DNA Fingerprinting - Simpson Trial

Janet Hays (BIO) jhays at chuma.cas.usf.edu
Fri Feb 24 09:00:49 EST 1995



On Mon, 20 Feb 1995 U27111 at uicvm.uic.edu wrote:

> JK> 1. If it is an RFLP test, where did the DNA that's restricted
> JK> come from?  If from a PCR reaction, in my opinion it can be
> JK> discarded. It could just as easily be from contamination, and
> JK> in all likelihood is.
> 
> How's that if it's a positive result with negative internal
> controls?
Because the internal controls are irrelevant to whether or not a second 
genome (i.e. a few strands of NBS or RG) have been inadvertantly left on 
a blood sample (i.e. Bronco, OJ blood) in the field during collection. 
Both genomes will be isolated when the DNA is extracted and both genomes 
will be amplified in the PCR reaction. All the internal control tells you 
really is that you haven't contaminated any of your reagents, since the 
blank you run is run without template (no DNA from any of your test 
subjects). 

It is even possible through pipette aerosol contamination when 
putting the master mix (no template) into the various tubes with the 
template DNA. You mix a master for several samples, and then apportion it 
between them. If on putting part of the master into one sample, say NBS, 
and then put the next portion into the OJ tube, poor technique may allow 
carryover of the DNA in the first tube to the second tube. At this point 
the second tube will show as a having two genomes though it wasn't 
extracted as such. There are methods and equipment that can avoid this 
kind of error, and perhaps these labs use them, but I for one am 
unwilling to assume that they have.

Just a note, the biggest thing I have learned about being a scientist is 
never, ever, ever trust anyone's results until you have done a thorough 
examination of his materials and methods. Results are meaningless if they 
were not obtained under proper expermental procedures.

Gotta run. Will address the rest of this later.

jk



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