What is PCR?

PCR is a method of copying DNA molecules. DNA replication is common in life; for example it takes place inside your own cells every time they divide. An enzyme known as polymerase uses one strand of DNA as a template to create a complementary strand. The result is that one double stranded DNA molecule is converted into two, both identical to the first.

PCR, or the polymerase chain reaction, adds two components to this process. The initial reaction yields twice the number of starting molecules, but then is immediately followed by a subsequent reaction, which yields twice the molecules as the first reaction. This is why PCR is known as a chain reaction. Commonly 25-40 reactions are chained together, theoretically resulting in 225 – 240 more molecules of DNA then were initially present.

Additionally, the goal of a PCR reaction is commonly to replicate only a portion of the genome of interest. For example, somewhere between 75-1000 bases, instead of the entire human genome of 3 billion bases. As PCR produces billions of copies of only the DNA of interest, this process is known as “amplification”.

Why is PCR important?

The amplification provided by PCR is very powerful. For example, suppose we want to detect whether a dangerous E. Coli pathogen is present in a sample of meat. That meat sample contains a huge amount of DNA from the meat source, and many non-pathogenic bacteria. Looking for the DNA from the pathogenic E. Coli, is akin to searching for a needle in a haystack.

However a PCR reaction can be designed to amplify only the DNA from a portion of this pathogenic E. Coli. If the pathogen is present, we can make billions of copies of its targeted DNA, which will come to outnumber the overall DNA originally present in the sample, and allow us to easily detect it. If no such signal is amplified by a properly controlled reaction, we can conclude the pathogen was not present.

How is it used?

PCR and related techniques have many applications. Here are just a few

Human Diagnostics

  • Detecting viral infections (HIV, etc.)
  • Detecting bacterial infections (Tuberculosis, etc.)
  • Genotyping (detecting genetic variants, which can indicate predisposition to disease)

Environmental Monitoring

  • Water quality monitoring
  • Food safety testing

Scientific Research

  • Preparing DNA to sequence
  • Monitoring gene expression levels
  • Manipulating DNA in genetic engineering and synthetic biology

How does PCR work?

Sometimes a picture is worth a thousand words, so check out this animation: