Yeah, except the parts are not exactly the same from one organism to the next...
Of course not, because a higher organism requires different DNA. If it was exactly the same you would have the same species would you not? Guess, you must have been sick when that class was given.
Oh, gee, where do I start? DNA consists of 4 nucleotides, abbreviated G, A, T, C, and all living things use them. Let's see... I use bacteria to grow DNA containing human genes, which I insert into mouse cells and test whether the human genes affect a mouse protein by measuring the amount of firefly enzyme that the cells produce... which is impossible, according to what you said. Yet I, and countless others, have published papers in peer-reviewed journals detailing just this kind of experiment.
And American taxpayers have spent tens of thousands of dollars for me to do this, too. Thank you, taxpayers!