Its possible that they would look at your marks and let you in, but only an academic advisor could tell you for sure. This is a busy time of they year if you email them, so if at all possible you should go to drop in counselling in BSB (at Mac). If not, phoning is the next best option.
If you can't get in you still have a chance of being able to go into a science program second year, just make sure you take the right pre-reqs for the program you're interested in. The departments are here, and the programs in that department are accessible by clicking the links.
http://www.science.mcmaster. ca/main/departments.html
In general, you'll need:
Bio 1M03
Bio 1A03
Chem 1A03
Chem 1AA3
Psych 1X03
Physics 1L03 or 1B03
Calc 1A03 or 1LS3
Some of them you won't have problems getting into, some you might need to get a waiver, and some you'll need to take during the summer. Again, an academic advisor will help you more.