This won’t be exactly applicable to the OP, since he’s evidently in the natural sciences, but for the benefit of others who may be looking in, I’ll add this:
Most master’s programs expect you to build and further specialize in what you’ve already learned in undergraduate college. Whether you’re doing chemistry or Sanskrit, you’re expected to have an undergraduate major in that subject on which you can build, or in a closely related one. But certain programs usually reject applicants who have their undergrad majors in those fields. MBA and MLS (library school) programs both seem to follow the “any major but” rule. If you have a bachelor librarian degree, if such an animal still exists, you can’t get into a graduate library school. Accordingly, in that type of master’s program, which usually lasts two years, you’ll usually spend the first year taking a dozen or so core content courses much like doing your major sequence as an undergraduate. Therefore your first year of grad school in that type of program will bear a strong resemblence to your junior undergraduate year, when you began working on your upper division major courses.