Jack Horner, a paleontologist at the Montana State University, Bozeman, and curator of the Museum of the Rockies, replies:
"We are a long, long way from being able to reconstruct the DNA of extinct creatures, and in fact it may be impossible to resurrect the DNA of dinosaurs or other long-extinct forms. We have DNA for living creatures, including ourselves, and yet we cannot clone any living animal (from DNA alone). As for extinct taxa, it is unclear whether or not DNA actually can survive more than a few thousand years. No one has been able to demonstrate incontrovertibly, as of yet, that they can retrieve DNA from an extinct species.
"The cloning that has been achieved recently has been accomplished at the cellular level, not at the level of a strand of DNA. And to date we do not have any living cells of any extinct taxa. Without cells we cannot accomplish the same kind of cloning that has been done with sheep, and without being able to acquire ancient DNA or being able to clone from DNA, it is at present not possible even to forecast when such a thing might be possible.
"And, even if we could get DNA from an extinct taxon, and if we knew how to clone the DNA, we would still have another hurdle in creating the exact embryonic conditions. For example, how would we synthesize a dinosaur egg or its exact environment? Too many hurdles!"
Tomas Lindahl is the director of research at the Imperial Cancer Research Fund in South Mimms, England. He adds:
"The DNA recovered from long-extinct creatures is severely degraded, and therefore unsuitable for any future reconstruction attempts. The oldest DNA sequences that have been reproducibly recovered are a few short fragments of mitochondrial DNA from mammoths present in Siberian permafrost regions. These fragments of DNA are 50,000 to 100,000 years old. No DNA has been retrieved from mammoth cell nuclei, but the mitochondrial sequences were sufficient to show that the mammoth was closely related to the elephant (which was already known from the fossil record) and that three subspecies of mammoths apparently existed.
"Early anecdotal reports on recovery of DNA fragments many millions of years old from a dinosaur bone or from insects entombed in amber have turned out not to be reproducible in recent extensive investigations. A large, careful study published in April 1997 (in the Proceedings of the Royal Society) from a research group at the Natural History Museum in London is particularly convincing. The preliminary positive reports can now be ascribed to trivial contamination of reagents or equipment with non-ancient DNA. The really ancient DNA once present in those fossils is so completely degraded that it cannot be recovered even in fragment form.
So, 'Jurassic Park' remains science fiction only!