Hii,
Yes , it perfectly fine to choose other subject than master in Phd some college allow some not. In some colleges for Phd Biotechnology the eligibility criteria is the candidate Must pass their PG course in Biotechnology or Bio Informatics.
But this doesn't seem like the good decision , you did your Bachelor in bio related field and then did master with computer Science and now again to want to do Phd in Biotechnology.

If you are interest in life Science discipline the the better decision can be to take phd Bioinformatics which is related to computer Science, mathematics, biology, and Statistics.

All the best!!