CBSE Class 12 Biology Pre-Board Sample Paper 2025 With Solutions, Download PDF

General Instructions:

1. All questions are compulsory.
2. The question paper is divided into five sections, containing 33 questions.
3. Section–A has 16 questions of 1 mark each; Section–B has 5 questions of 2 marks each; Section– C has 7 questions of 3 marks each; Section– D has 2 case-based questions of 4 marks each; and Section–E has 3 questions of 5 marks each.
4. Internal choices are provided in some questions. Attempt only one alternative in such cases.
5. Wherever necessary, neat and properly labelled diagrams must be included.

Section – A (1 Mark Each)

(Each question includes the Unit or Chapter Name)

1. Unit: Reproduction
Signals for parturition in human females originate from:
a) Fully developed foetus only
b) Both placenta and fully developed foetus
c) Placenta only
d) Oxytocin released from maternal pituitary

Answer: b) Both placenta and fully developed foetus

2. Unit: Genetics and Evolution
To produce 1600 seeds, the number of meiotic divisions required will be:
a) 2400
b) 2000
c) 1600
d) 1800

Answer: b) 2000

3. Unit: Genetics and Evolution
A sample of normal double-stranded DNA was found to have thymine content of 27%. What will be the expected proportion of guanine in this strand?
a) 23%
b) 32%
c) 36%
d) 73%

Answer: a) 23%

4. Unit: Biotechnology and its Applications
If E. coli has 4.6 X 10⁶ base pairs and completes replication in 18 minutes, then the average rate of polymerization is approximately:
a) 2000 bp/s
b) 4000 bp/s
c) 3000 bp/s
d) 1000 bp/s

Answer: c) 3000 bp/s

5. Unit: Genetics and Evolution
Suresh and Rajesh have defective haemoglobin due to genetic disorders. In Suresh, the problem is qualitative as he is having incorrectly functioning globin molecules, while in Rajesh the problem is quantitative, having very few globin molecules. Identify the disorder they are suffering from.
a) Sickle Cell Anaemia
b) Thalassemia
c) Haemophilia
d) Hemoglobinopathy

Answer: b) Thalassemia

6. Unit: Genetics and Evolution
In E. coli, the lac operon gets switched on when lactose is:
a) Present in the medium and it binds to the repressor
b) Not present in the medium and the repressor binds to the operator
c) Not present in the medium and RNA polymerase binds to the operator
d) Active lactose present in the medium binds to RNA polymerase

Answer: a) Present in the medium and it binds to the repressor

7. Unit: Reproduction
Which of the following features shows the mechanism of sex determination in honey-bees?
a) An offspring formed from the union of a sperm and egg develops as a female
b) Males have half the number of chromosomes than that of female
c) The males are haploid having 32 chromosomes
d) All workers and males are diploid having 16 chromosomes

Answer: a) An offspring formed from the union of a sperm and egg develops as a female

8. Unit: Biotechnology and its Applications
The following diagram shows a fragment of DNA which is going to be transcribed, the upper strand with polarity 3’ to 5’ is the template strand:
3’ ATTGCC 5’
What will be the sequence of mRNA after transcription?
a) 5’ AUUGCC 3’
b) 5’ UAACGG 3’
c) 5’ GGCAAU 3’
d) 5’ ATTCGG 3’

Answer: b) 5’ UAACGG 3’

9. Unit: Ecology and Environment
Idli – dosa dough rises due to the production of which of the following gas?
a) CO
b) CO₂
c) NO
d) NO₂

Answer: b) CO₂

10. Unit: Genetics and Evolution
Adaptive radiation leads to:
a) Increased competition among species
b) Decreased speciation rates
c) Limited morphological diversity among species
d) Rapid divergence of traits among populations inhabiting a given geographical area

Answer: d) Rapid divergence of traits among populations inhabiting a given geographical area

11. Unit: Biotechnology and its Applications
Which of the following correctly represents the manner of replication of DNA

Answer: (d)

12. Unit: Biology and Human Welfare
During the secondary treatment of sewage, which of the following changes in the effluent occur due to flocs?
a) Reduction in BOD
b) Increase in BOD
c) Decrease in DO
d) No change in DO or BOD

Answer: a) Reduction in BOD

13. Unit: Reproduction
Assertion (A): Cells of tapetum have more than one nucleus.
Reason (R): They undergo meiosis without cytokinesis.
Select the correct option:
a) Both A and R are true and R is the correct explanation of A.
b) Both A and R are true and R is not the correct explanation of A.
c) A is true but R is false.
d) A is false but R is true.

Answer: c) A is true but R is false

14. Unit: Genetics and Evolution
Assertion (A): Deoxyribonucleoside triphosphates serve dual purposes.
Reason (R): They act as proof readers and provide energy.
Select the correct option:
a) Both A and R are true and R is the correct explanation of A.
b) Both A and R are true and R is not the correct explanation of A.
c) A is true but R is false.
d) A is false but R is true.

Answer: c) A is true but R is false

15. Unit: Biotechnology and its Applications
Assertion (A): A floating cover placed over the slurry in a biogas plant keeps on rising.
Reason (R): This cover keeps on rising due to the gas produced in the tank by the microbial activity.
Select the correct option:
a) Both A and R are true and R is the correct explanation of A.
b) Both A and R are true and R is not the correct explanation of A.
c) A is true but R is false.
d) A is false but R is true.

Answer: a) Both A and R are true and R is the correct explanation of A

16. Unit: Genetics and Evolution
Assertion (A): DNA fragments can be isolated by Gel electrophoresis on the basis of their size.
Reason (R): The larger the fragment size, the faster it moves.
Select the correct option:
a) Both A and R are true and R is the correct explanation of A.
b) Both A and R are true and R is not the correct explanation of A.
c) A is true but R is false.
d) A is false but R is true.

Answer: c) A is true but R is false

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