Question

If your sample DNA contains 1 copy of the target sequence to amplify, and you run the PCR for only 10 cycles because of an instrument failure, about how many copies of the target sequence should exist in the PCR reaction tube?

a) 1 copy, PCR requires at least 20 cycles to amplify the target DNA

b) 100 copies

c) 1000 copies

d) 1,000,000 copies

e) Impossible to determine

Answer 1

PCR is the process of selective multiplication of a specific region of DNA molecule. The process is used to produce DNA fragments for cloning and is used to amplify certain segment of DNA. If the process of replication of DNA is repeated many times, the segment of DNA can be amplified to approximately billion times ie 1 billion copies are made at the end of 30 PCR cycles. The extension product of one cycle can serve as a template for subsequent cycles and each cycle essentially doubles the amount of DNA from the previous cycle. As a result, from a single strand template molecule,. it is possible to generate 2n molecules after n number of cycles.

But this is only possible after amplifying the target DNA for a specific number of cycles, because the number of cycles for amplification depends on the number of copies of template DNA present initially and the efficiency of primer extension and amplification. When one of the components become limiting, the reaction proceeds in a geometric phase. At this point the yield of amplification products is maximum. So atleast 20-25 cycles are required to amplify the target DNA so that acceptable levels of amplification of single copy of DNA can be achieved.

Here initially, the DNA sample contains 1 copy of target sequence to amplify, but there is only 10 cycles of PCR done. So, there will be 1 copy of target sequence exist in the PCR reaction tube, as PCR requires atleast 20 cycles to amplify the target DNA.