CBSE Class 12 Biology Sample Paper Set A

Sample Paper Class 12

See below CBSE Class 12 Biology Sample Paper Set A with solutions. We have provided CBSE Sample Papers for Class 12 Biology as per the latest paper pattern issued by CBSE for the current academic year. All sample papers provided by our Class 12 Biology teachers are with answers. You can see the sample paper given below and use them for more practice for Class 12 Biology examination.


Q1. Name any two techniques that serve the purpose of early diagnosis of some bacterial and viral human diseases. 
Answer : PCR and ELISA

Q2. What type of cut ends are formed when both the strands of DNA is cleaved at exactly the same nucleotide position? 
Answer : Blunt or flush ends.

Q3. What would be the molar concentration of human DNA in human cell? 
Answer : In human beings the concentration of DNA is 0.4%. It contains 6.6 × 109 bp.

Q4. Write the dual purpose served by deoxyribonucleoside triphosphates in polymerisation. 
Answer : Two functions of deoxyribonucloside triphosphates in polymerisation are:
• Acts as substrate
• Provides energy

Q5. Some allergens trigger sneezing and wheezing in human beings. What causes this type of response by the body? 
Answer : Body produces IgE antibodies on exposure to allergens and release of chemicals like histamine and serotonin from the mast cells.

Q6. What are ‘true breeding lines’ that are used to study inheritance pattern of traits in plants? 
Answer : Homozygus plants which are obtained after self pollination.

Q7. What makes the Nucleopolyhedrovirus a desirable biological control agent? 
Answer : These are species specific, narrow spectrum insectcides and do not harm non-target organisms.

Q8. How is Agrobacterium tumifaciens able to transform a normal plant cell into a tumor? 
Answer : Plasmid of Agrobacterium tumifaciens carries the genes for tumour, ‘Ti’. Agrobacterium transfers these cancer causing genes in plants thus causing tumour.

Q9. Why is it essential to have a selectable marker in a cloning vector? 
Answer : A selectable marker helps us to distinguish between transformed and non-transformed cells.

Q10. Why do children cured by enzyme replacement therapy for adenosine deaminase deficiency need periodic treatment? 
Answer : The lymphocytes in which ADA gene has been introduced are not immortal and patient needs repeated transfusion of such cells, so there is a need of periodic treatment.

Q11. Assertion (A): The bulbourethral gland is a male accessory gland. 
Reason (R): Its secretion helps in the lubrication of the penis, thereby facilitating reproduction.
(a) Both assertion and reason are true, and reason is the correct explanation of assertion.
(b) Both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both assertion and reason are false.
Answer : (a) 


Assertion (A): Each seminiferous tubule is lined on its inside by three type of cells.
Reason (R): These cells are male germ cells, Sertoli cells and Leydig cells.
(a) Both assertion and reason are true, and reason is the correct explanation of assertion.
(b) Both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both assertion and reason are false.
Answer : (d) 

Q12. Assertion (A): Amniocentesis is often missemployed. 
Reason (R): Amniocentesis is meant for determining the genetic disorders in the foetus, but it is being used to determine the sex of the foetus, leading to death of the normal female foetus.
(a) Both assertion and reason are true, and reason is the correct explanation of assertion.
(b) Both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both assertion and reason are false.
Answer : (a) 

Q13. Assertion (A): Gametes receives only one allele of a gene. 1
Reason (R): During gamete formation, mitosis takes place leads to formation of haploid cells.
(a) Both assertion and reason are true, and reason is the correct explanation of assertion.
(b) Both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both assertion and reason are false.
Answer : (c) 

Q14. Assertion (A): The matrix used in gel electrophoresis should have controllable pore size. 
Reason (R): Agarose concentration can be changed to change pore sizes.
(a) Both assertion and reason are true, and reason is the correct explanation of assertion.
(b) Both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both assertion and reason are false.
Answer : (b) 

Q15. The Watson and Crick observed that the nitrogenous bases form complementary pair between the two polynucleotide chains of DNA. Based on the X-ray diffraction data, they proposed that DNA consisted of a double helix with two chains having sugar, phosphate group on the outside and nitrogen bases on the inner side. Further, they proposed that the two chains are antiparallel with 5′ – 3′ orientation of the other. The two chains are twisted helically just as a rope ladder with rigid steps twisted into a spiral.
This property of double helix model of DNA led them to hypothesize semiconservative mode of DNA replication, where the two strands separate and act as a template for the synthesis of new complementary strand. 

(i) Which form of DNA is mentioned in Watson and Crick model?
(a) A-DNA
(b) Z-DNA
(c) B-DNA
(d) Quadraplex DNA
Answer : C

(ii) Thermal denaturation of DNA is not affected by
(a) Temperature
(b) Humidity
(c) Ionic bond strength
(d) All of these
Answer : B

(iii) The supercoiling of circular DNA is termed as
(a) Cross-linking
(b) Twist number
(c) Linking number
(d) None of these
Answer : C

(iv) Which of the following is used to introduce negative supercoiling in DNA?
(a) SYBR gold
(b) Ethidium bromide
(c) Gyrase type 2 topoisomerase
(d) Cobalt
Answer : C

(v) Assertion (A): The two chains of DNA have antipolarity.
Reason (R): In one chain of DNA, ribose sugar at 5′ end consists of a free phosphate
moiety while at the other end the ribose has a free 3′ OH group.
(a) Both assertion and reason are true, and reason is the correct explanation of assertion.
(b) Both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both assertion and reason are false.
Answer : A

Read the following and answer any four questions from 16(i) to 16(v) given below:

Q16. A sacred grove or sacred woods are any grove of trees that are of special religious importance to a particular culture. They were important features of the mythological landscape and cult practice of celtic,Estonian, Baltic, Germanic, Roman, Indian and Japanese.
They are the sacred forests patches around the places of worship. They are held in high esteem by tribal communities. Tribals do not allow to cut even a single branch of a tree in sacred groves. They are found in several parts of India e.g., Rajasthan (Aravalli hills), Madhya Pradesh (Sarguja, Bastar), Meghalaya and Maharashtra.

Sacred groves help in the conservation of many endemic species (rare species) flourishing in  that area. 

(i) Which of the following is/are benefits of sacred groves?
(a) Recharging aquifers
(b) Biodiversity hotspot
(c) Sources of various Ayurvedic medicines
(d) All of these
Answer : D

(ii) Sacred groves is an example of
(a) In-situ conservation
(b) Ex-situ conservation
(c) Both (a) and (b)
(d) Neither (a) nor (b)
Answer : A

(iii) Sacred groves are specially useful in
(a) Conserving rare and threatened species
(b) Preventing soil erosion
(c) Promoting environmental awareness
(d) All of these
Answer : A

(iv) In which culture sacred groves is known as Alka?
(a) Japanese
(b) Lithuanian
(c) Latvian
(d) Chinese
Answer : B

(v) In Kerala sacred groves is known as
(a) Deori
(b) Sharma
(c) Sharpakavu
(d) Madaico
Answer : C


Q17. Given below is one of the strands of a DNA segment: 
(a) Write its complementary strand.
(b) Write a possible RNA strand that can be transcribed from the above DNA molecule formed.
Answer : (a) 5′
ATGCATGCATGCATGC→ 3′ (polarity = ½, nucleotide sequence = ½)
(b) 5′
AUGCAUGCAUGCAUGC→ 3′ (polarity = ½, nucleotide sequence = ½)

Q18. Write the full form of PCR. What are the three basic steps involved in a single PCR amplification cycle? 
Answer : — Polymerase Chain Reaction
— Denaturation, Annealing and Extension.

Q19. It is often observed that the chances of a person suffering from measles in his or her lifetime are low if he or she has suffered from the disease in their early childhood. Justify the
Answer : Memory cells develop during measles in early childhood, subsequent encounters with the same pathogen elicits a highly intensified secondary / anamnestic response.

Q20. List the three hormones produced in women only during pregnancy. What happens to the levels of estrogen and progesterone during pregnancy?
Answer : • Human chorionic gonadotropin / hCG
• Human placental lactogen / hPL
• Relaxin
(The level of progesterone and estrogen) increases.


Write the function of each of the following:
(a) Middle piece in human sperm.
(b) Luteinising hormone in human males.
Answer : (a) Provides energy for movement of tail.
(b) LH hormone act on leydig cells and stimulate the synthesis and secretion of androgen testosterone.

Q21. A student on a field trip suddenly felt breathlessness and started to sneeze very badly. Name this response and explain what it is due to. 
Answer : Allergy: Allergens (dust / pollen / mites etc.), due to release of chemicals like histamine / serotonin (from mast cells).

Q22. Define the term addiction. Name some drugs which are normally used as medicines but are often abused. 
Answer : — Addiction is a psychological attachment to certain effects – such as euphoria and a temporary feeling of well-being associated with drugs and alcohol.
— Barbiturates, amphetamines, benzodiazepines and LSD (Lysergic acid diethyl amides) are normally used as medicines to help patients cope with mental illness, are often abused.


Define auto-immune diseases. Give two examples.
Answer ; Instead of destroying foreign molecules when immunity starts attacking the body’s own cells leading to serious diseases called auto-immune diseases.
Examples: (a) Multiple sclerosis and (b) Rheumatoid arthritis.

Q23. Name the genus of baculovirus that acts as a biological control agent in spite of being a pathogen. Justify by giving three reasons that make it an excellent candidate for the job. 
Answer : Genus –Nucleopolyhedrovirus, species-specific, Narrow spectrum, No negative impact on (plants / mammals / birds / fish) non-target organisms.

Q24. What is vector? Which cloning vector was discovered first? 
Answer : — Vector is a DNA molecule that can carry a foreign/desired DNA segment and replicates inside the host cell.
— pBR322 vector.

Q25. What is aminoacylation? State its significance. 
Answer : When an amino acid is linked to its cognate tRNA in the presence of ATP is called charging of tRNA or aminoacylation of tRNA. If two such charged tRNAs are brought close enough, the formation of peptide bond between them would be favoured energetically.


Q26. Explain the changes that milk undergoes when suitable starter / inoculum is added to it. How does the end product formed prove to be beneficial for human health? 3
Answer : Lactobacillus / lactic acid bacteria (LAB) present in inoculum, grows in milk at a suitable / optimum temperature, multiplies converting milk to curd, produce acids that coagulate and partially digest the milk proteins.
This improves its nutritional quality by increasing Vitamin B12, LAB check diseases causing microbes in our stomach.

Q27. During the secondary treatment of the primary effluent how does the significant decrease in BOD occur? 
Answer : During secondary treatment the aeration allows vigorous growth of useful aerobic microbes into flocs (masses of bacterial cells in association with fungal filaments). As they grow, the microbes consume a major part of the organic matter in the effluent ; so, BOD is significantly decreased.

Q28. Name any two natural cloning vectors. Give reasons that make them act as cloning vectors.
Write the two characteristics the engineered vectors are made to possess. 
Answer : Plasmids, bacteriophages ability to replicate within bacterial cells, high copy number within the bacterial cells. Characteristics of engineered Vectors: easy linking of foreign DNA, Selection of recombinants from non-recombinants / selectable marker.

Q29. What are gemmules and conidia? Name one organism each in which these are formed? 
Answer : Gemmule-asexual reproduc tive structure in sponges Conidia-asexual reproductive structure in Penicillium.

Q30. Study the age pyramids ‘A’, ‘B’ and ‘C’ of the human population given below and answer the questions that follow: 
(a) Identify pyramids ‘B’ and ‘C’.
(b) Write the basis on which the above pyramids are plotted.
Answer : (a) B–Stable population
C–Declining population
(b) Age Distribution of male and female of human population.

Study the population growth curves in the graph given below and answer the questions which follow? 
(i) Identify the growth curves ‘a’ and ‘b’.
(ii) Which one of them is considered a more realistic one and why?
(iii) If dN/dt = rN K – N/K=is the equation of the logistic growth curve, what does K stand for?
(iv) What is symbolised by N?
Answer : (i) (a) T-shaped, exponential growth curve
(b) S-shaped logistic/sigmoid growth curve
(ii) curve b is more realistic because food for the animals is finite and so will soon become a limiting factor.
(iii) ‘K’ is the carrying capacity of the area.
(iv) ‘N’ symbolises the population density at time t.


Q31. (a) IVF is a very popular method these days that is helping childless couples to bear a child.
Describe the different steps that are carried out in this technique.
(b) Would you consider Gamete Intrafallopian Transfer (GIFT) as an IVF? Give a reason in support of your answer. 
Answer : (a) Ova from the wife/donor (female) and sperms from the husband/donor (male), are collected and induced to form zygote, in simulated conditions in laboratory, the zygote/early embryos
(upto 8 blastomeres) then transferred into the fallopian tube (ZIFT), and embroyos with more than 8 blastomeres, into the uterus (IUT) to complete its further development.
(b) No, GIFT cannot be considered as IVF technique because fert ilisation takes place in the female body / in vivo.


(a) Draw a sectional view of a human ovary and label primary follicle, tertiary follicle, Graafian follicle and corpus luteum in it.
(b) Name the gonadotropins and explain their role in oogenesis and the release of ova.
Answer : (a) 
(b) Both LH and FSH increase during follicular phase and stimulate follicular development,secretion of estrogen by the growing follicles, LH surge (rapid secretion of LH) during mid
cycle / 14th day induces rupture of graffian follicle, and release of ova / secondary oocyte.

Q32. Describe the experiment carried out by Hershey and Chase. Write the conclusion they arrived at.
Answer : They grew viruses on a medium containing radioactive phosphorus / P32, and some on radioactive sulphur / S35, radioactive DNA contained in viruses grown on radioactive
phosphorus, radioactive protein contained in virus grown in radioactive sulphur, radioactive phages were allowed to attach to E.coli bacteria, the virus coats were removed from bacteria
by agitating in a blender, the virus particles were separated from the bacteria on centrifugation, Bacteria infected with viruses containing radiactive DNA were radioactive, where as bacterial cells infected with virus containing radioactive protein did not show radioactivity. 
Conclusion: DNA is therefore the genetic material that is passed from virus to bacteria.


What is meant by R cells and S cells with which Frederick Griffith carried out this experiments on Streptococcus pneumoniae? What did he prove from these experiments?
Answer : The bacterium Streptococcus pneumoniae has two strains virulent and non-virulent. The virulent strains causes pneumonia, whereas the non-virulent strains do not produce the disease.
Its virulent strain is known as S-type because when grown on a suitable medium they form smooth colonies. The non-virulent type of bacteria form irregular or rough colonies and
therefore called R-type. By his experiments Griffith concluded that the R-strain bacteria (nonvirulent) has somehow been transformed by the heat killed S-type (virulent type), which must be due to the transfer of the genetic material. Griffith proposed that the transforming principle is a chemical substance released by heat killed bacteria which changed the R-bacteria into S-bacteria.

Q33. Elaborate on the key abiotic elements that contribute to the variation in habitats. 
Answer : The important abiotic components that contribute to the variation in habitats are:
(i) Temperature: It affects the kinetics of enzymes activity and other physiological functions of the organism. The levels of thermal tolerance of different species also determine their geographical distribution to a larger extent.
(ii) Water: The productivity and distribution of plants is heavily dependent on water. For aquatic organisms the quality (chemical composition, pH and salinity) of water becomes important. Some organisms are tolerant to a wide range of salinities while others are
restricted to a narrow range (stenohaline). Many freshwater animals cannot live for long in sea water and vice versa because of the osmotic problems that they would face.
(iii) Light: It is required for (a) photosynthesis, (b) flowering, and (c) diurnal and seasonal migrations of organisms, for e.g., photoperiodism and hibernation.
(iv) Soil: It is a very important ecological factor as it provides water, minerals and nutrients and support to the producers.


(a) Why are herbivores considered similar to predators in the ecological context? Explain.
(b) Differentiate between the following interspecific interactions in a population:
(i) Mutualism and competition
(ii) Commensalism and Amensalism.
Answer : 
(a) Herbivores are considered similar to predators in ecological context because they transfer the energy fixed by plants to the next tropic level of carnivores. They also maintain the
plant population under control.
(b) (i) In mutualism, both the species are at benefitted so it is beneficial whereas in competition, both the species are at loss. In this life the competition occurs when closely related species compete for the same resources that are limiting.
(ii) In commensalism, one species is benefitted whereas the other is neither benefitted nor harmed. In ammensalism, one species is harm ed and the other remains unaffected.