Hi Adwaith,

As per the NEST eligibility criteria students with any of the three main subjects of main Science stream,i.e Physics, Chemistry, Mathematics and Biology can apply for the test where Mathematics is compulsory. So you having PCM can easily apply for it. Now coming to your second question, see when we talk of pure Science it includes Physics, Chemistry and Mathematics. That is the reason why they are conducting the first part in one session and the Biology part in another session. If you want you may skip the second session test. For further eligibility criteria doubts you can go through the link below:-

https://www.google.com/amp/s/university.careers360.com/articles/nest/amp

You may also comment below for any more questions.