Hi Arpit

Dear you can do biology ,taking it as the additional subject as a private candidate after completing 12 th  .There is this one thing that CBSE allows that one can appear for a subject as a private candidate.So, you don't need to reapper for 12th.

For more details and guidelines visit the the link below :

http://www.cbse.nic.in/faq/private_candidate.htm

Hope this helps you.

All the best.