@jillpink All classes are taught by a professor, even the larger ones. That goes for all subjects. TA’s and tutors are used in a complementary capacity, and will likely help with labs and recitations. For general biology/chemistry courses, you may find 200-300 person lectures, but they will also have recitations and labs in smaller groups of 20-30. After those bigger classes, expect more in the range of 20-50.
Every professor is required to hold office hours, no matter how big the class. You can always find them there if you need a professor specifically.