When crime scene technicians find DNA such as semen at the scene of a crime and collect it, they must undergo the task of determining whose it is whether it is the victim's or the suspects. You have seen crime lab technicians on CSI inject fluids into this rectangular medium containing a colorless gel. Then you see these little bands spread out within the gel. Just what exactly are those bands within the gel and how do they tie a criminal to the scene of a crime?
In this article, I will explain what those bands of DNA are and how forensic scientists use these bands to make comparisons of unknown genetic material found at the crime scene to known suspects.
When a forensic scientist comes across unknown bodily fluids containing DNA, they use a gadget called an electrophoretic gel. This gel is divided into several parallel columns called lanes. A DNA sample is then injected at the beginning of each lane just like you see on CSI. The scientist then runs the gel on an electric current which causes DNA fragments in each lane to move creating these bands are spread out through the entire length of each lane. Each sample of DNA separates into a series of bands determined by the size of the various fragments in each sample. The unique bands are then compared, and the unknown bands in one lane are compared to the known bands in another lane. If a lane containing unknown DNA has the same band pattern as that of a lane containing known DNA, this is a positive match. Technicians can make this comparison visually or with the help of a computer.
The samples put at the beginning of each lane are variable. Some are control samples taken from bacterial, viral, or lab-synthesized DNA. These samples contain DNA fragments of known sizes and can be used in determining the sizes of fragments in any unknown samples. Furthermore, one column contains the crime-scene sample, and the other columns contain samples taken from any suspects. For instance, suppose that an offender cut himself at the scene of a homicide, and crime scene technicians found and collected it. The offender's blood would be considered the unknown sample of DNA because nobody knows whose it is. When a list of suspects involves three people, DNA samples are taken from each one of them because they are considered known samples. The process of DNA fingerprinting compares the suspects' DNA with that found at the crime scene.
No two people have the same DNA fingerprint except for identical twins. If this is the case, authorities can still determine which twin is which by their unique fingerprints.
Now that you know what electrophoretic gel and DNA bands are, you will have a better understanding of them the next time you watch your favorite CSI TV program.
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