Sophomore year is the earliest that you may take either sequence. When you take one of these sequences will depend on many things, including your interest in it as a possible major, when you would like to take the MCAT, overseas plans, etc. The laboratories, Biology 44X and 44Y, are designed to be taken concurrently with or subsequent to either core. Although you may find that it is easier to take the lab and the complementing courses together, the ability to take the labs at a later time does give you some flexibility. The labs do not contribute significantly to your preparation for the MCAT, and therefore do not have to be completed before you take the exam.
Many medical schools require one upper division biology course beyond the one-year core sequence.
Physics To satisfy the requirement for a year of physics with lab, most students take one of two suggested series of courses. As with all premedical courses, you will want to plan your schedule individualy. Many students who choose the 20 series find that junior year is the most convenient time to take it. However, it can be taken earlier or later to suit your interests. Some Biology faculty believe a background in physics enhances your understanding of Biology.