DNA synthesis and PCR

An important attribute of DNA is its ability to replicate itself, using the same base complementarity that allows the two strands to bind together. Guanine (G) and cytosine (C) can only bind with one another, and adenine (A) and thymine (T) can only bind with each other. When replication occurs the strands of the DNA molecule separate, and a complement to each strand is synthesized. Thus one strand of DNA can be used as a template for a new strand of DNA. The natural environment of the cell contains the necessary ingredients for this to occur: free floating nucleotides (G, C, A, and Ts that will make up the new strand), an enzyme called DNA polymerase that help the nucleotides find the correct location along the template strand, and other proteins that assist in the process. This process is called “semi-conservative” because the each new DNA molecule consists of one new strand and one original strand that served as the template.

The Polymerase Chain Reaction, or PCR, discovered in the 1980s, uses this natural ability of DNA to replicate itself, but allows the amplification of specific segments of the DNA, such as a gene or chromosomal regions of interest. Scientists mix together the ingredients necessary for the reaction, and use a machine called a thermalcycler, which produces cycling temperatures that encourage the process.

PCR is now applied extensively in many fields, including medicine and forensics. One use that is widely known, due to the popularity of TV crime shows, is genetic fingerprinting used to identify criminals.  In scientific research there are many additional applications of PCR (many of which are discussed in later sections), including:

* Finding and using molecular markers
* Gene mapping
* Cloning
* Studying genetic diversity
* Understanding evolution