DNA Testing: How It Is Done

Without going into extraordinary detail, I wanted to discuss two of the most common ways forensic scientists analyze DNA. The first way I want to talk about is restriction fragment length polymorphism (RFLP). This method has almost been fully replaced by the other method I will discuss for cost, sample state conditions, and time reasons. However, this method is slightly more accurate and might be used if ample amounts of "good" DNA are available. By good DNA I mean fresh and not contaminated because the longer DNA sits at room temperature the more denaturation, or degradation, will occur.

The way RFLP analysis works is by comparing DNA found at the scene with a suspect's DNA. This method uses special enzymes called restriction enzymes that cut double stranded DNA at a particular base sequence, called a restriction site. Different enzymes recognize different sequences, but each enzyme only recognizes one sequence. Therefore, the DNA at a crime scene can be introduced to a restriction enzyme in the lab and it will cut the DNA into sections of a certain length that is unique to every person. Everyone's DNA contains restriction sites but the distance between restriction sites is variable amongst us all. These distances are so variable that this method is 99.999% accurate. When conducting a DNA comparison the DNA from the crime scene will be spliced in one container and the DNA from the suspect in another one. The two samples will be "run" on an electrophoresis gel. This method is very similar to TLC. As you may know DNA is a negatively charged molecule so the samples are loaded into tiny holes, or wells, in the gel. Then an electric current is applied between the end the samples are loaded into (negative) and the opposite end (positive). The positive end attracts the DNA and the negative end repels it, but the key is that the current is uniform over the entire gel. Therefore, the only factor manipulating the distance the DNA travels is size, the longer fragments travel a shorter distance due to friction and the shorter fragments travel farther.

But back to comparing the DNA. If the crime scene DNA is loaded in a well next to the suspect's DNA and the gel is run, then a positive match would mean that both columns show DNA bands at the same lengths from the beginning of gel. Just like you would compare an unknown to a known in TLC.

The other method for DNA analysis involves polymerase chain reaction (PCR) and short tandem repeats (STRs). This method is much less expensive and can be conducted with a sample size of only a few cells. This method has almost completely replaced RFLP analysis.

The first part of this test is PCR. Without going into extreme detail just think of this process as xeroxing a portion of DNA many times over. Theoretically, you could start this process with only one strand of DNA and after 20 short cycles you would have over 1 million copies available. PCR uses primers to section off portions of DNA that have STRs present. A STR is a section of DNA that has a repeating section such as ATATATATATATATAT. Everyone has STRs located at key points in their genetic code, but the lengths of these sections are variable. Therefore, a crime lab could receive a sample of DNA from only a few cells and amplify that amount with PCR then analyze the length of those sections with gel electrophoresis (discussed previously) and compare that length to the suspects DNA.

These methods are extremely interesting and much more complex than what I have given you here. There are many resources that can go into great detail on this topic so if you have any questions please consult them or post your questions in the comments section of this blog.