Kinetics of PCR product formation: After the 4th cycle of PCR, how many double-stranded DNA molecules comprise precisely the target-length amplicon (bounded by both primers)?

Introduction / Context:
In early PCR cycles, products include over-extended strands. Target-length amplicons (bounded on both ends by primer termini) first appear after the second cycle and accumulate in a predictable way. Understanding this helps explain exponential amplification and band purity.

Given Data / Assumptions:

• Two primers flank the target.
• Cycle counting starts at the initial denaturation/extension sequence.
• We are counting only the exact-length double-stranded products.

Concept / Approach:
Exact-length product appears starting in cycle 3 and then doubles each cycle thereafter. A classic rule: after cycle n, the number of precise-length ds molecules equals 2^(n-2) (for n ≥ 3). Thus cycle 3 yields 2, cycle 4 yields 4, and so on.

Step-by-Step Solution:
At cycle 1 and 2: products include over-extended strands; no exact-length ds product at cycle 2 end.Cycle 3: first time both ends are primer-defined → 2 exact-length ds molecules.Cycle 4: doubling from cycle 3 → 4 exact-length ds molecules.

Verification / Alternative check:
Derive using strand-tracking diagrams showing how primer-defined ends accumulate and then dominate the reaction in later cycles.

Why Other Options Are Wrong:

• Two (A) corresponds to cycle 3, not cycle 4.
• Three (B) does not follow doubling logic.
• Eight (C) corresponds to cycle 5 for exact-length molecules.

Common Pitfalls:
Counting total DNA molecules instead of exact-length amplicons; totals include heterogeneous lengths early on.

Final Answer:
Four ds DNA molecule