Question

In the polymerase chain reaction, DNA is heated to 95 degrees Celsius and nucleotides, enzymes and DNA primers are added to the mixture. Why is the temperature altered several times during one cycle?

Answer 1

The Polymerase Chain Reaction(PCR) consists of a number of cycles in which the temperature is changed at regular intervals. Each temperature alteration has an associated function within the cycle. The DNA is first heated at 95 degree Celsius to promote denaturation of the double stranded DNA.This is required for the primers to bind to the individual DNA strands. This is called as the "denaturation" stage.

In order for the primers to bind to the DNA, the temperature has to be lowered (this is dependent on the primer size and its GC content), the range of which is usually 55-60 degree Celsius. This is called the "annealing" stage.

Once the primers have bound to the DNA strands, the temperature is again altered, this time increased to 72 degree Celsius, the optimum temperature for the DNA polymerase to function. The DNA polymerase then acts to form new DNA strands from the template.

This process is repeated for several cycles.