Describe a typical cycle of a PCR reaction.

Polymerase Chain Reaction or PCR is the process of amplifying a sample of DNA. The first stage of a PCR cycle is known as the denaturation phase. This occurs at a temperature of 95C and lasts for approximately 30 seconds. During this phase the hydrogen bonds that hold the base pairs of the double stranded DNA are seperated due to the high temperature forming 2 single strands of DNA. The second phase is known as the annealing stage of the reaction. This is when the forward and reverse DNA primers bind to the two seperate DNA strands creating a location for the DNA nucleotides to bind. This occurs at a temperature of around 55C for 30 seconds, depending on characteristics of the primer it may however vary. 

The final phase is the extension stage of the reaction which occurs at 75C for a time of 60 seconds. This involves DNA nucleotides binding att he site of the primers and extending the DNA chain, unlike normal DNA extension a special enzyme known as Taq polymerase must be used instead of regular DNA polymerase due to its thermal resistant properties. The taq polymerases then run in a 3'-5' manner extending the DNA strands until 2 identical DNA strands remain. This entire process is repeated multiple times in orderto amplify the initial strand of DNA