Here are the key points regarding your query about appearing for NEET or IISER exams after passing 12th board in 2023 with 53% overall marks and appearing for biology improvement through NIOS:

1. NEET Eligibility:

- For NEET (National Eligibility cum Entrance Test), the minimum eligibility criteria is 50% marks in Physics, Chemistry, and Biology/Biotechnology subjects in the 12th board exam.

- Since your overall 12th board marks are 53%, you would be eligible to appear for NEET, provided you score at least 50% in the individual subjects of Physics, Chemistry, and Biology.

- Appearing for biology improvement through NIOS should not affect your NEET eligibility, as long as you meet the 50% criteria in the individual subjects.

2. IISER Eligibility:

- For admission to the Indian Institutes of Science Education and Research (IISERs), the eligibility criteria includes scoring a minimum of 60% marks in the 12th board exam (55% for SC/ST/PwD categories).

- With your overall 12th board marks being 53%, you would not meet the general category eligibility criteria for IISER.

- However, if you are able to improve your overall 12th board marks to 60% or above (55% for SC/ST/PwD) through the biology improvement exam, then you would become eligible to appear for the IISER Aptitude Test (IAT) 2024.

- For NEET, your 53% overall 12th board marks along with the 50% individual subject criteria should make you eligible, provided you clear the biology improvement exam.

- For IISER, your current 53% overall 12th board marks do not meet the 60% (55% for SC/ST/PwD) eligibility criteria. You would need to improve your overall marks to 60% or above to be eligible for the IISER Aptitude Test 2024.

I hope this helps provide clarity on your eligibility for NEET and IISER exams. Let me know if you have any other questions.