Question 11 of 20 Fundamentals Concept
How do SYBR Green and hydrolysis probe chemistries differ in qPCR?
Short answer
SYBR Green is a dye that fluoresces when bound to any double-stranded DNA, so it reports every product the primers make. A hydrolysis probe, such as a TaqMan probe, only gives signal when it anneals to the target and is cut by the polymerase, which adds a second layer of sequence specificity.
Why
With a dye, specificity comes from the primers alone. Primer dimers and off-target products fluoresce as brightly as the real amplicon. That is why a melt curve is run at the end of every dye-based assay.
A hydrolysis probe carries a reporter dye and a quencher. While the probe is intact the quencher absorbs the reporter's energy. When the polymerase extends through the probe, its 5' nuclease activity cleaves it, the reporter is freed and fluorescence rises.
Signal therefore needs three matching oligonucleotides. Because each probe can carry a different reporter, several targets can be read in one well. Dyes are cheaper and work on any amplicon, which suits primer screening, library quantification and early research.
Probes cost more and must be designed and validated, but they suit diagnostic tests, genotyping and multiplexing. A probe does not make a bad primer pair good. Primer dimers still consume reagents and lower efficiency, even though they stay invisible.
Several dye molecules bind each amplicon, so a longer product gives more signal per copy than a short one.
A strong answer covers
- Dye binds any double-stranded DNA, including primer dimers
- Probe signal needs annealing and 5' nuclease cleavage
- Melt curve is required for dye-based assays
- Probes allow multiplexing with different reporters
- Cost and design effort versus specificity
Common mistakes
- Believing a probe assay cannot suffer from primer dimers because they are not seen
- Skipping the melt curve on a SYBR Green assay because the amplification curves look clean
- Assuming dye signal per copy is the same for amplicons of very different length
The interviewer might ask next
What would you check before combining two probe assays into one multiplex well?