11. What is the polymerase chain reaction? List the principles 
       behind the test and two examples of how it might be applied to 
       disease of the eye. 
Polymerase Chain Reaction is a technique in which any genetic sequence can be quickly amplified. The starting material for PCR is a solution of double stranded DNA containing the nucleotide sequence that is targeted for copying. It is important to know the DNA sequence of the interested gene.

The enzyme DNA polymerase, which catalyses the reaction, is then added together with nucleotides and primers. The primers determine the DNA sequence to be amplified. The DNA strand is denatured (separated) when heated to a certain temperature, and this allows the primers to bind to a single stranded DNA. 

Within a short time the amount of target DNA sequence has been doubled. By repeating this cycle, we can generate an unlimited number of copies of the DNA sequence. 

Examples in the use of polymerase chain reaction:

a. A patient has a severe posterior uveitis. The clinician is concerned about 
    acute retina necrosis caused by herpes simplex. The vitreous fluid can 
    be aspirated and sent for PCR detection of herpes simplex.

b. A contact lens wearer has an indolent ulcer and repeated culture has 
    been negative for Acanthoamoeba. The scrapped tissue can be sent for 
    PCR to look for any evidence of Acanthoamoeba.

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