CBSE Class 12 Biology Pre Board Sample Paper 2025-26 with Solutions: Download PDF

Category

Details

Conducting Body

Central Board of Secondary Education (CBSE)

Exam Date

27 March 2026 (Friday)

Exam Mode

Pen-and-paper (offline)

Total Marks

70

Duration

3 hours

Official website

cbse.gov.in

Question Number

Questions

Marks

Questions 1 to 12 are multiple-choice questions. Each question has only one correct option. Select and write the correct option along with its answer.

1

Which of the following statement/s are correct example/s of the degenerate feature of the genetic code?

P.  UAA and UAG act as terminator codons.

Q. CUA codes for leucine in bacteria as well as humans.

R. Valine is coded for by GUU and GUC.1

A. only Q                                     B. only R

C. only P and Q                           D. only P and R

1

2

Which cellular process is shown below?

A. DNA Replication                       B. Translation - Initiation 

C. Translation - Elongation            D. Translation – Termination 

1

3

Ginkgo biloba is also known as a living fossil as it has changed very little over time. It is resistant to disease and pests, is tolerant of a wide range of environmental conditions and is the last-standing member of its botanical family. 

The above is an example of which of the following phenomena? 

1. A. speciation                                  B. fossilization 

1. C. adaptive radiation                      D. survival of the fittest 

1

4

In a science fiction movie, scientists find fossilised pollen grains of Archaefructus and use them to fertilise a modern genus of Archaefructus. Nitya thinks that these pollen grains can be found under polar ice sheets where the temperature is around -40°C. 

Is she correct and why? 

1. A. Yes, because -40°C is enough to keep pollen grains viable. 

2. B. No, because the pollen grains will get wet and won't function. 

3. C. Yes, because pollen grains are viable at any temperature for    

4.     several years. 

5. D. No, because pollen grains need to be stored at much lower 

6.      temperatures to be viable. 

1

5

As a part of increasing immunity against COVID-19, a small part of the virus is administered to people as a vaccine. 

Which of the following types of immunity is the above an example of? 

1. A. Naturally acquired active immunity 

2. B. Artificially acquired active immunity

3. C. Naturally acquired passive immunity

4. D. Artificially acquired passive immunity 

1

6

Which of the following methods of assisted reproductive technologies (ART) can be a more viable process of pregnancy in a woman after menopause? 

1. A. Gamete intra fallopian transfer 

2. B. Zygote intra fallopian transfer 

3. C. Artificial insemination 

4. D. Pregnancy is not possible after menopause 

1

7

Which one of the following is an incorrect statement with regard to pedigree analysis? 

A. It verifies that DNA is the carrier of genetic information. 

B. It helps to understand whether the trait depicted in the chart is dominant or recessive. 

C. It confirms that the trait is linked to one of the autosome. 

D. It helps to trace the inheritance of a specific trait. 

1

8

Some events of pregnancy in humans are written below in a sequence. 

(i) complete development of foetus 

(ii) uterine contraction 

(iii) dilation of cervix 

(iv) delivery of the baby 

(v) lactation 

Between which of the following events does the shedding of the placenta happen? 

1. A. (i) and (ii)                                          B. (ii) and (iii) 

2. C. (iii) and (iv)                                       D. (iv) and (v)

1

9

Industrial production of which of these products can be negatively affected by the presence of Saccharomyces cerevisiae? 

1. A. Beer          B. Wine           C. Fruit juice           D. Wheat bread 

1

10

Transgenic mice are being developed to replace the use of monkeys for laboratory testing of vaccines or drugs meant for humans. 

Which of the following is/are POSSIBLE reasons for this move? 

P) Mice and humans have similar physiology. 

Q) The reproduction rate is faster in mice. 

1. A. only P                                B. only Q 

2. C. both P and Q                      D. neither P nor Q

1

11

In mice, brown fur is dominant and white fur is recessive. A biologist crossed abrown fur male with a white fur female and obtained two brown fur and twowhite fur offspring. Which of the following statements is MOST likely to be true?

A. Both parents are heterozygous.

B. Only the male parent is homozygous.

C. Only the male parent is heterozygous.

D. Only the female parent is heterozygous.

1

12

The main reason for the presence of both a leading and a lagging strand during DNA replication is, 

A. DNA polymerase can read only in the direction of 3' to 5' 

B.DNA polymerase can only synthesize one strand at a time 

C. Only one strand is available to be read at any given time 

D. There are not enough RNA primers to have both strands be synthesized simultaneously 

1

Questions No. 13 to 16 contain two statements — Assertion (A) and Reason (R). Answer each question by selecting the most appropriate option from the choices 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): The liver is the highest affected organ by the consumption of alcohol as compared to the other organs. 

Reason (R): Alcohol received by the liver gets chemically reduced to produce alkanes, which are very harmful.

14

Assertion (A): Transposons cause insertional mutations that can be treated using gene silencing. 

Reason (R): Transposons are mobile genetic elements that self-replicate via an RNA intermediate. 

15

Assertion (A): Genetic makeup serves as the primary factor in shaping the phenotype of a species and not vice versa. 

Reason (R): Adaptation involves the phenotype's interaction with the environment, leading to changes in genetic makeup over generations.

16

Assertion (A): Endosperm in a flowering plant is formed before the formation of the embryo. 

Reason (R): The endosperm provides food to the developing embryo. 

SECTION-B

17

The Medical Termination of Pregnancy (Amendment) Act, 2017 dictates that a pregnancy may be terminated on certain considered grounds within the first 12 weeks of pregnancy on the opinion of one registered medical practitioner. If the pregnancy has lasted more than 12 weeks, but fewer than 24 weeks, two registered medical practitioners must be of the opinion, formed in good faith, that the required ground exist.

[Note: Grounds refer to circumstances or conditions.]

Describe two grounds under which medical termination of pregnancy is permitted.

                                OR

Write down two ways by which an adult human can contract the HIV. 

2

18

Can you think of a situation where we deliberately want to make a species extinct? How would you justify it?

                                OR

With the help of two examples, enumerate defence mechanisms evolved by prey species to lessen the impact of predation.

2

19

State two ways in which sex determination in humans is different from that in honeybees.

2

20

Why it is essential to have selectable marker in a cloning vector? Cloning vector should ideally have one recognition site for the commonly used endonuclease. Justify.

2

21

What happens when: 

(a) A newborn is not breastfed. 

(b) A tumour cell enters the bloodstream. 

                                                                 OR

Mark the following statements as TRUE or FALSE, and support your answer with a reason. 

(i) Vector-borne diseases are caused only by protozoans. 

(ii) All infectious diseases caused by bacteria spread through air. 

2

SECTION-C

22

A scientific group aims to compare the quality of water from various water bodies. They collected samples from 3 water bodies and calculated their BOD amongst other parameters and tabulated them as follows. 

(a) Arrange the sample names with respect to their level of pollution (highest to lowest). 

(b) Explain how BOD can indicate the level of pollution in water and hence the water quality.

3

23

State ONE characteristic of a pollen grain that can help students identify: 

(a) a water-pollinated pollen grain 

(b) an animal-pollinated pollen grain. 

(c) a wind -pollinated pollen grain.

                                                              OR

Draw a neat and labelled diagram of the female gametophyte in a flowering plant. 

The filiform apparatus at the micropylar end forms an important part of the embryo sac. What is the importance of the filiform apparatus? 

3

24

With the help of experiments done by various scientists over 40 years, it was finally concluded that DNA is the genetic material. 

(a) Before DNA, which molecule was considered to be the genetic material? 

(b) What was concluded from Griffith's experiments using S and R strains of mice? 

(c) Briefly describe an experiment that led to the conclusion that DNA is the genetic material.

3

25

X is the first restriction enzyme isolated from strain H of Bacillus

amyloliquefaciens. This enzyme cuts between two guanine bases of the following palindromic sequence:

5’-G G A T C C-3’

3’-C C T A G G-5’

(a) Name the restriction enzyme X. Explain how you arrived at this name.

(b) Draw the DNA fragments produced due to the action of enzyme X on the palindromic sequence shown above.

                                                           OR

Why must a cell be made competent in biotechnology experiments. Explain two methods by which a host cell is made competent to take the foreign DNA.

3

26

Sudha cracked open a coconut and found the following content as shown in the image below: 

(a) Identify the parts of the seed labelled P and Q. 

(b) What is most likely to have happened to the coconut water? 

(c) What is the ploidy of the coconut water that we drink from the tender coconut? Justify.

3

27

In 1953, Stanley Miller performed an experiment that may explain what occurred on primitive Earth billions of years ago. His experiment was based on a hypothesis that stated life could have originated from basic molecules present on the early Earth. 

1. a- Label ‘A’, ‘B’, and ‘C’ 

2. b- What is the role of ‘A’ 

3. c- What was the conclusion made by Miller after this experiment? 

                                                                   OR

A. Explain the type of evolution that has resulted in the development of structures like the wings of butterflies and birds. 

B. Identify the examples of homologous and analogous structures from the following, also give reason for it:

1. (i) Thorns in Bougainvillea and tendrils of Cucurbita. 

2. (ii) Vertebrate hearts 

3

28

a. Represent diagrammatically the age -pyramid for human population of India of present day.

b. How does an age -pyramid for human population at a given point of time helps the policy – makers in planning for future?

                                                                OR

Differentiate between the following: (one difference for each with example) 

1. Mutualism and competition

2. Commensalism and amensalism

3

SECTION D (CASE BASED QUESTIONS)

29

Two patients - X and Y, who had recently consumed drugs, were brought to a rehabilitation centre. While it is established that each of them consumed either morphine or cocaine, their specific drug consumption remained undisclosed. During the mandatory tests, X exhibited heightened alertness with bouts of hallucinations while Y appeared drowsy. Further, the resting heart rate of X was high and that of Y was slower than normal. 

(a) Based on these observations, identify the drug that each patient is likely to have consumed. 

(b) Which category of drugs does each drug identified in (a) belong to? 

(c) Mention any TWO ways in which society's overall well-being is likely to get affected by the misuse of such drugs. 

4

30

A biology student after studying about the different levels of hormones during the menstrual cycle was comparing 2 subjects (Patients). A table was created after looking at the levels of hormones A and B for Subject 1 and 2. Read the information in the table and answer the questions that follow

(i) The peak observed in Subject 1 and 2 is due to 

     A. Estrogen                               B. Progesterone 

     C. Luteinizing Hormone           D. Relaxin 

(ii)  The Subject 2 has higher level of hormone B, which is 

     A. Estrogen                             B. Progesterone 

     C. Luteinizing Hormone         D. Follicle Stimulating Hormone 

(iii) Which structure in the ovary will remain functional in subject 2? 

     A. Corpus Luteum                   B. Tertiary follicle 

     C. Graafian follicle                  D. Primary follicle 

(iv) Which of the following statements is true about the subjects? 

       A. Subject 1 is pregnant 

       B. Subject 2 is pregnant 

       C. Subject 1 and 2 both are pregnant 

       D. Subject 1 and 2 both are not pregnant 

4

SECTION-E

31

Mendel crossed a pea plant having inflated pods with another pea plant having constricted pods. Both parents were pure-lines. Next, he self-crossed the progeny obtained in the F1 generation.

(a) Depict the cross and observations using a Punnett square.

(b) How did his findings lead to the inference of the law of segregation?

(c) State two findings that Mendel would have made if there was no

     segregation.

                                                      OR

Explain the process of protein synthesis from processed m-RNA.

5

32

1. Differentiate between GRAZING food chain and DETRITUS food chain (2 points) along with example of each food chain

2. Construct a pyramid of biomass starting with phytoplankton. Label three trophic levels. Is the pyramid upright or inverted? Why?

                                           OR

Explain in situ conservation and ex situ conservation of biodiversity with examples of each. Why do we need ex-situ conservation along with in-situ conservation?

5

33

What is meant by RNA interference (RNAi)? Describe in correct sequence the use of this phenomenon to produce nematode – resistant tobacco plant.

                                                          OR

1. Represent by means of flow chart the steps involved in the process of recombinant DNA technology

2. What is meant by insertional inactivation?

5

Answer Number

Answers

Marks

1

B. only R

1

2

C. Translation - Elongation 

1

3

D. survival of the fittest

1

4

D. No, because pollen grains needs to be stored at much lower temperatures to be viable. 

1

5

B. Artificially acquired active immunity 

1

6

B. zygote intra fallopian transfer 

1

7

A. It verifies that DNA is the carrier of genetic information. 

1

8

D. (iv) and (v) 

1

9

1. C. Fruit juice 

1

10

1. C. both P and Q 

1

11

C. Only the male parent is heterozygous. 

1

12

A. DNA polymerase can read only in the direction of 3' to 5' 

1

13

C. A is true, but R is false. 

1

14

C. A is true, but R is false. 

1

15

B. Both A and R are true, but R is not a correct explanation for A. 

1

16

A. Both A and R are true, and R is the correct explanation of the assertion. 

1

17

1 mark each for any two of the following:

• Continuing the pregnancy may pose a risk to the woman’s mental, emotional, or physical health.

• There is a possibility that the baby may be born with serious physical or mental abnormalities.

• The pregnancy is unwanted by the woman, such as in the case of a rape victim.

OR

1 mark each for any two of the following modes of transmission:

(i) Sharing injection needles or surgical instruments with an infected person.
(ii) Receiving blood through transfusion from an infected person.
(iii) Having sexual contact with an infected person. 

2

18

Species that are harmful to humans, such as HIV and the polio virus, can be eliminated. These microorganisms are not part of any food chain, so their extinction would not negatively impact the ecosystem.

OR

Prey defence mechanisms (any two):

(a) Some insects and frogs have cryptic colouring (camouflage), which helps them avoid being easily detected by predators.

(b) The Monarch butterfly is avoided by birds because it is highly distasteful. This is due to a special chemical it obtains while feeding on a poisonous weed during its caterpillar stage.

(c) Some plants have thorns or spines as a defence mechanism, such as Acacia and cactus.

(d) Some plants produce poisonous chemicals like cardiac glycosides, quinine, strychnine, opium, nicotine, and caffeine. These chemicals protect them from grazing animals.

2

19

In humans, sex determination follows the XY system. Males have two different sex chromosomes (X and Y), while females have two X chromosomes. In bees, sex determination is haplodiploid—females are diploid (two sets of chromosomes), and males are haploid (one set of chromosomes).

In bees, males are produced by parthenogenesis (development from unfertilized eggs), and females are produced by fertilization. In humans, both males and females are produced only through fertilization.

In bees, sperm cells are produced by mitosis, whereas in humans, sperm cells are produced by meiosis.

2

20

Selectable markers are important because they help identify and remove non-transformants. They do this by allowing only the transformants to grow.

To insert foreign (alien) DNA, vectors should have very few restriction enzyme recognition sites, usually just one. If multiple sites are present, the restriction enzyme will cut the vector into many fragments, making cloning difficult.

Ligation of foreign DNA is usually done at a restriction site located within one of the two antibiotic-resistance genes of the vector.

2

21

(a) When a newborn is not breastfed, they receive less passive immunity. This can make them more vulnerable to infections because breast milk contains important protective factors.

(b) A tumour cell that enters the bloodstream can travel to another part of the body and may form a secondary tumour there. This process is known as metastasis.

OR

(i) 0.5 marks for each:

• FALSE

• Elephantiasis is a vector-borne disease caused by parasitic worms.

(ii) 0.5 marks for each:

• FALSE

• Typhoid is caused by a bacterium and spreads through contaminated food and water.

2

22

(a) The pollution level in the samples is as follows: Sample A > Sample C > Sample B.
(Appropriate marks should be deducted if any part of the order is incorrect.)

(b) BOD (Biological Oxygen Demand) measures the amount of oxygen required by microorganisms to break down the organic matter present in one litre of water. [1 mark]

A higher BOD value means that microorganisms need more oxygen to decompose the organic matter, indicating that the water is highly polluted and of poor quality. [1 mark]

3

23

1 mark for each correct answer:

(a) The pollen grain will have a mucilaginous covering to prevent it from getting wet.
(b) The pollen grains will have a sticky outer surface.
(c) The pollen grains are light, dry, and may be winged.
(Any other valid answer may also be accepted.)

OR

2.5 marks for the labelled diagram (at least four correct labels)

The filiform apparatus helps guide the pollen tube into the synergids.

3

24

(a) Proteins

(b) The transfer of genetic material can change a cell’s function by transforming it to perform a new activity.

(c) 1.5 marks for any of the following explanations:

• Avery, MacLeod, and McCarthy purified DNA, RNA, and proteins from heat-killed S cells to determine which molecule could transform live R cells into S cells. They found that only DNA from the S bacteria was able to transform the R bacteria.
  
  

• Hershey and Chase used bacteriophages labelled with radioactive ³²P (DNA) and ³⁵S (protein coat) to infect bacteria. They observed that only the ³²P-labelled DNA entered the bacterial cells, proving that DNA is the genetic material.

3

25

(a) 0.5 mark for each:

Name of the enzyme: BamHI

The naming convention for a restriction enzyme is as follows:

• The first letter comes from the genus name (Bacillus).

• The next two letters come from the species name (amyloliquefaciens).

• The fourth letter indicates the strain (strain H).

• The Roman numeral shows the order in which the enzyme was isolated from that strain (I = first enzyme).

• Recognition sequence:

(b)

5’-G           G A T C C-3’

3’-C C T AG             G-5’

OR

DNA is hydrophilic and cannot pass through the cell membrane on its own. Therefore, the cell must be made competent to allow it to take up DNA from the surrounding medium.

Methods of making cells competent (any two):

• Gene gun: DNA-coated particles are shot into cells.

• Heat shock: Sudden temperature changes help DNA enter the cell.

• Microinjection: DNA is directly injected into the cell using fine needles.

• Disarmed pathogen: A modified pathogen delivers DNA into the cell.

• Electroporation: Electric pulses create temporary pores in the membrane to allow DNA entry.

3

26

(a) 0.5 marks for each correct answer:

• P: Endosperm

• Q: Embryo

(b) The coconut water would have been consumed by the developing embryo.

(c) 0.5 marks for each correct answer:

• 3n

• It is a free-nuclear endosperm.

3

27

A = Electrode, B = Boiling water, C = Methane, ammonia, hydrogen, and water vapour

The role of A (electrode) is to create an electric discharge, simulating the conditions of the primitive atmosphere of early Earth.

Conclusion drawn by Miller from the experiment:

It provided experimental evidence supporting the theory of chemical evolution.

OR

Explanation of Convergent Evolution:

(i) These are homologous organs because both are modifications of the stem.

(ii) They are homologous structures as they share a similar anatomical structure.

3

28

1. Diagram of Expanding (triangular shaped) age pyramid

3. Age pyramid helps in planning the healthcare programmes, the education policies and the infrastructure of the area. Analysis is of age pyramid of a population can give the correct information about the status of the people in the area so that appropriate budget can be allocated to different sectors.

4.                                          OR

5. Any one difference of each with example

3

29

(a) 0.5 marks each for the following:

• X: Cocaine

• Y: Morphine

(b) 0.5 marks each for the following:

• Cocaine: Coca alkaloid

• Morphine: Opioid

(c) 1 mark each for any two of the following impacts:

• Increase in crime rates

• Rise in the number of unhealthy individuals due to short-term and long-term drug effects

• Greater burden on healthcare systems

• Strained relationships with family and friends

• Decrease in educational achievement and workforce productivity

4

30

C. Luteinizing Hormone 

B. Progesterone

A. Corpus Luteum 

B. Subject 2 is pregnant 

4

31

(a) Draw the correct Punnett squares for the F₁ and F₂ generations, using proper symbols and showing accurate ratios.

(b) Since the recessive parental trait reappears unchanged in the F₂ generation, it shows that the alleles separate or segregate during gamete formation, and each offspring receives only one allele from each parent.

(c) 0.5 mark each for the following:

• He would have observed a 1:1 ratio in the F₁ generation.

• A trait would not disappear in F₁ and then reappear in F₂.

OR

Translation

Definition: Translation is the process by which the genetic code on mRNA is used to synthesize proteins.

Diagram: (Figure to be drawn by student)

Three steps of translation:

1. Initiation

2. Elongation

3. Termination

5

32

Two differences – 1 mark
• One example of each food chain – 1 mark
• Labelled pyramid – 2 marks

The pyramid is inverted because the biomass of fishes is much greater than the biomass of phytoplankton.

OR

In situ conservation of biodiversity

Protecting species in their natural habitats.
Example: National parks, wildlife sanctuaries, biosphere reserves.

Ex situ conservation of biodiversity

Protecting species outside their natural habitats.
Example: Botanical gardens, zoos, seed banks.

Need for both methods (correct explanation):

Both approaches are important because in situ conservation maintains species in their natural ecosystem, preserving ecological relationships, while ex situ conservation safeguards endangered species that may not survive in the wild and helps in their long-term preservation and breeding.

5

33

Nematode Infection and RNA Interference

• The nematode Meloidogyne incognita infects the roots of tobacco plants, reducing crop yield.

• This infection can be controlled using RNA interference (RNAi), a process in which the expression of a specific mRNA is suppressed by a complementary double-stranded RNA (dsRNA).

• The dsRNA binds to the target mRNA and prevents its translation, effectively silencing the gene.

• Using Agrobacterium vectors, nematode-specific genes were introduced into the host plant so that the plant produces both sense and antisense RNA.

• These two RNA molecules are complementary and pair to form dsRNA, which triggers RNAi and silences the nematode’s specific mRNA.

• Sources of complementary RNA include retroviruses and transposons.

• As a result, the parasite cannot survive in the transgenic plant, thereby protecting the plant from pests.
  (Any other valid points on RNAi may also be accepted.)

OR

Steps Involved in Recombinant DNA Technology

1. Isolation of DNA

2. Fragmentation of DNA using restriction endonucleases

3. Isolation of the desired DNA fragment

4. Amplification of the gene of interest by PCR

5. Ligation of the DNA fragment into a vector

6. Insertion of recombinant DNA into the host

7. Culturing the host cells on a suitable large-scale medium

8. Extraction of the desired gene product

9. Downstream processing to obtain the finished product ready for marketing

(b) Insertional Inactivation

• Recombinants and non-recombinants can be identified using colour production in the presence of a chromogenic substrate.

• When recombinant DNA is inserted into the coding region of the β-galactosidase enzyme, the enzyme becomes inactive.

• Bacterial colonies containing the recombinant plasmid do not produce colour, whereas colonies without the recombinant plasmid produce blue-coloured colonies.

5

CBSE Class 12 Biology Pre-board Sample Paper 2025-26 with Solutions PDF