Jurassic park lecture

Bryan L. Ford fordb at bcc.orst.edu
Wed Feb 26 20:43:51 EST 1997


Blarne Flinkard wrote:
> 
> On Tue, 25 Feb 1997, Maarten van Roosmalen wrote:
> 
> > Hi,
> >
> > 'Could you search the internet for more information on ancient
> > DNA sequencing and retrieving DNA from insects captured in amber?'
> >
> This scenario is unlikely as DNA has a natural decay rate. I don't know
> what it is, but just losing a few base pairs a year will lead to very
> short average chain lengths which essentially means that recovering a
> complete genome from a dead organism, especially a long-dead one, is
> exceedingly unlikely (i.e. impossible). Remember, in live beings, DNA is
> continually being repaired--a luxury that any dead thing can't afford.

Friends:

This objection to ancient DNA survival (cytosine to uracil by
deamination, if I recall the essence of it) is certainly a factor
degrading DNA from many fossil sources. However, it has been shown to be
quite inappropriate in the case of preservation within amber. The
thinking on this subject has had to undergo profound changes since the
very strong evidence that bacterial spores can remain viable for
millions of years when "entombed" in amber (check the work of R.J. Cano
and M.I. Borucki, in Science [1995] v. 268(5213): pp.1060-1064, with
other work doubtless following). There is also evidence that relatively
unprotected DNA can survive surprisingly long times if the conditions
are appropriate, including and especially those such as exist within
many ambers. The solidification and polymerization of amber effectively
dehydrates any trapped organisms and their contents/constituents,
including their DNA, and then holds them in this anhydrous condition
indefinitely. Work on the rate of racemization of amino acids (normally
predictable enough to be a dating technique) shows that in amber the
rate is effectively zero.  Solution chemistry kinetics and reaction
mechanisms, particularly for macromolecules such as DNA, are generally
vastly different from those in the solid state. I do not have the
figures but my recollection from reading somewhere in the last 2 years
that the C->U rate in amber or amber-like conditions was also
effectivley zero. One can be sure that the amber entombed creatures are
in an anoxic if not mildly reducing environment as well. Of course DNA
repair mechanisms are moot here not only because the organism is
typically dead (although we might debate the bacterial spores status)
but also because any biochemical activities are effectively "frozen" by
the anhydrous and anoxic conditions.

Twenty million year-old bacteria surviving as spores in the guts of
ancient bees in amber are of course a long way from sixty million
million year old megafauna "surviving" as fossilized and DNA containing
blood cells in sucking insects found in amber. But let's not be so hasty
citing inappropriate criticisms of the notion.



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