Your personal statement you get to write whatever you want -- It's just a blank spot on your AMCAS. Your secondary essays will have specific questions, such as "Why do you want to go to school X" "What contribution would you like to make to the field of medicine" "Discuss an ethical dilemma you have personally been involved in, and how you handled it." "How do you deal with stress" "What is your personal belief system. Discuss influences." and so on.
I'd start writing your AMCAS personal statement as soon as possible. Worry about your secondary essays as soon as you get them. Be warned that U Chicago's has four longish essay questions. Other schools are just "send a check."
Anka