Compare and contrast PCR and qPCR.

• A. PCR quantifies DNA in real time; qPCR only amplifies.
• B. PCR amplifies DNA; qPCR quantifies DNA in real time using fluorescent signals; Ct values correlate with initial template amount.
• C. PCR uses RNA templates; qPCR uses DNA templates.
• D. PCR uses reverse transcription; qPCR does not.

Answer: (B)

The essential idea is that standard PCR is about making more DNA and checking the result at the end, while qPCR adds a real-time readout to measure how much DNA is present as amplification happens.

In regular PCR, you cycle to amplify DNA and then assess the product afterward (often by gel), so you don’t know the quantity during the run. It’s primarily qualitative or semi-quantitative. In qPCR, fluorescence is measured during each cycle, so you can track how quickly the DNA is amplified. The key metric is the threshold cycle (Ct): when fluorescence crosses a predefined threshold, which happens earlier if there was more starting template. Therefore, Ct values inversely reflect the amount of initial DNA—lower Ct means more starting material.

Because of this, qPCR can quantify DNA amounts in real time, either absolutely (using a standard curve) or relatively (comparing samples). This combination of real-time detection and quantitative readout is what sets qPCR apart from standard PCR.