Like I said in another thread, my class alone has some people who didn't finish an undergrad degree and are doing well while others who had a masters' degree coming in are now gone. Nothing's a given, but I think that you would really be helping your chances and knowledge base if you at least stuck in undergrad to take biochem, micro, cell and molec bio, anat/phys, etc. It's only one more year, and you are only young once... a bit more of the college experience (partying, sports, being single, etc) certainly never killed anyone (ok, well, probably not too too many people).