First: Are all courses really closed, or just the sections or instructors you prefer? We will not overfill one course section if there are others you could take.
Second: Contact your faculty advisor. Your advisor may be able to help you find alternatives.
Third: If all else fails, contact the department chair for assistance. Keep in mind that there are 800 Biology and Molecular Biology majors and only one department chair. Contact the chair only when your faculty advisor is unable to resolve your problem.
NOTE: The above apply only to biology courses. Other departments have different policies. Contact the department office in question for further information.