Divya, as i can see you want to join ISRO/DRDO basically something great like serving the country. Well, i would suggest, you can go for MBBS after BSc zoology, but you should not have crossed the age limit of 17-25 years.

If you are going for MBBS, you can apply at Armed Forced Medical College (AFMC), Pune. This is an armed force college where you would be trained to work as a doctor in the army. Check out the official website

https://www.afmc.nic.in/

You can also get into DRDO, as BSc Zoology falls under one of the educational qualifications for CEPTAM exam. You can apply and write for CEPTAM, if you qualify you can surely get into DRDO. You can check official website for further notice

https://www.drdo.gov.in/ceptam-notice-board

For joining ISRO, you educational qualification should be BSc or MSc is Physics, Chemistry or Maths only.

I hope this helps you!