You apply when you apply to get into undergrad. You can apply to Penn's college of arts and sciences, or the other affiliated undergrad programs. You take 3 years at your undergrad school doing bio major stuff, and then the last year you start dental school. The basic sciences you take in d-school finish your BS. You need to maintain a certain GPA (I believe it's 3.5) and you still need to take the DAT (I don't know what the score has to be) in order to stay in the program.