What is the main difference between endpoint PCR and quantitative PCR (qPCR)?

• A. Endpoint PCR identifies whether a product is amplified beyond a threshold after cycles, while qPCR measures product accumulation in real time to quantify starting template.
• B. Endpoint PCR and qPCR both measure real-time product accumulation.
• C. qPCR identifies a product only after all cycles are completed.
• D. Endpoint PCR quantifies the starting template more accurately than qPCR.

Answer: (A)

The main idea here is how detection differs between endpoint PCR and qPCR. In endpoint PCR, you run all the cycles and only read out the product at the end, usually by gel or another end-point readout. This tells you whether the target was amplified enough to be visible, but it doesn’t reliably quantify how much starting template you had because amplification efficiency levels off in later cycles (the plateau effect).

In qPCR, the fluorescence is measured during each cycle, so you see the amount of product accumulate in real time. This real-time data lets you relate the cycle at which the signal crosses a threshold (the Ct value) to the amount of starting template, enabling accurate quantification over a wide dynamic range.

So, the best choice says: endpoint PCR detects if amplification passes a threshold after finishing cycles, while qPCR tracks product buildup in real time to quantify the starting material.

The other statements don’t fit because endpoint PCR isn’t measuring in real time, qPCR isn’t defined as simply identifying products only after all cycles, and endpoint PCR isn’t more accurate for quantification than qPCR.