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Bodies ... The Exhibition
Frequently asked questions about DNA
Sunday, December 16, 2007
In recent years DNA has become a leading crime-fighting tool. We asked Denise Rankin, DNA lab supervisor at the Miami Valley Regional Crime Lab, to explain why.
Q What is DNA?
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A DNA, or deoxyribonucleic acid, is in nearly every human cell in the body. It's acquired from your mom and your dad, and while humans share 99.9 percent of DNA, the other 0.1 percent makes us unique.
Q How do you keep track of and store all the DNA evidence?
A Each criminal case is given a lab case number, and each piece of evidence submitted in that case also gets a number and are tracked by a computer inventory system. Samples dating back to 1983 are kept in six large freezers at the crime lab.
Q How do forensic scientists determine if there is DNA evidence and how do they examine it in the lab?
A Generally, we're looking for a body fluid, mainly blood, semen and saliva. There are tests to identify these body fluids. We can also take samples on evidence that might contain skin cells. Usually, we are looking for evidence discarded at the scene which might contain DNA from the perpetrator. Next we take these samples through the DNA process to see if we obtain a DNA profile.
Q How can you tell that DNA probably came from one particular person?
A Scientists examine 15 locations on a strand of DNA that are highly variable between individuals. The building blocks of DNA are adenine, thymine, cytosine and guanine (A, T, C, G). These pair with one another to form DNA's double helix, a twisting genetic ladder. Those 15 locations give us a unique picture of someone's DNA that can be compared against DNA taken from crime scenes. The combination of all 15 locations are used to estimate how often that exact profile would be found in the population. Often, when all 15 locations on a suspect's DNA sample and a crime scene sample match, the numbers can be one in a quadrillion or higher that the samples came from the same person. If just one location doesn't match, then the DNA didn't come from the same person. If some locations are unreadable because the original DNA sample was degraded, the numbers are presented as to how often this particular combination of markers would appear in the population.
Q How long does it take your lab to analyze DNA and how much does it cost?
A It's not as quick as on TV. Generally, it takes a month and costs a few hundred dollars per sample. A check and balance to ensure accurate data, the entire file has to be reviewed by a second DNA analyst who must agree with the conclusions before a report can be issued.
Q Can DNA from very small or very old samples still be useful in investigating crimes?
A Yes. We have the technology to get DNA on items that have been handled by perpetrators and we routinely get DNA profiles on evidence from old cases.
Q How many criminal cases has the DNA lab helped investigate?
A Since 2003, when we started tracking the numbers, DNA has been helpful on 578 cold and current cases – giving law enforcement more information for their investigations.
Q How can DNA be an important tool in solving crimes?
A DNA left at a crime scene may give police suspects to investigate or witnesses to interview. We can also link crimes together when the same DNA is found at different crime scenes, such as in burglaries or serial rapes. DNA gives police information but there should be a lot to an investigation other than just one piece of evidence.
