Quantitative PCR (qPCR/RT-qPCR) uses a fluorescent probe strategy. In TaqMan assays, a dual-labeled probe is used. What is the mechanism by which fluorescence increases only during legitimate amplification?

• A The probe (5'-fluorophore, 3'-quencher) anneals to target sequence; Taq polymerase's 5'→3' exonuclease activity degrades the probe during extension, separating fluorophore from quencher, generating signal only when probe hybridizes to authentic amplicon ✓
• B The probe forms a hairpin loop that keeps fluorophore quenched unless the specific PCR product opens the hairpin by strand displacement
• C SYBR Green intercalates into double-stranded PCR product, generating fluorescence proportional to total dsDNA mass
• D The quencher molecule is cleaved from the probe by a restriction enzyme added to the PCR reaction mixture after amplification

Explanation

TaqMan probes use FRET (fluorescence resonance energy transfer) quenching: when the 5'-reporter fluorophore is in close proximity to the 3'-quencher, fluorescence is suppressed (quenched). When Taq polymerase extends the upstream primer and reaches the bound probe, its 5'→3' exonuclease activity hydrolyzes the probe nucleotides one by one, separating the reporter from the quencher. The freed fluorophore fluoresces, and signal accumulates proportionally to amplicon number. This ensures signal only from probes hybridized to genuine target sequence (not from primer dimers, unlike SYBR Green). Option B describes the molecular beacon probe mechanism.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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