qPCR (real-time PCR) protocol explained

Understanding qPCR: A Key Technique in Viral Detection

Overview of qPCR

• Quantitative PCR (qPCR), also known as real-time PCR, is a technique used to quantitatively assess the presence of specific gene sequences in samples. It is currently the gold standard for testing viral infections like COVID-19.

• The process begins with collecting a swab sample that may contain viral RNA, which is then extracted and reverse transcribed into DNA for analysis.

Mechanism of qPCR

• Unlike conventional PCR, where DNA amplification is detected post-reaction, qPCR allows for real-time monitoring of the amplification process using fluorescent reporters.

• The cycle threshold (Ct value) indicates when the amplified product's fluorescence surpasses background levels, marking sufficient accumulation for detection.

Assay Methods in qPCR

SYBR Green Assay

• In the SYBR Green assay, an intercalating dye binds to double-stranded DNA. Fluorescence increases as more DNA is synthesized during each cycle.

TaqMan Assay

• The TaqMan method utilizes specific probes that bind to target sequences. Upon synthesis of complementary DNA strands, these probes are cleaved, releasing a fluorescent signal detectable after each cycle.

Analysis and Interpretation

• During denaturation in both assays, primers bind to their complementary sequences while fluorescent signals are monitored throughout amplification cycles.

• The Taq polymerase enzyme has 5-prime exonuclease activity that degrades the probe during amplification, leading to increased fluorescence as products accumulate.

Applications and Implications

• Gene expression analysis can compare multiple genes by assessing Ct values; lower Ct values indicate higher concentrations of target DNA.

• qPCR has diverse applications including diagnostics for pathogen detection and gene expression studies.