Each cycle of PCR doubles the amount of DNA synthesized in the previous cycle. A pair of primers is used to direct the specific synthesis of a desired DNA segment. The following diagram shows the products of PCR amplification after three cycles. The DNA strands of the desired segment are indicated in yellow boxes in the diagram. As shown, at the end of the third cycle, 8 out of 16 strands have the desired length and are boxed. After four more cycles (i.e. at the end of the seventh cycle), how many strands of DNA are expected to be synthesized? How many of them have the desired length? Write down the two numbers and separate them with a comma, e.g. 16,8.

Question

Quizplus · Accepted Answer

The total number of DNA strands grows exponentially, whereas the number of longer-than-desired strands grows linearly. After seven cycles, there would be a total of 256 strands (i.e. 2 × 2?) among which 16 strands are longer than desired (i.e. 2 original strands plus 2 × 7 longer primer extensions) and the remaining 240 have the desired length. After several more cycles, essentially all of the DNA strands will be the desired length.

Each Cycle of PCR Doubles the Amount of DNA Synthesized