qPCR for gene expression analysis can be done with primers and intercalating dyes such as SYBR® Green (Thermo Fisher) or with primers and fluorescent probes such as PrimeTime™ qPCR probes. Understanding the key benefits of each method will help you design the best assay for your research experiment.
Quantitative PCR, also known as real-time PCR, is a typical PCR reaction that amplifies a target sequence from a sample of DNA or RNA, but instead of assaying the amount of PCR product as an end-point value, qPCR assays monitor and measure the amount of PCR product produced in each cycle of amplification based on a fluorescent signal. The fluorescence is produced using intercalating dyes, such as SYBR® Green dye (Thermo Fisher) or using 5’ nuclease probes such as PrimeTime™ qPCR Probes. The accumulation of a PCR amplicon in each cycle is plotted on a graph where the x-axis has the cycle number, and the y-axis plots the relative fluorescence, a measure of the amount of PCR amplicons after subtracting any background fluorescence. qPCR reactions can be coupled with reverse transcriptase (RT) to quantify the abundance of an RNA transcript, which is sometimes referred to as RT-qPCR, qRT-PCR, or real-time RT-PCR. These assays are used to assess gene expression, a key measure for understanding biological systems, including development, splice variant-specific gene expression, or disease progression. Real-time PCR can also be used in genotyping applications, such as identifying copy number variations, determining frequency of SNPs in populations, multi-species variations, or association studies to determine genes relevant to specific traits or diseases.
The second and more specific method of quantifying the accumulation of the PCR amplicons is called a probe-based or 5’ nuclease assay, which is also known as a PrimeTime™ or TaqMan® assay (Thermo Fisher) (Figure 2). These assays include a fluorescently labeled oligonucleotide probe in addition to the forward and reverse PCR primers. The combination of probe and primers increases the specificity for the amplification reaction since all three must bind to the correct target for fluorescence signal to be generated. In addition, probe-based assays allow users to combine multiple primer and probe sets in one reaction. These multiplex qPCR assays include unique 5′ fluorophores for each of the probes, and therefore, the wavelength of fluorescence signal indicates which transcript is present and its relative abundance. For more information on selecting the different dyes for multiplex assays, see Recommended dye combinations for multiplex qPCR. Of course, multiplex PCR assays need to have real-time or qPCR instruments and software capable of recognizing the different fluorophores. IDT has a Multiplex Dye Selection Tool that offers guidance on what fluorophores are recommended for use with many qPCR instruments, and also alternative fluorophores.
PrimeTime qPCR probes are 5' nuclease probes with a 5′ fluorophore and a 3′ quencher that are designed to anneal between the two PCR primers. Since the quencher and fluorophore are in proximity, no fluorescence is recorded. During annealing, the probe and PCR primers attach to their complementary sequences in the target region. During extension/elongation, Taq DNA polymerase uses its exonuclease activity to hydrolyze or degrade the probe. Once hydrolyzed, the 5’ fluorophore is released from the quencher. Without a quencher in proximity, the fluorescence is recordable by the qPCR instrument. The amount of fluorescence correlates with the amount of PCR products that are synthesized.
It is important that probe length is short so the quencher is close enough to absorb the fluorescence energy. Longer probes will have high background fluorescence since the quencher is less effective. For more information on strategies to design qPCR probes with low background, see How to design primers and probes for qPCR and PCR,
Double-quenched probes increase qPCR sensitivity and precision, and Improve assays with customizable oligos and probes containing cost-effective locked nucleic acids.
SYBR Green and other types of intercalating dyes are more cost-effective for the researcher than 5′ nuclease probe assays but can pose issues if the primers form primer-dimers, bind to non-specific sites, or anneal to cross-reacting genes. PrimeTime qPCR Primer Assays include predesigned assays for human, mouse, or rat. These assays have been designed with advanced bioinformatics to optimize the assay to avoid these issues. The primer pair is designed and premixed for real-time PCR, which makes the reaction setup easy. In addition, the primer sequences are identical to a coordinating PrimeTime qPCR Probe Assays, which means you can easily transition from primer assays to 5′ nuclease assays to increase the assay specificity or establish a new multiplex assay. For more information on converting assay design, see SYBR to 5’ nuclease assays.
Familiarize yourself with critical parameters of qPCR assay design. This complete 62-page guide covers every aspect of probe-based qPCR assays.
Digital PCR (dPCR) includes the same reagents found in a typical qPCR assay and amplified in a similar manner, but dPCR divides the reaction into nano-sized droplets or wells prior to amplification. The partitions are so small that either 1 or 0 templates are in each. After amplification, the fluorescence in the well or droplet represents the genotype.
PrimeTime qPCR Probe Assays are 5′ nuclease assays that include a library of predesigned assays for human, mouse, and rat sequences. Custom assays may be created for these or other species using the PrimerQuest Tool. Predesigned and custom assays are available in tubes or plates.
PrimeTime qPCR Primer Assays for use with intercalating dyes, such as SYBR® Green dyes, include predesigned assays for human, mouse, or rat sequences. Custom assays may be created for these and other species using the PrimerQuest Tool. Predesigned and custom assays are available in tubes or plates.
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