Peter S. Evans evans at
Sat Jun 25 15:06:06 EST 1994

In article <h08QwTQ.brunker at>, brunker at wrote:

> 	I've only been doing PCR's for about a month and obviously am not 
> completely up on all aspects of it.
> 	However in our lab, it takes around 20 minutes or so to set up a PCR, 
> a couple-three hours for it to cycle, followed by around 20 minutes or so to 
> set up a restriction enzyme digest, a couple-three hours for the digest, then 
> 20 minutes or so to set up a gel and a couple-three hours for the gel to run.  
> 	So if all goes smoothly we can get results in one longish day, or in 
> two days if we decide to go home at a decent hour. Southern blots might take 
> a few days, DNA extractions take about two hours for a small batch.
> 	While waiting for these procedures to complete, my colleagues
> and I will, among other things, look at the daily newspapers. And the
>  papers (The Daily News and New York Newsday) claim that DNA testing in the OJ 
> Simpson case will take two months; the evening news says the same.
> 	Two months? Are the papers and TV news just being silly again
> or are there some applicable forensic PCR procedures that commonly take that 
> long?
> 	Marla Brunker
> 	brunker at

Dear Marla,

I heard the same thing on CNN, and was perplexed also.  The forensic test
(ie DNA "finger printing") uses Southern analysis of DNA extracted from
blood (or skin or semen, etc) found at the scene of the crime.  The DNA
probe recognizes Alu repeats found throughout the Human chromosome. 
Everybody (or, almost everybody) has different numbers of these Alu
sequences, so everybody will have a unique pattern on this southern blot,
and thus the test can tell if OJ was at the scene of the crime.

Still, the time necessary to do these tests is similar to what you
described, so why the 2 month lag?  My guess is that the results must be
repeated several times in order to be admitted as legal evidence.  Does
anybody have more than a guess?


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