You may only repeat a course for credit if you earned less than a C- (i.e. D+ or lower). If you retake the course and earn a higher grade, the old grade will be dropped from your GPA calculation, but will still show up on your transcript as a replaced grade (i.e. "RF").
If you repeat a course for which you earned at least a C-, you waste your time and money. The new grade will not be entered into your GPA and it will show up on your transcript as "REPEAT NO CREDIT".
Note that you must have at least a 2.0 major GPA to graduate with a BS in Biology or Molecular Biology.