When working on any project, being accurate is critical. One of the ways to promote accuracy is to have multiple trials of your research. If you have multiple times of working through every procedure and multiple different sets of results, any discrepancies can be more easily found and analyzed. Duplication allows for more trials and thereby more comparisons to see if the result is normal or if there was increased likelihood of something having gone wrong with that trial.
Polymerase Chain Reaction is a method for duplicating DNA segments. It allows for exponential duplication in a short space of time (Fig. 1). With this cloning effect, researchers are better able to take small initial concentrations of the DNA but have plenty of material to work with and test.
Figure 1. A brief description of the process of Polymerase Chain Reaction. (http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1993/illpres/pcr.gif)
The process of replicating DNA by PCR involves denaturing, annealing, and elongating. As shown in the figure, these steps take place at different temperatures. Denaturing is carried out at 95°C (203°F), and causes the two strands of DNA to separate. Once the strands are separated, the solution is cooled to 50°C (122°F) to allow the DNA to reassemble, or anneal. The solution has a plethora of DNA primers, which crowd the DNA strands, preventing all the DNA from rejoining exactly how it was broken apart in the first heating step. Once a primer is attached to the DNA strand, the complementary strand cannot attach, but a new strand can be made to compliment the first strand. The solution is then brought to 72°C to allow the DNA polymerase to bind to the primers. The DNA polymerase adds nucleotides from where it binds to the end of the DNA strand. This creates a copy of the DNA including the desired fragment and some additional DNA. The cycle is repeated. On the third cycle, the strands of DNA which are made only of the desired fragments begin being produced. The more cycles after this point which are carried out, the greater the number of desired-fragment only segments of DNA are produced. To see this process explained in more detail step by step, go to: http://learn.genetics.utah.edu/content/labs/pcr/. This website involves a stepwise demo in which you can interact with it in some instances and it also shows what is being explained while explaining it.
Another great resource for if you are interested in learning more specifics about PCR is located at http://www.ncbi.nlm.nih.gov/genome/probe/doc/TechPCR.shtml. The NIH not only describes the basic process of PRC, but also identifies the applications of this method, current project examples, technologies, and locations to purchase equipment for the technique.
In general, PCR is a fast and effect method for replicating precise DNA segments. It allows for more flexibility in research because less original DNA is needed than is required for most analyses with the DNA. It applies simple concepts and takes advantage of natural replicating processes. A themocycler is used for PCR, which could be a challenge to invest in. The PCR tubes are cheaper, but disposable so add a constant maintenance amount.
The Human Genome Project and Cancer 
