There is no definitive weight age but the topics can be divided into 3 categories :
1.Must be done
A. Living world
B. Biological classification
C. Principles of inheritance
D. Molecular basis of inheritance
E. Chemical control...
F. Cell
G. Sexual reproduction in flowering plants
H. Microbes in human welfare
I. Anatomy of flowering plants
J. Environmental issues
K. Plant kingdom
L. Morphology of flowering plants

2 should be done:
A Sturactural organizations in plants
Transport in plants
Human reproduction
Evolution
Respiration in plants
Biotechnology
Human physiology
Morphology of animal kingdom

3.Do if you have time

Remaining chapters

See..each chapter doesn't carry a particular weight age in an examination but still I would tell you that definitely more questions are asked on chapters regarding Human Physiology , Genetics and the zoological aspects of animal kingdom classification and morphological systems and life cycles of different lower organisms. So it would be good if you give more stress on them but apart from that chapters like plant physiology and even animal physiology for that matter we studied for Applied type questions where you need to analytically answer one out of several options which are given - either there may be a twist in the options given and you might be asked as to which one is true or false or you might have to pay. The odd one out in the matching agenda which day give..
So try paying more attention to these chapters on which logical questions were more students who don't study carefully make a mistake and that will eventually make a difference in the marks obtained at the end of the examination as well because other chapters where you can just learn some facts like the chapters based on environmental science and animal husbandry they are not very important same regarding the biochemical chapters regarding biomolecules but as for the application part of it the chapters eligibility said start preparing to answer questions which require analysis based on those topics that will help you regarding the time management for the other questions in your examination.

HI Sonika, See I found something that might help you.

  BOTANY 

Unit	Number of Question	Total Marks	Weightage Percentage
Biological Classification	3	12	6.00%
Cell Biology	4	16	8.00%
Ecology	6	24	12.00%
Genetics	5	20	10.00%
Molecular Basis of Inheritance	6	24	12.00%
Morphology	1	4	2.00%
Morphology of flowering plants	1	4	2.00%
Plant Anatomy	3	12	6.00%
Plant Kingdom	6	24	12.00%
Plant Morphology	2	8	4.00%
Reproduction in Flowering Plant	5	20	10.00%

ZOOLOGY 

Unit	Number of Questions	Total Marks	Weightage Percentage
Animal Kingdom	3	12	7.50%
Animal Morphology	1	4	2.50%
Animal Physiology	1	4	2.50%
Biomolecules	4	16	10.00%
Biotechnology	3	12	7.50%
Evolution	1	4	2.50%
Human Health and Disease	3	12	7.50%
Human Physiology	16	64	40.00%
Human Reproduction	1	4	2.50%
Microbes in Human Welfare	3	12	7.50%
Reproductive Health	2	8	5.00%
Strategies for Enhancement of Food Production	2	8	5.00%

Hope this answer Helps.

Biology in NEET-UG entrance comprises 90 questions. And since that's quite a good number, demarcation of the important topics from Botany and Zoology becomes a little. I've mentioned the level of importance of topics under Botany and Zoology subheadings:

BOTANY:
TOPICS THAT HAVE MAXIMUM WEIGHTAGE:
1) Ecology
2) Genetics
3)Molecular basis of inheritance
4)Plant kingdom
5)Reproduction in flowering plants

TOPICS THAT HAVE INTERMEDIATE WEIGHTAGE:
1) Biological classification
2) Cell biology
3) Plant anatomy

TOPICS THAT HAVE RELATIVELY LESS WEIGHTAGE:
1) Living world
2)Morphology of flowering plants
3)Photosynthesis in plants
4) Respiration in plants
5)Transport in plants.

ZOOLOGY :
TOPICS THAT HAVE MAXIMUM WEIGHTAGE:
1) Human physiology
2)Biomolecules

TOPICS THAT HAVE INTERMEDIATE WEIGHTAGE:
1) Animal kingdom
2) Biotechnology and it's applications
3)Microbes in human welfare
4)Human health and diseases

TOPICS THAT HAVE RELATIVELY LESS WEIGHTAGE:
1)Evolution
2) Human reproduction
3) Strategies for enhancement on food production

There is no definitive weight age but the topics can be divided into 3 categories :
1.Must be done
A. Living world
B. Biological classification
C. Principles of inheritance
D. Molecular basis of inheritance
E. Chemical control...
F. Cell
G. Sexual reproduction in flowering plants
H. Microbes in human welfare
I. Anatomy of flowering plants
J. Environmental issues
K. Plant kingdom
L. Morphology of flowering plants

2 should be done:
A Sturactural organizations in plants
Transport in plants
Human reproduction
Evolution
Respiration in plants
Biotechnology
Human physiology
Morphology of animal kingdom

3.Do if you have time

Remaining chapters

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