What does the threshold cycle (CT) in classical PCR indicate?

The threshold cycle (CT) in classical PCR is a crucial concept, especially in quantitative PCR (qPCR) where it allows for quantification of the target DNA. The CT value indicates the cycle number at which the fluorescence signal produced by the amplified DNA crosses a predetermined threshold level. This threshold is set above the background noise to ensure that the detected signal is a true representation of the amplification occurring during the PCR process.

As the PCR cycles progress, the amount of target DNA increases exponentially, leading to a corresponding increase in fluorescence. The CT is significant because it correlates inversely with the initial amount of target DNA: lower CT values indicate more starting target DNA, while higher CT values suggest less initial target content. This property makes CT a valuable parameter for quantifying nucleic acids in various diagnostics and research applications.

In contrast, the other choices do not accurately describe the specific function of the CT value in PCR. The cycle at which amplification begins refers to a concept but does not precisely capture what the CT value represents. The total number of cycles completed doesn’t account for the fluorescence signal’s level or provide any information about quantification. Finally, the maximum fluorescence level achieved is not what the CT measures, as it focuses strictly on the point where the