Some unis actually require an A2 in chemistry but don't mind if you have biology to AS as all the import at stuff from biology is covered again in first year, but it's a good idea to have both biology and chemistry to A2 if you want to increase your chances of getting in. Have a look at the entry requirements of all the unis you're considering, but I'd definitely advise doing chemistry, I did biology, chemistry and maths to A2 and did AS psychology and got into medicine