In the very first round of PCR using genomic DNA, the DNA primers prime synthesi

Question

In the very first round of PCR using genomic DNA, the DNA primers prime synthesis that terminates only when the cycle ends (or when a random end of DNA is encountered). Yet, by the end of 20 to 30 cycles - a typical amplification - the only visible product is defined precisely by the ends of the DNA primers. (see attached figure). In what cycle is a doulble-stranded fragment of the correct size first encountered?

A. Cycle 10

B. Cycle 6

C. Cycle 3

D. Cycle 15

In the very first round of PCR using genomic DNA, the DNA primers prime synthesis that terminates only when the cycle ends (or when a random end of DNA is encountered). Yet, by the end of 20 to 30 cycles - a typical amplification - the only visible product is defined precisely by the ends of the DNA primers. (see attached figure). In what cycle is a doulble-stranded fragment of the correct size first encountered? ****For your answer please expain your rationale**** A. Cycle 10 B. Cycle 6 C. Cycle 3 D. Cycle 15

Explanation / Answer

The answer is C, cycle 3. The first cycle generates the products shown in the diagram you provided. The second cycle sees the primers bind to the new DNA strands (the ones in grey, not black) and extend to the ends. These new, new products are the correct length, but only one is produced since the other strand is still too long. The third cycle sees the new, new DNA strand that is the correct length bind to a primer, synthesizing another DNA strand of the correct length. You officially have the product seen 20 cycles later. Hope this helps!

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