Well you don't have to apply until this time next year, so you have a year to make up your mind on courses and Unis. Also, your Uni course doesn't have to lead straight into a particular job. What you do at Uni doesn't usually trap you into one specific thing. Just do something you enjoy, be it Biology, Chemistry, both, or something else. You can think about careers when you're at Uni.
I'm a third year and a lot of people I know still don't know what they want to do. Most people are applying for a few different things too, to increase their chances of getting something.
In conclusion: you don't need to have your whole life planned out at 16/17! There's plenty of time.