MadSci Network: Molecular Biology

Re: what is the procedure for DNA testing?

Date: Mon Apr 10 12:02:44 2000
Posted By: Dale L. Laux, Staff, Serology/DNA, Ohio Bureau of Criminal Identification
Area of science: Molecular Biology
ID: 955205271.Mb

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 

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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