|MadSci Network: Molecular Biology|
Hello Bazil, To do a DNA test on a sample of blood or a hair left at a crime scene, you must first ISOLATE the DNA, then QUANTITATE the sample, and finally TYPE the DNA and obtain a DNA profile. To isolate the DNA from the sample, we place the sample in solutions containing enzymes and detergents that lyse the cell membranes and remove proteins. This procedure is generally done in an oven or hot block to accelerate the process. After isolation, the DNA must be quantitated to determine how much there is and most importantly, how degraded the sample is. If the DNA is too degraded, i.e., broken up into small pieces, typing may be impossible. Generally, even very small samples can be typed successfully as long as they have remained dry. Moisture contributes to bacterial degradation of the DNA and don't forget, bacteria have DNA too, and this leads to contamination problems. Quantitation is generally performed using commercial kits that are available. The isolated DNA samples are denatured (the two DNA strands are separated) attached to a membrane and then exposed to a human specific "probe" that attaches to specific areas. The probes carry with them chemicals that can be detected using X-ray film. The darker the band, the more DNA that is present. By comparing the band intensities with known standards, the amount of DNA present can be determined. Most labs "amplify" the isolated DNA by a process called the polymerase chain reaction or PCR for short. Essentially, we make copies of the DNA that is present so that we have a sufficient amount of sample. DNA is a very long molecule and we only amplify areas of the DNA that we know are highly variable in people. Finally, we "type" the samples that we've amplified. We can do this in various ways but most labs now look at STRs which stands for short tandem repeats. These are pieces of DNA that we all have that consist of multiple repeats of three or four of the units of DNA such as ATGC. This sequence of letters may repeat 16 times in your DNA and 12 times in mine. We separate these pieces of DNA by a procedure called capillary electrophoresis. The DNA passes through a long thin column with the shortest pieces coming off first and the longer pieces coming off last. The information is deciphered using a computer and results in a DNA profile. Now we take this DNA profile of the sample left at the crime scene and compare it with DNA standards from suspects and victims. If your DNA profile is different from the crime scene sample, then you are excluded as a source of the material. If your profile is the same, then you are included in the group of people that could have left the material. The DNA profiles that we obtain are generally one in several billion and although not as good as a fingerprint, are very informative. Best regards, Dale L. Laux Forensic Scientist
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