Dear Jithin, for appearing for IISER, you need to have Mathematics, Physics, Chemistry and Biology as your subjects because these are the subjects the exam is conducted in. And they all carry equal weightage. There are a total of 60 questions, 15 from each subject. So in order to be eligible for IISER, you need to have studied Physics, Chemistry, Mathematics and Biology as your subjects. Sorry to disappoint you but you cannot write IISER with just PCM.