How Dna Works

In the last few years, DNA evidence has started to play a big part in many nations' criminal justice systems. It has been used to prove that suspects were involved in crimes and to free people who were wrongly convicted. In the United States, it has been integral to several high-profile criminal cases, including the trial of Orenthal James (O.J.) Simpson and the investigation of the 1996 murder of JonBenet Ramsey.

Most people have a basic idea of what DNA is. It's essentially an instruction manual and blueprint for everything in your body (see How Cells Work for details). A DNA molecule is a long, twisting chain known as a double helix. DNA looks pretty complex, but it's really made of only four nucleotides: Adenine, Cytosine, Guanine, Thymine .

These nucleotides exist as base pairs that link together like the rungs in a ladder. Adenine and thymine always bond together as a pair, and cytosine and guanine bond together as a pair. While the majority of DNA doesn't differ from human to human, some 3 million base pairs of DNA (about 0.10 percent of your entire genome) vary from person to person.

In human cells, DNA is tightly wrapped into 23 pairs of chromosomes. One member of each chromosomal pair comes from your mother, and the other comes from your father. In other words, your DNA is a combination of your mother's and your father's. Unless you have an identical twin, your DNA is unique to you. This is what makes DNA evidence so valuable in investigations -- it's almost impossible for someone else to have DNA that is identical to yours.

The key to DNA evidence lies in comparing the DNA from the scene of a crime with a suspect's DNA. To do this, investigators have to do three things:

Collect DNA at the crime scene and from the suspect (see How CSI Works for more information)

Analyze the DNA to create a DNA profile

Compare the profiles to each other

Authorities can extract DNA from almost any tissue, including hair, fingernails, bones, teeth and bodily fluids. Sometimes, investigators have DNA evidence but no suspects. In that case, law enforcement officials can compare crime scene DNA to profiles stored in a database. The most commonly used database in the United States is called CODIS, which stands for Combined DNA Index System.