qPCR Calculators

0 calculators tagged with “qPCR”

Quantitative PCR (qPCR) — also called real-time PCR — is a molecular biology technique that measures the amount of a specific DNA or RNA sequence in a sample by monitoring PCR amplification in real time. Unlike conventional PCR which simply detects presence or absence, qPCR quantifies the initial amount of target by tracking how quickly the fluorescent signal rises above background. It is one of the most powerful, sensitive, and widely used tools in molecular biology, clinical diagnostics, gene expression analysis, and pathogen detection.

All Calculators

No calculators found for this topic.

How qPCR Works

In qPCR, fluorescent reporter molecules are included in the reaction. As the target DNA is amplified, fluorescence increases proportionally to the amount of PCR product. A fluorescence detector monitors the reaction in real time at each cycle.

The Cq value (quantification cycle; also called Ct, Cp, or Cq) is the cycle number at which the fluorescence signal crosses a defined threshold above background. Cq is inversely proportional to the starting amount of target: more starting template = fewer cycles to reach threshold = lower Cq.

Detection Chemistries

  • SYBR Green: Intercalating dye that fluoresces when bound to any dsDNA. Simple and inexpensive; requires melt curve analysis to verify specificity. Can detect non-specific products.
  • TaqMan probes: Dual-labeled hydrolysis probes (fluorophore + quencher) designed to hybridize specifically to the target sequence. Digested by Taq polymerase during extension, releasing fluorophore. Highly specific; no melt curve needed; more expensive.

Absolute vs. Relative Quantification

Absolute Quantification

Uses a standard curve of known copy numbers to calculate the absolute number of target molecules in each sample. Requires accurate standards. Used for viral load, GMO quantification, and copy number variation analysis.

Relative Quantification (2^−ΔΔCt)

Expresses target gene expression relative to a reference (housekeeping) gene and a control condition. No absolute copy number needed. The Livak method (2^−ΔΔCt) is the standard approach — fold change = 2^−(ΔCt_sample − ΔCt_control).

qPCR Efficiency

A perfectly efficient qPCR reaction doubles product every cycle (E = 2, 100% efficiency). Real reactions typically achieve 90–105% efficiency. Efficiency is determined from the slope of a standard curve: E = 10^(−1/slope). Slope = −3.32 corresponds to 100% efficiency. Efficiencies outside 90–110% indicate primer problems, inhibitors, or suboptimal conditions.

Key Quality Controls

  • No template control (NTC): No DNA added — detects reagent contamination
  • Melt curve analysis (SYBR): Verifies single specific product
  • Reference gene validation: Confirm reference gene stability across conditions
  • Efficiency check: Standard curve slope should be −3.2 to −3.5

Glossary

Cq (Quantification Cycle)
The PCR cycle at which fluorescence crosses a defined threshold. Inversely proportional to starting template quantity. Also called Ct or Cp. A difference of ~3.32 Cq cycles corresponds to a 10-fold difference in starting copy number at 100% efficiency.
qPCR Efficiency
The fraction by which PCR product increases per cycle. 100% efficiency = perfect doubling each cycle. Determined from standard curve slope: E = 10^(−1/slope). Acceptable range 90–110%. Required for accurate quantification and ΔΔCt calculations.
TaqMan Probe
A dual-labeled oligonucleotide probe (fluorophore + quencher) used in qPCR for sequence-specific detection. The probe hybridizes to the target; Taq polymerase cleaves it during extension, separating fluorophore from quencher and generating fluorescence only when the specific target is amplified.

Frequently Asked Questions

Cq (quantification cycle, also called Ct) is the PCR cycle at which the fluorescence signal crosses a defined threshold above background. It is inversely proportional to starting template amount: a sample with 10× more starting DNA reaches threshold ~3.32 cycles earlier (since log₂(10) = 3.32 for 100% efficient PCR). Lower Cq = more starting material. Every ~3.32 cycle difference represents a 10-fold difference in starting copy number at 100% efficiency.

SYBR Green is an intercalating dye that fluoresces with any double-stranded DNA — simple, inexpensive, but non-specific (detects all dsDNA, including primer-dimers). Requires melt curve analysis to confirm a single specific product. TaqMan uses sequence-specific probes with a fluorophore and quencher — only the correct target generates signal. More specific and does not require melt curves, but more expensive. TaqMan is preferred for clinical diagnostics and multiplexing.

PCR efficiency describes how closely each cycle doubles the target sequence — 100% means perfect doubling. Determined from a standard curve: E = 10^(−1/slope); slope = −3.32 at 100% efficiency. Acceptable range: 90–110%. Low efficiency (flat slope) suggests inhibitors, poor primer design, or template issues. High efficiency (steep slope) may indicate primer-dimer formation. Efficiency deviations between target and reference genes invalidate the 2^−ΔΔCt calculation.

Essential controls include: (1) No template control (NTC) — water instead of sample, detects contamination; (2) No RT control (for RT-qPCR) — no reverse transcriptase, detects genomic DNA contamination; (3) Positive control — sample with known target, confirms assay works; (4) Reference gene sample — all experimental samples should have similar Cq for the reference gene, confirming equal RNA input. Melt curve analysis (SYBR) should show a single sharp peak confirming product specificity.