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Re: DNA at last


>>    Paleontologist Jack Horner, and advisor on Steven Spielburg's hit 
>> about dinosaurs, managed to isolate fragments of DNA from the 
>femur of a
>> Tyrannosaurus Rex, the Sunday Times reported.(me: what Sunday 
>>times?  The London Sunday Times?)

>The method used is PCR (polymerase chain reaction).  This method is 
>prone to contamination.  I have heard of one case where an organism 
>being studied in an adjacent lab was able to contaminate a sample.  

        This is most certainly true and quite possible. Even pico-gram 
quantities of contaminating DNA are easily amplifiable using this method.

>The sample was contaminated anyway by being touched, being in 
>contact with airborne organisms, and having been taphonomically ??
>altered by bacteria.  Basically, the PCR "amplifies" fragments of DNA 
>so that they can be identified.  The problem is that the contaminents 
>are also "amplified" or multiplied, and it is virtually, if not wholly, 
>impossible to distinguish what is a fragment of dino-DNA and 
>contaminent DNA.

        This is rather inaccurate. The fragments amplified will almost 
certainly be sequenced and compared to avian sequences. A sequence as
ancient as T. rex would certainly be very different from any extant bird sp. 

        Mixing two templates may yield _some_ recombinant molecules but there 
numbers would be low. PCR is more or less asexual process. (Note: See Nature 
Aug 4th for a discussion of Sexual PCR.) This assumes of course that you 
actually have _two_ templates to amplify (a contaminant (yourself?) and the 
target you're after). 

        The real problem is that obtaining amplifiable tissue from dinosaurs 
is virtually impossible. Tissues which have been frozen or otherwise 
preserved stand some chance. (Unfortunately amber is also oxygen permeable 
and therefore a poor preserver of DNA). I agree that Horner and his group 
have almost certainly amplifed themselves or some other extant sp. 
If not I expect to see a sequence of T. rex in the next issue of Science. ;-).

>I took part in a course on taxonomy at Glasgow recently where PCR 
>was covered.  The conclusion was that the results obtained for fossil 
>material over 10,000 years old are unreliable.>
        Probably an accurate estimate.

Les Willis
Research Station 
Agriculture Canada