RT-PCR Calculators

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RT-PCR (reverse transcription PCR) is a molecular biology technique that converts RNA into complementary DNA (cDNA) using the enzyme reverse transcriptase, followed by PCR amplification of the cDNA. It enables detection and quantification of RNA sequences — gene expression in cells, viral RNA (HIV, SARS-CoV-2), and mRNA splice variants. When combined with real-time quantitative PCR detection (RT-qPCR or qRT-PCR), it allows precise measurement of mRNA levels in a sample. RT-PCR is the gold standard for gene expression analysis and was the basis of COVID-19 diagnostic tests.

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RT-PCR Steps

  1. Extract RNA from cells or tissue (TRIzol, column-based kits)
  2. Reverse transcription: RNA → cDNA using reverse transcriptase enzyme, oligo-dT primers (poly-A tail binding) or random hexamers
  3. PCR amplification of the cDNA with gene-specific primers
  4. Detection: conventional RT-PCR → gel electrophoresis; RT-qPCR → real-time fluorescence

RT-qPCR (Real-Time Quantitative RT-PCR)

Combines reverse transcription with real-time PCR (qPCR) for absolute or relative quantification of mRNA. Two formats:

  • One-step RT-qPCR: RT and PCR in the same tube with the same primers; simpler; less sensitive to variation; better for high-throughput
  • Two-step RT-qPCR: Separate RT reaction then qPCR; more flexible (cDNA can be used for multiple targets); more sensitive; standard for gene expression analysis

Primers for RT-PCR

Design primers spanning an exon-exon junction: amplicons crossing splice sites will not amplify from contaminating genomic DNA (different band size if present). Use no-RT (−RT) control to confirm absence of genomic DNA contamination. Primers should have Tm of 58–65°C; amplicon 80–200 bp for RT-qPCR.

Reference Genes

GAPDH, β-actin, B2M, HPRT1 are commonly used reference (housekeeping) genes for normalization. Validate stability in your experimental conditions (geNorm, NormFinder algorithms). MIQE guidelines require ≥2 validated reference genes.

Glossary

RT-PCR
Reverse transcription PCR: RNA → cDNA (by reverse transcriptase) → PCR amplification; detects and quantifies RNA including mRNA and viral RNA; basis of gene expression analysis.
RT-qPCR
Real-time quantitative RT-PCR; measures mRNA abundance by combining reverse transcription with real-time fluorescence PCR; uses ΔΔCt for relative or standard curve for absolute quantification.
No-RT Control (−RT)
RT-PCR reaction lacking reverse transcriptase; detects genomic DNA contamination in RNA samples; should be negative; positive result requires DNase treatment of RNA.

Frequently Asked Questions

Regular PCR amplifies DNA from a DNA template. RT-PCR amplifies a DNA copy made from an RNA template. The extra step is reverse transcription: RNA is converted to complementary DNA (cDNA) using reverse transcriptase enzyme and specific or random primers. Then PCR amplifies the cDNA. RT-PCR is required when: (1) the target is RNA (mRNA expression analysis, viral RNA detection); (2) you want to detect only transcribed regions (exons, not introns — assuming exon-exon junction spanning primers); (3) measuring how much a gene is expressed. Regular PCR detects DNA — whether a gene is present, not whether it is transcribed.

RT-PCR: two-stage process — reverse transcription of RNA to cDNA, followed by conventional PCR. Products are analyzed by gel electrophoresis after amplification — qualitative (is the target present?) or semi-quantitative (band intensity comparison). RT-qPCR (real-time quantitative RT-PCR): includes real-time fluorescence detection during PCR amplification. Ct values allow precise quantification. The ΔΔCt method or standard curve method gives relative or absolute mRNA levels. RT-qPCR is the standard for gene expression studies due to its sensitivity, dynamic range (up to 10⁷-fold), and quantitative accuracy.

A no-RT control (also called -RT or NRT control) is an RT-PCR reaction that contains all components except reverse transcriptase. If a positive signal appears in the −RT control, the sample contains genomic DNA contamination (RT-PCR with exon-exon junction primers would normally not amplify genomic DNA or produce a larger band due to the intron, but not all primer designs span junctions). No-RT controls should be included for every RNA sample to confirm that signals in the real RT-PCR come from RNA (cDNA), not residual genomic DNA. Treat samples with DNase I before RT if no-RT controls are positive.

One-step RT-qPCR: reverse transcription and PCR occur sequentially in the same tube using the same gene-specific primers; simpler workflow; fewer pipetting steps; better for single-target high-throughput; less flexible. Two-step RT-qPCR: first perform a separate RT reaction (using random hexamers, oligo-dT, or gene-specific primers) to create cDNA; then use an aliquot of this cDNA in qPCR; allows multiple genes to be analyzed from the same cDNA preparation; more flexibility; more sensitive when RNA input is limiting; preferred for comprehensive gene expression profiling. Two-step is generally preferred for research applications; one-step suits clinical diagnostics (like COVID-19 testing).