CBSE Class 12 Sample Paper 2025 for Biology

CENTRAL BOARD OF SECONDARY EDUCATION

SAMPLE PAPER
CLASS 12
2025

Sample Question Paper
Class XII
044 Biology (2024-25)
Maximum Marks: 70 Time: 3 hours

General Instructions:

(i) All questions are compulsory.
(ii) The question paper has five sections and 33 questions.
(iii)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.
(iv) There is no overall choice. However, internal choices have been provided in
some questions. A student has to attempt only one of the alternatives in such
questions.
(v) Wherever necessary, neat and properly labeled diagrams should be drawn.

Section – A
Q. No. 1 to 12 are multiple choice questions. Only one of the choices is correct.
Select and write the correct choice as well as the answer to these questions.

Q. Question Mark
No s

1 Signals for parturition in human female originate from 1
A. Fully developed foetus only
B. Both placenta as well as fully developed foetus
C. Placenta only
D. Oxytocin released from maternal pituitary

2 To produce 1600 seeds, the number of meiotic divisions required will be 1
A. 2400
B. 2000
C. 1600
D. 1800

3 A sample of normal double-stranded DNA was found to have thymine 1
content of 27%. What will be the expected proportion of guanine in this
strand?
A. 23%
B. 32%
C. 36%
D. 73%

4 Observe the schematic diagram that depicts a small section of nucleic 1
acid. The bases in two strands are paired through hydrogen bonds that
are shown by the dark lines. Identify the correct sequence of nucleotide
in the 5’-3’ direction.

A. GCAT
B. CGTA
C. TAGC
D. ATCG
For Visual Impaired Students
E. coli has 4.6 X 10 6 base pairs and completes the process of 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

5 Suresh and Rajesh have defective haemoglobin due to genetic disorders. 1
In Suresh, the problem is qualitative as he is having incorrectly functioning
globin molecules while in Rajesh the problem is quantitative as he is
having very few globin molecules. Identify the disorder they are suffering
from.

Suresh Rajesh

A Thalassemia - Autosomal Sickle Cell Anaemia - Autosomal
Dominant blood disorder linked Recessive trait

B Sickle Cell Anaemia - Thalassemia - Autosomal
Autosomal linked Dominant Recessive blood disorder
trait

C Sickle Cell Anaemia – Thalassemia – Autosomal
Autosomal linked Recessive Recessive blood disorder
trait

D Thalassemia - Autosomal Sickle Cell Anaemia - Autosomal
Dominant blood disorder linked Dominant trait

6 In E.coli, the lac operon gets switched on when lactose is 1

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.

7 Which of the following features shows the mechanism of sex 1
determination in honey-bee?

(i) An offspring formed from the union of a sperm and egg develops
as a female.
(ii) Males have half the number of chromosomes than that of female.
(iii) The males are haploid having 32 chromosomes.
(iv) All workers and males are diploid having 16 chromosomes
A. (i) and (ii)
B. (ii) and (iii)
C. (i) and (iv)
D. (ii) and (iv)

8 The following diagram shows a fragment of DNA which is going to be 1
transcribed, the upper strand with polarity 3’ to 5’ is the template strand:
3´ ATTGCC 5´
5´ TAACGG 3´

After transcription the mRNA can be represented by:
A. 5´ AUUGCC 3´
B. 5´ AUUGCC 3’
C. 5´ UAACGG 3´
D. 5´ GGCAAU 3´

9 Idli – dosa dough rises due to production of which of the following gas? 1
A. CO
B. CO2
C. NO
D. NO2

10 Adaptive radiation leads to which of the following? 1
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.

11 Eco R1 cuts the DNA between bases G and A only when the sequence 1
of GAATTC is present. The number of nucleotides present in the resultant
sticky ends that will be formed in each of the two strands of DNA after this
enzyme cuts the DNA will be:
Vector DNA Foreign DNA
A. 1&5 5 &1
B. 2&4 4 &2
C. 2&5 5&2
D. 3&4 4&3

12 During the secondary treatment of sewage, which of the following change 1
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

Question No. 13 to 16 consist of two statements – Assertion (A) and Reason (R). Answer
these questions selecting the appropriate option given below:
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.

13 Assertion (A): Cells of tapetum have more than one nucleus. 1
Reason (R): They undergo meiosis without cytokinesis.

14 Assertion (A): Deoxyribonucleoside triphosphates serve dual purposes. 1
Reason (R): They act as proof readers and provide energy.

15 Assertion (A): A floating cover placed over the slurry in a biogas plant 1
keeps on rising.
Reason (R): This cover keeps on rising due to the gas produced in the
tank by the microbial activity.

16 Assertion (A): DNA fragments can be isolated by Gel electrophoresis on 1
the basis of their size.
Reason (R): The larger the fragment size, the faster it moves.

Section - B

17 Attempt either option A or B. 2
A.
(i) A blood test reported negative for hCG.
What does negative hCG imply? Name the tissue which
produces hCG?

(ii) If a blood test reported positive for hCG in a person, then which
other hormones would also be secreted by the tissue secreting
hCG?
OR
B.
(i) The human male ejaculates about 200 to 300 million sperm
during a coitus, however the ovum is fertilized by only one
sperm. How does the ovum block the entry of additional
sperms?
(ii) All copulations will not lead to fertilization. Why?

18 Attempt either option A or B. 2
A. The schematic representation given below shows a DNA strand and
two types of mutations in the DNA strand.

(i) Identify the type of mutation exhibited in I and II.
(ii) Which of the above mutation is more harmful? Give reason.

OR

B. Given below is a schematic representation of a mRNA strand

(i) In the above sequence identify the translational unit in mRNA.
(ii) Where are UTRs found and what is their significance?

19 Given below is the relationship between the HIV levels in the blood and 2
helper T cell count in a person detected with AIDS. Study the relationship
and answer the questions that follow.

A. What kind of relationship is observed in the virus levels and the
immune response after some days of the initial infection?
B. Does it completely clear the virus from the body permanently? Give
reason for your answer.
For visually impaired students.
Write the sequence of events that occur when a retrovirus enters a human
being, causing reduction in helper T-cells.

20 A culture plate of Lactobacillus shows blue-coloured colonies and 2
colourless colonies. Explain the principle involved in the formation of such
variance in the colour of colonies.

21 Attempt either option A or B. 2
A.
(i) It was estimated that if an evergreen forest has a GPP of 400
J/m2/day and 150 J/m2/day worth of carbon dioxide flows out of
that forest, what is the NPP in that forest?
(ii) Explain why pyramids of energy must always be upright.

OR
B.
(i) Assume that, GPP Forest A = GPP Forest B = GPP Forest C,
If Forest A has NPP = 1254 J/m2/day; Forest B, NPP =2157
J/m2/day; and Forest C, NPP = 779 J/m2/day, which one of
these forests has maximum energy loss by respiration? Give
reason.
(ii) Draw an ecological pyramid of number of the following food
chains
a. Grass — Animal —Fleas on the host animal
b. Tree — Insects — Woodpecker

Section - C

22 The image below shows two germinated seeds X and Y which belong to 3
the same species. Seed X is produced by apomixis whereas seed Y is a
product of sexual reproduction.

A. Write the number of embryo(s), embryo sac(s) and ovules in the
ovary of seed X.
B. How multiples embryos are formed in citrus fruits?
C. What advantage will plants developed from seed Y have over seed
X?
For visually impaired students
Each Mango fruit contains one seed. Two mango seeds, X and Y were
sown in the soil. From Seed X, 3 plant saplings germinated but from
seed Y only 1 plant sapling germinated.
A. For seed X which is apomictic, calculate the number of:
i) Embryo(s)
ii) Embryo sac(s)
iii) Ovules in this seed’s ovary
B. How multiples embryos are formed in citrus fruits?
C. What advantage will plants developed from seed Y have over seed
X?

23 Name the place in human ovary where the first meiotic division is 3
completed during oogenesis. What are the products of this division? Give
the chromosome number of each type of cells involved in the process.

24 The schematic representation given below shows the concept of Central 3
Dogma.

A. During the process of replication and transcription the pairing of
nitrogenous bases is not similar. Explain.
B. How is the above process modified in a retrovirus? Name the
process.
C. Justify why during the process of transcription only a segment of
DNA is copied into RNA.
For visually impaired students
Central Dogma explains the process of DNA transcription and translation.
From DNA mRNA is transcribed and then mRNA is translated into a
polypeptide.
A. During the process of replication and transcription the pairing of
nitrogen bases is not similar. Explain.
B. How is the above process modified in viruses? Name the process.
C. Justify why during the process of transcription only a segment of DNA
is copied into RNA.

25 Describe the steps involved in Southern blot hybridization using 3
radiolabeled VNTR as a probe.

26 Bio-fertilisers are organisms that enrich the nutrient quality in the soil. 3
Explain the role of three main sources of bio-fertilisers.

27 Explain how PCR technique can be used for amplification of a small 3
amount of DNA template.

28 A. Diagram given below depicts different species of Warbler birds 3
feeding on different regions on a Spruce tree. Explain the mechanism
which helps them to co-exist.

B. What does Gause‘s exclusion principle state? Does it apply in the
case shown above? Explain.
For visually impaired students
A. Name and explain the mechanism where two species competing for
the same resource co-exist.
B. What does Gause‘s exclusion principle state? Does it apply in the
above situation? Explain.

Section - D

29 Assuming that within a population of beetles where Hardy Weinberg 4
conditions are met, the colour black (B) is dominant over the colour red
(b). 40% of all beetles are red (bb).
Given this information, answer the questions below:
A. What is the frequency of red beetles? (1)
B. Calculate is the percentage of beetles in the population that are
heterozygous. (2)
Attempt either subpart C or D.
C. What is the frequency of homozygous dominant individuals? (1)
OR
D. Assuming that Hardy Wienberg conditions are met in the beetle
population consisting of 1500 beetles. How many beetles would you
expect to be black and red in colour respectively? (1)

30 Given below is the pattern of temperature in a person suffering from a 4
non-viral disease transmitted by mosquitoes. Study the graph and answer
the questions that follow:

A. Explain the factor(s) responsible for this pattern of temperature. (1)
B. How does this pathogen multiply in the human body? (2)
Attempt either subpart C or D.
C. How is this infection transmitted to humans? (1)
OR
D. Which stages of the life cycle of this pathogen are completed in the
mosquito’s gut? (1)
For visually impaired students
A. A non-viral disease that is transmitted by mosquitoes causes recurring
fever in an infected person. Explain giving reason(s). (1)
B. How does this pathogen multiply in the human body? (2)
Attempt either subpart C or D.
C. How is this infection transmitted to humans? (1)
OR
D. Which stages of the life cycle of this pathogen are completed in the
mosquito’s gut? (1)

Section - E

31 Attempt either option A or B. 5
A. Cryptorchidism is a condition in which the testes fail to descend into
the scrotum. It can also lead to compromised Sertoli cell function and
has an impact on Leydig cell function.
(i) Identify at least 3 parameters of male fertility which get affected due
to cryptorchidism.
(ii) Which process will be affected if mature spermatids are not
released from Sertoli cells?
(iii) Name and explain one assisted reproductive technology (ART
process) in which the sperm/semen is used to assist fertilization.
(iv) Name and explain the assisted reproductive technology that should
be used to complete the development of embryos I and II shown in
the figure given below.

Embryo I Embryo II
For visually impaired students
(iv) An infertile couple decided to use ART to conceive. After IVF they
decided for Embryo transfer of the following 2 embryos. Embryo 1
consisted of 8 blastomeres and Embryo 2 consisted of 16
blastomeres. Name and explain the techniques they should deploy
to complete further development of the given embryos.
OR
B.
(i) Explain the significance of each of the following features present in
plants given below:
a) In rose-bay plant the stamens ripen before the stigma.
b) In certain species of primrose, the flowers have short stamen
and long style.
c) The bisexual flower of mustard exhibits rejection of self-pollen
grain.
(ii) Explain how autogamy is prevented in castor and papaya plant
respectively.

32 Attempt either option A or B. 5
A. Explain how advent of biotechnology has helped in preventing
infestation by nematodes and thereby increasing crop yield.
OR
B. In the future, genetic therapies may be used to prevent, treat, or cure
certain inherited disorders in humans. Justify the statement with a
suitable example.

33 Attempt either option A or B. 5
A.
(i) Why is there a need to conserve biodiversity? (Any two reasons)
(ii) Name and explain any two causes that are responsible for the loss
of biodiversity.
OR
B.
(i) Name the two types of desirable approaches to conserve
biodiversity? Explain with examples bringing out the difference
between the two types.
(ii) State the features of a stable biological community?

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