Real-Time Quantitative PCR Basis

ABI Prism® 7900HT real-time PCR instrument

Content Principles of quantification Methods of quantification Applications

The polymerase chain reaction (PCR) has revolutionized the detection of DNA and RNA. As little as a single copy of a particular sequence can be specifically amplified and detected. Theoretically, there is a quantitative relationship between amount of starting target sequence and amount of PCR product at any given cycle. In practice, though, it is a common experience for replicate reactions to yield different amounts of PCR product. The development of real-time quantitative PCR has eliminated the variability traditionally associated with quantitative PCR, thus allowing the routine and reliable quantitation of PCR products.

PCR Reaction Phase

Quantification in Log Phase Initial template ,more significant Perfect reproducibility, less error Constant amplification efficiency, linear standard curve

Several Basic Concepts Threshold: The low limit when PCR reaction signal goes into log phase Ct value: The cycle number of PCR reaction when the fluorescence signal reaches to threshold

Standard Curve CT = -k lgX0+ b There is a linear relationship between CT and lgX0 (X0 stands for amount of initial DNA) Use standard sample which we have known its initial concentration to make standard curve. And then we can identify the initial concentration of unknown template through the standard curve when we obtain its CT value.

Methods of quantification Dye method: SYBR® Green I Probe methods: TaqMan Probe and TaqMan MGB Probe Molecular beacons Dual-oligo FRET pairs

SYBR® Green I SYBR® Green I dye is a highly specific double-stranded DNA binding dye. Its fluorescence increases when it bound to double-stranded DNA while disappears when DNA denatured. There is a direct ratio between fluorescence signal and the amount of double-stranded DNA.

Advantages and Disadvantages about SYBR® Green I SYBR® Green I assay chemistry will detect all double-stranded DNA, including non-specific reaction products. The advantage of SYBR® Green I assay chemistry is that no probe is required, thus reducing assay setup and running costs. And it can make dissociation curve analysis.

Fluorescent Resonant Energy Transfer (FRET) When the emission band of one fluorophore is overlapped with the absorption band of another, and simultaneously they are very near , then the energy will transfer from short wavelength (high energy) fluorophore to the longer wavelength (lower energy ) one. In other words, the fluorescence of short wavelength fluorophore is quenched by another.

TaqMan Probe One probe, two primers. One probe, two fluorophores: one is reporter, another is quencher . Hybridises with the target amplicon。 Is 3’ terminally blocked (cannot be extended by the polymerase)。 Has two fluorescent dyes attached: 1.Reporter(R) 2.Quencher(Q)

React Process

The production of one DNA strand will cut one probe into pieces simultaneously. The disjunction of one probe will produce one fluorescent signal. Signal intensity are proportional to the amount of DNA which is binded by probe.

Advantages about TaqMan chemistry Noiseless data Due to the second level of specificity provided by the probe. Multiplex compatible Each probe can be differently colored and thereby mixed with others. Signal proportional to products Signal related to amount of amplified product . Unreversible reaction, signal will not quenched Other probe chemistries use reversible hybridisation to generate signal.

TaqMan MGB Probe Non background fluorescence All probes will be short(13-20bp) Tm enhancer: MGB Minor Groove Binder attached at 3' end of probe

SNP Discrimination (Allele Discrimination )

Result Homozygote: wild type: FAM mutant type: VIC Heterozygote: FAM+VIC

Other Methods Based on Probe Molecular beacons Dual-oligo FRET pairs

Applications: Primer and Probe Design Using Primer Express® Software Primer Express® software uses a set of default parameters to automatically select primer and probe sets. Even though no probe is required for SYBR® Green I dye detection, it is still a good idea to use Primer Express® software to select a primer and probe set when designing a SYBR® Green I assay. Although no probe will be used, the primers will meet all the required criteria and if, in the future, there is the need to convert the assay to TaqMan® assay chemistry to obtain higher specificity, the probe can immediately be found in the original Primer Express® software document.

Primer and Probes selection guidelines for quantitative assays

Applications Absolute Quantification (AQ) Absolute--produces data with quantity amounts The absolute quantity of target gene is measured needs a standard template whose concentration is known absolutely The quantity of the standard template has to be measured precisely unnecessary for most studies

Applications Relative Quantification (RQ) Relative--makes comparisons of quantity (no units) relative standard curve or △△CT analysis any stock DNA or RNA containing the target gene can be used to prepare relative standard curve normalises for amount of sample added needs endogenous control target the most powerful and widely used method

Applications Allelic Discrimination (AD) Plus/Minus with IPC (+/-)

THANKS！