Reverse transcription before PCR

Reverse transcription before PCR is two enzyme steps in series. A reverse transcriptase makes complementary DNA from RNA. A DNA polymerase then amplifies a chosen stretch of that cDNA. When the gel is blank, smeared, or positive in a tube that never received reverse transcriptase, the fix depends on which step failed. This page is that troubleshooting path. It is not a quantification method. Relative expression, with cycle thresholds and reference transcripts, is RT-qPCR for relative expression. The PCR half of the work is still the endpoint cycle in how polymerase chain reaction works.

Two enzymes, even when the kit is one tube

In a two-step layout you finish reverse transcription, then move an aliquot of cDNA into a separate PCR. You can test several primer pairs, and you can see that the PCR works on a DNA control while this cDNA does not. In a one-step layout both enzymes share the tube. Handling decreases, and so does your ability to rerun only the PCR. A blank one-step reaction still has two suspects.

Many MMLV-type reverse transcriptases are used near 37 to 42 Celsius. Engineered enzymes often ask for a higher temperature so RNA structure melts. Follow that card. A 72 Celsius hold copied from PCR extension denatures the wrong enzyme. After a two-step reaction the PCR has its own polymerase and its own primers. Treat the RT primer and the PCR primers as the same pair only when you chose a gene-specific RT primer on purpose. An RNase inhibitor is a reagent class, not a substitute for clean handling. Use the RT card's dNTP and magnesium amounts. Do not import the PCR mix's magnesium.

Three ways to prime the RNA

Oligo-dT primers bind the poly(A) tail. They miss broken RNA that has lost the tail and transcripts that were never polyadenylated, and they bias cDNA toward the 3-prime end. A blank 5-prime assay beside a working 3-prime assay on the same oligo-dT cDNA is that bias, not proof the gene is off.

Random primers copy broadly, including the ribosomal RNA that dominates total RNA. That helps degraded or non-polyadenylated samples, and it means most of the cDNA is not your gene. A gene-specific primer, often the reverse PCR primer, copies one region and spends the RNA on one assay. A blank from that cDNA in an unrelated PCR is not a biological negative. Change priming class and polymerase in separate experiments.

The no-RT control is the branch point

A no-RT control contains the same RNA and the same PCR, without reverse transcriptase. Sometimes the practical version is an RT reaction with the enzyme omitted, then the same PCR. A band at the sample's size means DNA that was already there was amplified. Genomic DNA is the usual source. Amplicon contamination is the other. Either way, the sample lane cannot be called cDNA.

Intron-spanning PCR primers help, because a long intron makes the genomic product larger or impossible. A processed pseudogene can still match the spliced size. Keep a no-RT lane on every RNA sample you will interpret, not on one control RNA for the whole plate.

If sample and no-RT both show the band, treat the RNA with a DNase class and inactivate that DNase so it does not destroy the later cDNA or the PCR. Follow the DNase card. Then repeat both the sample and the no-RT tube. You may claim an RNA origin only when the no-RT lane goes blank and the sample remains at the expected size. If both go blank, the previous band was DNA, and you have not yet shown that the transcript is detectable.

A no-template PCR control, with no RNA and no cDNA, still belongs in the run. It catches primer-dimer and carry-over. It does not catch genomic DNA in the RNA prep. The no-RT tube is the one that does that.

Split a blank gel before you replace everything

A260/A280 near 2.0 is a cleanliness hint, not intact RNA and not proof that inhibitors are gone. Integrity is a separate trace or gel. How the RNA should have been protected is protecting RNA during extraction. Fragmented RNA makes oligo-dT under-represent the transcript and can make random priming smear. That is not a PCR-primer failure.

Next split the enzymes with a DNA control: a plasmid or amplicon the PCR primers should copy, run in the PCR mix only. If that control is blank, stop debugging the reverse transcriptase. Fix the PCR primers, the activation hold, or the PCR mix. If the DNA control is a clean band and every RNA sample is blank, including one you know should contain the transcript, the RT step or the RNA handoff failed. Check that the reverse transcriptase was added, that the RT primer class matches the transcript, and that the temperature was the RT card's temperature.

A positive RNA that works, beside blanks from the experimental samples, points at those samples: degradation, a failed extraction, a gene that is not expressed in that material, or an inhibitor. Spike a little of the positive RNA into an experimental sample. If the spike dies, the sample matrix is inhibitory and a blank is not "gene off." If the spike lives and the sample stays blank, absence of detectable transcript is the fairer description, still limited to this assay.

A smear from random priming of degraded RNA should shrink toward a discrete band if you repeat with a gene-specific primer on RNA you trust. If the smear is also in the no-RT lane, you are amplifying DNA fragments, not cDNA.

What this gel must not be asked to say

Do not rank samples by band brightness and call it expression. Do not omit the no-RT control because the primers span an intron. Do not report a clinical or diagnostic result from a research reverse transcription. A single endpoint band, even with a clean no-RT lane, is presence and size. Sequence it if identity matters. Move to the RT-qPCR method, with its own controls, if the claim is a relative amount.

One-step reactions that fail cannot be rescued by "running the PCR again" on the same tube. The RNA has already been through the heat of the programme. Start from RNA that is still cold, with a fresh mix, and keep the no-RT tube in parallel.

Safety, ice and a warm handoff

RNase control is an institutional habit. Copying RNA into DNA does not change the template's biosafety level. This guide is not a diagnostic viral assay.

RNA that crosses a campus in a warm bag is a common reason the later PCR is blank while a bench control still works. Ice or a cold pack is the handoff. Wet gloves on a shared bench do more harm than humidity itself. After a power cut warms a freezer of RNA, rerun a known positive RNA before you interpret a stack of blanks, and set up the no-RT control from the same tube as the sample.

What to say when you ask for enzymes

Name RNA versus already-made cDNA, the priming class, one-step or two-step, the amplicon length, and what the no-RT lane did. Say whether a DNA control for the PCR half succeeded. Enzyme families are in the molecular biology catalogue. Put that pattern on the quote request. A reverse transcriptase and a PCR polymerase are different lines on the list. If the RNA comes from a difficult matrix, use the nucleic acid analysis pathway so extraction chemistry is part of the question. The multiplex PCR enquiry reference applies only when several cDNA targets must share the later PCR. It does not turn this endpoint gel into a quantified panel.