Explanation

Principle of PCR The purpose of a PCR (Polymerase Chain Reaction) is to make a huge number of copies of a gene. This is necessary to have enough starting template for sequencing There are three major steps in a PCR, which are repeated for 30 or 40 cycles. This is done on an automated cyclers, which can heat and cool the tubes with the reaction mixture in a very short time (i) Denaturation at $\mathbf{95℃}$ During the denaturation, the double-strand melts open to single-stranded DNA, all enzymatic reactions stop (for example : the extension from a previous cycle) (ii) Annealing at $\mathbf{54℃}$ The primers are jiggling around, caused by the Brownian motion. Ionic bonds are constantly formed and broken between the single-stranded primer and the single-stranded template. The more stable bounds last a little bit longer (primers that fit exactly) and on that little piece of double-stranded DNA (template and primer), the polymerase can attach and starts copying the template. Once there are a few bases built in, the ionic bond is so strong between the template and the primer, that it does not break anymore (iii) Extension at $\mathbf{72℃}$ This is the ideal working temperature for the polymerase. The primers, where there are a few bases built in, a already have a stronger ionic attraction to the template than the forces breaking these attractions. Primers, that are on positions with no exact match, get loose again (because of the higher temperature) and don’t give an extension of the fragament The bases (complementary to the template) are coupled to the primer on the 3’ side (the polymerase adds dNTPs from 5’ to 3’, reading the template from 3’ to 5’ side, bases are added complementary to the template)