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It is mentioned early in the movie, that dinosaurs are more similar to birds than to reptiles. So at least in movie continuity, this is true as in reality. So why then wouldn't they use bird DNA instead of amphibian DNA?

In-universe explanations only please, please - I know the plot needed an excuse for the spontaneous sex-changing.

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In the book, they actually used a mix of DNA sources - only a handful of the species had the frog DNA specifically (including the compys and velociraptors). –  HorusKol Jul 10 '11 at 23:50
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up vote 9 down vote accepted

This is from memory and it's been a while since I've read the book and it is mostly mentioned there rather than the film but there are a number of relevant points linked together to form a vital plot point.

We can deduce from their success that the scientists must have found a way to succeed. In addition, and remembering that the book was written in the late 80s when the understanding of genetics was considerably less than nowadays, the science in Jurassic Park is not viable in the real world so we can also assume that genetics is less difficult to get working right. It's also viable to assume that the scientists experimented with several different theories before they found the one that worked. Using frog DNA was just the first theory that worked.

Dinosaur DNA is not complete so it must be 'filled in' with other DNA. Most of the DNA that makes up a genome is either 'junk' (we don't know what it does) or common (shared by different species). Frogs are amphibians which evolved before dinosaurs, therefore using their common or junk DNA would likely be a better fit than using lizard or mammalian DNA.

Certain frogs apparently have the ability to change their gender. Thus when the scientists used 'junk' Frog DNA to make dinosaurs it accidentally gave the dinosaurs the ability to change gender. This allowed the dinosaurs to breed which is revealed firstly when Grants finds a Velociraptor shell and then when Malcolm demonstrates a flaw in how the park measures populations of dinosaurs, thus justifying Malcolm's saying that 'nature finds a way'.

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Not entirely sure whether they knew this info or not when Jurassic park was set or not, but presumably if they are lifting genetic material from frogs they should know the size of the genome.

The mean sizes for frogs vs birds genome is 4.68pg ± 0.13 (pg is picograms of genetic material for one complete copy of the genome) vs 1.38pg ± 0.01, meaning that the frog genetic code has about 3 times as much data. They may have felt the extra genetic material would make it easier for them to find DNA that corresponds more closely to the dinosaur DNA better.

Keep in mind that having more genetic data in the genome doesn't always make an organism more complex, it may just mean that the organism hasn't "cleaned" it genetic code and there is a lot of older left-over material from the process of evolution.

The sizes of the genomes are from Gregory, T.R. (2005). Animal Genome Size Database. http://www.genomesize.com

Another fun factoid is that the largest frog genome is 13.40pg, Ceratophrys ornata (8n), Ornate horned frog while the largest bird genome is 2.16pg, Struthio camelus, Ostrich. This means the largest genome from the frog is ~6 times as large as the largest bird genome.

I know its kinda complicated, but I hope it helps.

Also from http://www.genomesize.com/statistics.php?stats=amphibs#stats_top you can see that amphibians other then frogs can have a significantly larger genome then just frogs, they may have just settled on frogs because they are a more easily managed genome then the Gulf coast waterdog, for example, with a genome of 120.60pg (9 times the size of the largest frog).

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There might be some-pseudo scientific explanations that could fit the bill.

  • Reptiles(including dinosaurs) evolved from amphibians, and birds from reptiles. So, a frog may not necessarily be that far off from a dinosaur.
  • Frogs are easier to experiment with (like rats and mice), so they are more easily used in experiments, and hence their DNA has been studied more.
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