You really should finish studying for your biology exam, but you just can't tear yourself away from the latest episode of CSI. After all, you will learn a little about biology just by watching, right?
CSI and similar programs have made forensics a part of pop culture. Although they aren't always 100% accurate in their portrayal, DNA analysis forensics has been immensely useful in solving crimes. Lucky for Team Science, these TV shows have also been useful in altering the stereotype of white-coat-wearing scientists.
If investigators are lucky, blood or bodily fluid may be left at a crime scene. Blood typing can be performed and may eliminate a suspect, but blood types are nowhere near as unique as an individual's DNA. Every person has a unique genetic profile, and this fact can be exploited to confirm or rule out a suspect when compared to the profile created by the DNA left at the crime scene.
As DNA technologies improve, so do forensic applications. When DNA first began to be used as evidence in 1988, forensic scientists combined the techniques of restriction fragment length polymorphisms (RFLP, remember rif lips?) with Southern blot to look for similarities in DNA sequences.
An even more sensitive technique has been developed using short tandem repeats
. These are repetitive sequences of DNA that are highly variable between people. PCR primers are used to amplify a particular STR, and the PCR product is then subjected to electrophoresis. The size of the product indicates the number of repeats and a comparison can be made between suspect and crime scene evidence. The beauty of this technique is due to PCR. Remember PCR is used to amplify DNA sequences, so only a small amount of starting material is needed.
Image from here
The DNA from as few as only 20 cells is all that is needed to perform STP analysis. 3
That's basically a sample the size of an amoeba.