Real-time PCR (RT-qPCR) is used for quantitative viral load testing in HIV-infected patients. The molecular mechanism that allows real-time quantification of PCR product without gel electrophoresis using TaqMan probes is:

• A TaqMan probe binds non-specifically to double-stranded DNA and fluoresces proportionally to DNA amount
• B SYBR Green dye intercalates into double-stranded DNA and is used as TaqMan equivalent
• C Molecular beacon forms a hairpin structure that fluoresces only when annealing temperature is exceeded
• D TaqMan probe has a fluorophore (reporter) quenched by a nearby quencher; 5'-to-3' exonuclease activity of Taq polymerase cleaves the probe during extension, separating reporter from quencher and generating fluorescence ✓

Explanation

TaqMan probes are dual-labelled oligonucleotides specific for an internal PCR target sequence. The probe has a 5' fluorescent reporter (e.g., FAM) held in close proximity to a 3' quencher (e.g., TAMRA/BHQ) — in the intact probe, the quencher absorbs the reporter's emission via FRET (fluorescence resonance energy transfer), suppressing signal. During PCR extension, the 5'-to-3' exonuclease activity of Taq DNA polymerase cleaves the probe hybridised downstream of the primer, physically separating the reporter from the quencher and allowing fluorescence emission. Fluorescence accumulates proportionally to amplicon quantity per cycle, enabling real-time quantification using a standard curve (for absolute quantification) or comparative Ct method. SYBR Green and molecular beacons are alternative chemistries, not TaqMan probes.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.