[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Cloning prehistoric animals...



My apologies. I thought that PCR usually used macroscopic samples of DNA and
magnified the signal by duplicating the molecules, not reading molecules. I
did not know that technology was available to pick a single molecule and
read it directly, without duplication. I know that PCR will work with a
single piece of a DNA strand, but picking the right stuff will be hard
unless you can read a molecule without going through that process.

Your note says "if we can find complete strands" but my understanding is
that even the human genome project, with tons of great, fresh material
(moles of complete strands) and huge resources (>$10^9) has crowed "success"
when they analyzed only about 90% or less of a single human chromosome
(Nature, vol. 402, p 489, 12/2/99). With much less material and the amount
of degradation expected in the ancient samples, it doesn't look good for
mammoths.

With regard to hydrogen bonding, I was trying to suggest that the statistics
of degraded spots on the molecules that had survived would be influenced by
the local strength of the bonds between strands, i.e. that areas of the
molecule that unzipped would degrade faster. If this occurs it will produce
a spatial correlation in the degradation funciton, i.e.the places that
degraded most through time would be pretty much the same from molecule to
molecule, so that even if one has a large sample of molecules, one does not
have full information.

I understand that the bonds along the strand are covalent. I also understand
that the information on the DNA is redundant and that only one side need be
available, but I don't believe that the complete information will be
available from sources of ancient DNA because of the correlation of degraded
spots. Unless single strands with excellent preservation in critical spots
can be selected (by a technology I believe may lie in the future), the
dominant input signal in a PCR sample taken from a macroscopic sample will
have correlated bad spots and will overwhelm the small fraction of molecules
with preserved information in the critical areas.


joe wrote:

> Um, actually, we can and do have the technical tools and have had for
> over 15 years. I have been doing so as my job for almost 10. Accurately
> replicating single strands of DNA is commonplace. That is not the
> problem. Hydrogen bonding is not a problem as you don't need double
> stranded DNA and the bonds within a single strand are not hydrogen
> bonds. If you have enough to analyze, you have enough to replicate.
> Actually, the process of analyzing requires replicating it numerous
> times. Denatured DNA is no problem and is part of the procedure for
> replicating it. I think what you actually mean is degraded, which is
> completely different. This involves actually breakage of the strands and
> loss of sequence. This is the problem. Once the strands start breaking
> up and actual sequence is lost, there is no way to get that back.
> Of course, simply having the sequence is only the first and simplest
> piece of cloning extinct DNA. You then have to put it in the right order
> and have it methylated properly and no one has the slightest clue how to
> do that. If we did, cloning would be commonplace and not the expensive
> time consuming shot in the dark procedure it is now. There is so much
> more to the function of DNA than the sequence it is almost beyond
> belief. This is where the "could we" comes into play.
> But, in answer to all this, yes, if we can find complete strands, or at
> least enough that we can deduce the proper arrangement, there is no
> reason why we shouldn't be able to clone a mammoth with enough work. We
> don't need much DNA, just a tiny piece of good quality that can be
> analyzed.
>
> Joe Daniel