I think some of them are just downright nerdy, don't you?
Of course, the best puns are the ones you come up with yourself...so let me take a crack at this:
"What did the DNA say to the RNA? We don't need U here."
Ok, I can't think of any others. :(
Perhaps I'm not cut out to be a comedian.
Oh well, pass me a gamma-radiation sterilized scalpel and I'll make a wet-slide of that apple I wanted to eat.