Introduction to DNA


DNA is often called the stuff of life. It’s what controls who we are, how we look, and what we are able to do. Every single cell in our body contains a sample of our DNA, except our red blood cells. DNA is not limited to humans, but is also found in most living things including bacteria, animals, and plants. DNA is essentially a long string made up of four basic building blocks called bases. These bases are chemical components called nucleotides. There are four types of nucleotides: adenine, thymine, guanine, and cytosine. For simplicity, they are normally labeled A, T, G, and C. DNA is really a double strand of nucleotides. The base pairs always bond in a certain way: A always bonds with T, and C always bonds with G. These bonded pairs are called base pairs.

Each strand of DNA has about 3 billion base pairs. (For comparison, the DNA of e coli bacteria has about 4 million base pairs). Three billion base pairs strung together make a strand of DNA about three feet long. To compact itself into a cell, the strand is twisted, resembling a twisted ladder, thus providing an alternative name for DNA–the Double Helix.

The isolation of DNA began in the 1800s, but the actual structure of DNA wasn’t really understood until 1953. Scientists James Watson and Francis Crick were the first to discover the double helix design of DNA, as well as base pairing. They were awarded the Nobel Prize for their contributions nine years later.

The concept of DNA testing came later still. In the 1980s, British geneticist Alec Jeffreys made a remarkable discovery about segments of DNA that were typically considered purposeless or “junk DNA.” These segments actually were genetic markers that were unique to each individual (except identical twins). They are present in every cell and remain the same throughout a person’s life.

Jeffreys found a way to process these segments and create a pattern on x-ray film. With this process, DNA samples from a crime scene could be compared to samples from a suspect. Jeffrey’s called this technique DNA fingerprinting. His technique was first used to solve two horrific cases of rape and murder of two girls in 1983 and 1986. Colin Pitchfork has the distinction of being the first criminal convicted using DNA evidence.

In the short time that DNA fingerprinting has been available, its use has spread throughout several fields. Not only is DNA testing used in forensic applications, it is now standard operating procedure in paternity cases. Even individuals with an idle interest in their genealogical roots can access ancestry DNA tests that match their DNA profile to particular ethnic groups.