Polymerase Chain Reaction
Polymerase Chain Reaction
Description:
A sample of amniotic fluid yields a small amount of cells and, therefore, DNA. Often when performing genetic testing on amniotic fluid, the first step is to use a technique known as polymerase chain reaction (or PCR) to make many copies of the region of DNA that is of interest, since there are too few to study in the original amniotic fluid sample. This process is called amplification and can be done on DNA extracted from any tissue type. In the case of sickle cell anemia, the beta-globin gene is amplified. 
To replicate DNA, PCR uses the following:
- Short DNA primers that are complementary to the DNA sequences on either side of the region of interest
- A DNA-copying enzyme called a polymerase
- Nucleotides, the building blocks used to make the DNA copies (A, C, T, and G)
Each of these components and the target DNA are added to the reaction mixture. The sample is heated to separate the double-stranded DNA into single strands. The mixture is then cooled allowing the primers to anneal, or stick, to complementary DNA sequences. The DNA polymerase elongates the strand of DNA starting at the primers. At the completion of the first cycle, there are 2 copies of the DNA sequence of interest. The process is then repeated so that there are 4 copies after the second cycle; each subsequent cycle doubles the number of DNA copies. Millions or even billions of copies can be accurately produced in a short period of time.
