RT-qPCR Calculators

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RT-qPCR (reverse transcription quantitative PCR) combines reverse transcription of RNA into complementary DNA (cDNA) with quantitative PCR amplification to measure gene expression levels. It is the gold standard for gene expression quantification due to its sensitivity (detecting as little as one copy), specificity, dynamic range (>10⁷-fold), and quantitative accuracy. Applications include measuring mRNA abundance, microRNA profiling, viral load quantification, and validating RNA-seq data. MIQE guidelines govern reporting standards to ensure reproducibility across laboratories.

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RT-qPCR Workflow

  1. RNA extraction: Extract total RNA (TRIzol, column-based kit); assess quality (RIN ≥ 7; A260/A280 ≈ 2.0)
  2. DNase treatment: Remove genomic DNA contamination (critical for intronless genes)
  3. Reverse transcription: Convert RNA → cDNA using reverse transcriptase (oligo-dT, random hexamers, or gene-specific primers)
  4. qPCR amplification: SYBR Green or TaqMan probe detection; 40 cycles typical
  5. Data analysis: ΔΔCt method or absolute quantification from standard curve

One-Step vs. Two-Step RT-qPCR

One-step: RT and PCR in a single tube; faster, less contamination risk; uses gene-specific primers only; less flexibility. Two-step: Separate RT then PCR reactions; cDNA can be used for multiple gene targets; more sensitive; recommended for most applications.

ΔΔCt (Livak) Method

ΔCt = Ct(GOI) − Ct(reference gene)

ΔΔCt = ΔCt(treatment) − ΔCt(control)

Fold change = 2^(−ΔΔCt) (assumes 100% PCR efficiency)

Validate reference gene stability (geNorm, NormFinder) before use. At least 2 reference genes required per MIQE.

SYBR Green vs. TaqMan

SYBR Green: intercalating dye binding all dsDNA; cheaper; requires melt curve to verify specificity. TaqMan: sequence-specific fluorescent probe; higher specificity; no melt curve needed; preferred for diagnostics and multiplexing.

Glossary

RT-qPCR
Reverse transcription quantitative PCR; converts RNA to cDNA then amplifies it quantitatively; the gold standard for gene expression measurement; quantified by ΔΔCt or standard curve methods.
ΔΔCt Method
Relative quantification method: fold change = 2^(−ΔΔCt); ΔΔCt = ΔCt(treated) − ΔCt(control); assumes 100% PCR efficiency; requires validated reference genes.
Ct (Cycle Threshold)
The PCR cycle at which amplification fluorescence crosses a set threshold; inversely proportional to starting template amount; the raw data unit of quantitative PCR.

Frequently Asked Questions

RT-qPCR converts mRNA to cDNA via reverse transcription, then amplifies and quantifies the cDNA by qPCR. Fluorescence is measured each cycle; the Ct (cycle threshold) is the cycle at which fluorescence crosses a threshold. Higher mRNA abundance → more cDNA → lower Ct. Gene expression is calculated relative to reference genes using the ΔΔCt method: fold change = 2^(−ΔΔCt). The technique is highly sensitive (detects single copies), specific, and quantitative over a dynamic range of >10⁷.

ΔCt = Ct(gene of interest) − Ct(reference gene), normalizing for RNA input differences. ΔΔCt = ΔCt(treated) − ΔCt(control), calculating the relative difference between conditions. Fold change = 2^(−ΔΔCt), assuming 100% PCR efficiency. Example: gene of interest Ct = 25, reference Ct = 20, ΔCt = 5 in control; in treated sample ΔCt = 3; ΔΔCt = 3 − 5 = −2; fold change = 2² = 4-fold upregulation. Always validate reference gene stability in your experimental system before applying ΔΔCt.

One-step RT-qPCR combines reverse transcription and PCR in a single tube with a single set of reagents and gene-specific primers only. It is faster, reduces contamination risk, and uses less RNA. However, it is less sensitive, cDNA cannot be reused, and it requires gene-specific RT primers. Two-step RT-qPCR performs RT separately (with random hexamers or oligo-dT priming all genes), then uses the cDNA for multiple qPCR assays. Two-step is more flexible, generally more sensitive, and recommended by MIQE for most gene expression studies.

SYBR Green intercalates into any double-stranded DNA and emits fluorescence — it is cheaper and works with any primer pair, but it detects all dsDNA including primer dimers. A melt curve after amplification is essential to confirm a single specific product. TaqMan uses a dual-labeled fluorescent probe with specific sequence complementary to the amplicon — probe hydrolysis during extension releases the reporter dye. TaqMan is more specific (no melt curve needed), suitable for multiplexing (different dyes per gene), and preferred for clinical diagnostics and viral load testing.