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In Sam Raimi's Spider-Man , Peter Parker is bitten by a genetically enhanced 'Super Spider' while on a field trip to an egregiously unsafe gene laboratory (lock your spider boxes, kids) and falls unconscious soon after returning home groggy and ill. In the following few moments, we are treated to an exciting sequence that shows tiny spiders  crawling around inside his cells, and his DNA being cut and spliced in the process that turns him into the arachnid Avatar. This (and the following scenes of him grappling with his new abilities and, importantly, checking himself out in the mirror) is a lot of fun to watch, but viewers who have indulged themselves in high-school biology (and stopped after a couple of years, as I did) may at this point jump out of their seats, throw their Heelys at the TV screen and shout "Hang on, that's not how genes work! You can't meaningfully change your body after  you've been born just by editing your DNA!". Now, friends, t