PCR z-bio 2025-10-11 2966

　　Are you always confused by PCR, RT-PCR, and qRT-PCR when doing experiments? As a "required introductory course" in biological research, the differences between the three actually have a clear logical line. Today, we not only use vernacular language to help you distinguish them right away, but also combine the latest technological breakthroughs and practical pain points to teach you to easily avoid the experimental pits you have stepped on over the years!

　　Ordinary PCR: DNA’s “copier”, don’t be careless about basic operations

　　Ordinary PCR is like a precise DNA copying machine. Its core task is to amplify target double-stranded DNA fragments in large quantities, making the originally trace amounts of fragments "visible and accessible." The principle is not complicated: double-stranded DNA is used as the template, dNTP is used as the synthetic raw material, and under the catalysis of Taq DNA polymerase, the target fragment is "positioned" through a pair of specific F/R primers, and through a cyclic reaction of denaturation, annealing, and extension, exponential replication of the fragment is achieved.

　　1. Core application scenarios

　　After the size of the product is confirmed by agarose gel electrophoresis, it can be used for basic molecular experiments such as target gene cloning, pathogen DNA detection, mutation analysis, etc. It is a "raw material preparation machine" for subsequent complex experiments.

　　2. Tools and practical guide to avoid pitfalls

　　Primer design: It is recommended to use the NCBI website to confirm the target sequence and design with Primer Premier software. Focus on checking whether the primers form dimers. The brightness of the electrophoresis bands of the two primers must be roughly the same to avoid amplification failure due to concentration imbalance.

　　Key reagent control: Taq enzyme needs to be stored in aliquots to prevent inactivation. Mg²⁺ concentration is a "double-edged sword" - too high will reduce specificity, and too low will directly affect yield. It is recommended to optimize according to gradient concentration pre-experiment.

　　Frequently Asked Questions: If there is no amplified band, check the template first (whether it is degraded or contains inhibitors). If non-specific bands appear, you can increase the annealing temperature or reduce the number of cycles. The flaky drag is mostly caused by too much enzyme or too high a dNTP concentration.

　　RT-PCR: RNA’s “identity converter”, preventing degradation is the key

　　If the research object is RNA (such as gene transcription), ordinary PCR cannot do anything - because it cannot directly use RNA as a template. This is when RT-PCR comes on the scene. It has an additional step of "reverse transcription" magic, which can first convert RNA into stable cDNA and then perform subsequent amplification.

　　1. Core process

　　Total RNA extraction→genomic DNA removal→reverse transcription synthesis of cDNA→PCR amplification→gel detection. This process can accurately reflect the transcriptional activity of the gene and is often used to verify whether the target gene is expressed, determine the promoter interval, etc.

　　2. RNA operation “line of life and death”

　　RNA is easily degraded by RNase, so the “anti-pollution string” needs to be tightened during the entire operation, and the process should be optimized based on experimental techniques:

　　1) Prevent degradation during the entire process: In addition to treating the table with enzyme-free consumables and RNase scavenger, it is necessary to ensure that the sample is fully lysed during Trizol extraction, and the chloroform shaking must be vigorous and rapid to avoid RNA retention in the organic phase.

　　2) Quality inspection upgrade: While observing the three bands of 28S, 18S, and 5S with electrophoresis, use a UV spectrophotometer to measure the OD value - the 260/280 ratio is 1.8-2.0, and the 260/230 ratio is greater than 2, which is considered qualified RNA.

　　3) Control settings are essential: a “no reverse transcription control” must be added (use non-reverse transcribed RNA as a template to amplify the housekeeping gene) to confirm that the genomic DNA has been removed, otherwise the results will all be false positives.

　　qRT-PCR: Accurate “dose scale”, fluorescence tracking + quantitative upgrade

　　qRT-PCR (real-time quantitative PCR) is the “most accurate” player among the three. It can not only amplify fragments, but also track the amplification process in real time through fluorescence signals, ultimately achieving quantitative analysis of the target sequence. The technological breakthrough of the Guangzhou laboratory in 2025 will change it from "slow cooking" to "stir-frying".

　　1. New breakthroughs in principles and technologies

　　Conventional qRT-PCR requires three steps: "RNA→cDNA→fluorescence quantitative amplification". The core is to reflect the amount of DNA synthesis through the intensity of the fluorescence signal. The ultra-fast qRT-PCR technology developed by Professor Xu Qiang's team increases the temperature rising and cooling speed to 50-100 degrees Celsius per second, compressing the original 2-hour amplification to 12 minutes. The entire detection process only takes 30 minutes, but the cost is comparable to traditional methods.

　　2. Mainstream fluorescence detection methods

　　1) Fluorescent dye method (represented by SYBR Green I): The dye can specifically embed into double-stranded DNA and emit strong fluorescence. During the PCR extension stage, double-stranded DNA is synthesized, and the fluorescence signal intensity is proportional to the content of double-stranded DNA.

　　2) Fluorescent probe method (represented by TaqMan): The probe is an oligonucleotide chain targeting the target gene, and the two ends are labeled with a fluorescent reporter group and a quenching group respectively. When the probe is intact, the fluorescence is quenched. During amplification, Taq enzyme hydrolyzes the probe to separate the two groups and release fluorescence. The instrument detects the fluorescence intensity once per cycle, forms an amplification curve, and achieves relative or absolute quantification through the Ct value.

　　3. Three major high-frequency application scenarios

　　1) Quantification of intestinal flora: Extract fecal genomic DNA, amplify it with 16S rRNA variable region-specific primers, and use ultra-fast qPCR to achieve rapid sample screening. Specific primers can be designed based on the variable region of the 16S rRNA gene of the bacteria (it is recommended to refer to the verification primers of published literature).

　　2) Detection of small RNA: Small fragments such as miRNA need to be reverse transcribed using the stem-loop primer method. The TaqMan probe method is preferred for quantification. LNA modified primers can further improve the specificity.

　　3) Circular RNA analysis: First use RNase R to degrade linear RNA, design divergent primers across BSJ, and select 18S rRNA as the internal reference (to avoid easily degradable molecules such as GAPDH).

　　4. Key points to avoid pitfalls in quantitative results

　　1) Troubleshooting abnormal Ct values: If the Ct value is too large, it may be due to insufficient template. If the Ct value is too small, be wary of contamination. It is recommended to set up a negative control (no template) and a positive control (known concentration standard).

　　2) Data standardization: Relative quantification requires the selection of stable internal controls, and absolute quantification requires the preparation of a standard curve to avoid result deviations due to operational differences.

　　Three minutes to distinguish between the three: one table to resolve the core differences

　　Experience pool for scientific researchers: I have stepped through these "hidden pits" for you

　　Primer storage: freeze and store in high-concentration aliquots to avoid repeated freezing and thawing. Centrifuge before use to prevent insufficient concentration due to adhesion to the tube wall.

　　RNA dissolution: After washing with 75% ethanol, avoid excessive drying, and incubate with RNase-free water at 37°C for 10 minutes to aid dissolution.

　　Contamination treatment: Amplification product contamination is the main cause of false positives. It is recommended to operate the experiment in separate zones, use filter tips for pipettes, and regularly illuminate the table with ultraviolet light.

　　The implementation of ultra-fast qPCR technology in 2025 will bring nucleic acid detection into the "minute-level" era, and a solid grasp of basic operations is still the foundation for accurate experiments. What difficult problems have you encountered in your experiments? For example, the Ct value of qPCR fluctuates or RNA extraction fails repeatedly? Welcome to share in the comment area and unlock more pit-avoiding skills together!