 2ⁿ. With N₀ = 12 and n = 35:

$$N = 12 \times 2^{35} = 12 \times 34{,}359{,}738{,}368 = 412{,}316{,}860{,}416 \approx 4.12 \times 10^{11}\ \text{DNA molecules}$$

So after 35 cycles of perfectly efficient amplification, the 12 starting template copies would theoretically yield about 4.12 × 10¹¹ double-stranded DNA molecules (roughly 412 billion). In practice, real PCR reactions never sustain perfect exponential doubling for 35 full cycles — reagents (primers, dNTPs, polymerase) are depleted and the reaction plateaus well before this theoretical yield is reached — but the calculation illustrates why even a handful of starting template copies is sufficient for PCR to produce an easily detectable amount of product.