What is observed on a graph when target copy number is plotted against the number of PCR cycles?

When plotting target copy number against the number of PCR cycles, the graph typically demonstrates an exponential curve during the amplification phase. This is because, during PCR, each cycle ideally doubles the amount of the target DNA, leading to exponential growth in the quantity of DNA. This exponential phase is critical as it reflects the phase where the primers are bound specifically to the target sequences, and the enzyme is optimally functioning.

Initially, the target DNA is present in limiting amounts, leading to a rapid increase in copy number with each cycle. This continues until the reagents (such as primers, nucleotides, or polymerase) start to become exhausted as the reaction approaches its limits, at which point the curve may begin to plateau. In this plateau phase, the rate of amplification slows down significantly, and the increase in target DNA becomes less pronounced, reflecting the depletion of resources necessary for continued amplification.

Understanding this exponential phase is essential for quantifying the initial amount of target DNA, as it can directly correlate with the cycle number at which a signal crosses a certain threshold. Thus, the graph's shape during the amplification process is characterized by its exponential nature, making this the correct choice.