I'm a college senior and I've been accepted to multiple Biostat PhD programs without taking Real Analysis. I don't say that to mean you shouldn't take it (you should! I think it is still a huge plus on an application and am not looking forward to taking it next year while in grad school), but rather that I was freaking out a bit while applying that nobody would accept me because I didn't have the course. Of course I'm no expert, but based on my experience I would say it's probably fine to take it in the fall of your senior year. From my understanding, most application committees don't meet until after the semester ends anyway and you'd be able to send your updated grades in.