In one-step RT-qPCR or reverse transcription qPCR, cDNA synthesis and qPCR are performed in a single reaction tube. In two-step qPCR, cDNA is synthesized in one reaction tube, and aliquots of the cDNA is then used for a subsequent qPCR experiment.

• Accurate representation of target copy number
• Simple and rapid
• Fewer pipetting steps (reducing possible errors and contamination)
• Best option for high-throughput screening
• Best method when only a few assays are run repeatedly
• Multiplex qPCR of gene of interest and control can be done in single well, from same RNA sample

There are several other advantages of one-step reactions, these include limited sample handling and reduced bench time, which helps to decrease chances for pipetting errors and cross contamination between reverse transcription and qPCR steps. This method is quick to set up and makes processing multiple RNA samples easy (especially when using liquid handling robotics), when you are amplifying only a few genes of interest. It is therefore ideal for high throughput screening laboratories where only a few reverse transcription qPCR assays are run repeatedly, using well-established reaction conditions, with the added advantage that multiplex qPCR of the gene of interest and control genes can be done in single well, from same RNA sample.

Gene-specific primers are required for generating the cDNA and for subsequent amplification in one tube, however this reduces experimental variation, since both enzymatic reactions take place under the same conditions, making one-step qPCR highly reproducible.
One-step RT-PCR requires careful evaluation to prevent primer dimer formation, because the primers will be present during the lower temperature conditions of the RT reaction as well as the qPCR cycling. This is particularly important in multiplex qPCR assays.

As qPCR can amplify tiny amounts of DNA, preventing contamination is essential, as even small amounts of qPCR contamination can produce false positives. qPCR contamination can include cross-contamination from other samples, contamination from elsewhere in the laboratory, and carryover contamination from amplification products and primers used in prior qPCR experiments. The latter causes most of the false positive results seen in qPCR. Preventing contamination can be difficult and so laboratory procedures must be in place to ensure DNA from one assay is not re-amplified in subsequent assay.

Uracil DNA Glycosylase can specifically degrade products that have already been through the qPCR process and have incorporated dUTP, leaving native nucleic acid templates intended for amplification intact. Uracil DNA Glycosylase activation occurs as the first step of qPCR at a 50°C incubation for 2 minutes and then the Uracil DNA Glycosylase is denatured when the temperature increases to 95°C during hot-start activation.

Uracil-DNA glycosylase (UDG or UDGase) is a family of enzymes comprising six sub-families. Family I UDGase enzymes are called UNG (uracil-N-glycosylase gene), so there is no difference, the terms UDG and UNG are commonly used interchangeably because they perform the same function in qPCR—namely to prevent carryover qPCR contamination.