There are two kinds of answers to such a question.
1 The "legal" answer is the one offered corrrectly by the previous responders.
2 The "common sense" answer. If you have a beautiful A record in the sciences, it is to be expected that will continue into the senior year, but there is no way to guarantee that. Depending on what your record looks looks like, it may ot may not be safe to postpone those courses until your senior year. If you are taking only the minimum science requirements, you may be running some risk of not being looked at early.
Other considerations:
a By not finishing the requirements until your senior year you might leave the impression that you have little confidence in your ability to earn competitive grades.
b You cannot be sure of the future. If something unexpected happens, and you don't finish the required courses before medical school classes begin (on the assumption that you will have an acceptance), your acceptance will be rescinded.
c If because of the late finish, you end up on a wait-list, there is no guarantee that will morph into an acceptance.
d Where you attend college might make some difference, as will the quality of your recommendations.
You must decide on the risk/benefit ratio.
If you have a premedical advisor, consult her/him.