Class 12 PT II Question Paper 2025-26 Biology

CBSE CLASS 12
PT II Question Paper
Session 2025-26

BIOLOGY

Exam CBSE Class 12
Subject Biology
Session 2025-26
Document Type PT II Question Paper

Notes · Sample Papers · Previous Year Papers · Mock Tests

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Question Paper

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SECOND UNIT TEST
SESSION: 2025-2026
CLASS: XII
SUBJECT: BIOLOGY TIME: 01 HOUR 30 MINUTES
General Instructions:
(i) All questions are compulsory.
(ii) The question paper has five sections and 19 questions.

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(iii) Section–A has 8 questions of 1 mark each; Section–B has 4 questions of 2 marks each; Section– C has 5

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questions of 3 marks each; Section– D has 1 case-based questions of 4 marks; and Section–E has 1

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(iv) There is no overall choice. Answer all 19 questions.

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(v) Wherever necessary, neat and properly labeled diagrams should be drawn.
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SECTION-A
1. Which of the following best illustrates natural selection? 1
a) A single oak tree growing taller in one season.
b) Bacteria developing resistance to an antibiotic after repeated exposure.
c) A person building muscle by exercise.
d) Formation of a fossil in sedimentary rock.

2. The protein encoded by the genes cryIAc and cryIIAb controls which insect pest, respectively? 1
a) Nematodes and bollworms

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b) Cotton bollworms and corn borer
c) Corn borer and cotton bollworms

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d) Mosquitoes and flies

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3. Assertion: Vaccination is an example of active immunity. 1
Reason: A vaccine introduces pre-formed antibodies into the body to rapidly neutralize the
pathogenic agents.
a)� Both assertion and reason are true and reason is the correct explanation of assertion.
b)� Both assertion and reason are true and reason is not the correct explanation of assertion.
c)� Assertion is true but reason is false.
d)� Assertion is False but reason is true.

4. Assertion: When DNA from two different sources is cut by the same restriction enzyme, the 1

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resultant fragments have different kinds of 'sticky ends".

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Reason: These can be joined end-to-end using DNA ligases.

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Select the correct option: -

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a)� Both assertion and reason are true and reason is the correct explanation of assertion.

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b)� Both assertion and reason are true and reason is not the correct explanation of assertion.

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c)� Assertion is true but reason is false.
a d)� Assertion is False but reason is true.

5. If a foreign gene is ligated at the PvuI site in pBR322, which of the following option is correct? 1
a)� Recombinants will grow in ampicillin-containing medium.
b)� Recombinants will grow in tetracycline-containing medium.
c)� Recombinants will not grow in medium lacking antibiotics.
d)� Recombinants will grow in the presence of ampicillin and tetracycline.

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6. Select the correct match with respect to population interaction. 1
Column I Column II
a.� An orchid-mango association i.� Mutualism
b.� Five related species of warblers ii.� Ectoparasites
c.� Mycorrhizae iii.� Commensalism
d.� Fishes and copepods iv.� Competitive co-existence

a)� a-ii; b-iii; c-i; d-iv
b)� a-iv; b-ii; c-i; d-iii
c)� a-iii; b-iv; c-i;d-ii
d)� a-iii; b-iv; c- ii; d-i
7. A graph shows the concentration of antibodies in the blood after two successive injections of the 1
same antigen. Curve A shows the response after the first injection and Curve B after the second.
Which statement is best supported by the high and rapid peak of Curve B?

a) The response is solely due to the activation of Innate Immunity.
b) The immune system failed to recognize the second dose of antigen.
c) It is slower and less intense than Curve A because the antigen was weak.
d) It is a quicker and more intense response due to immunological memory.

8. For a potential permanent cure for ADA deficiency, a different strategy is employed, focusing 1
on cells that are capable of self-renewal. Arrange the following steps for this advanced, long-
term gene therapy strategy:
1.� Introduction of the ADA gene into bone marrow stem cells or cells at the early
embryonic/fetal stage.
2.� Isolation of the functional ADA gene.
3.� Transfer of the genetically corrected cells back into the patient.
4.� Isolation of bone marrow stem cells from the patient.
a) 2, 4, 1, 3 b) 4, 3, 2, 1 c) 1, 2, 3, 4 d) 2, 1, 4, 3
SECTION- B
9. A population of sheep is in Hardy-Weinberg equilibrium. White wool (W) is dominant over 2
black wool (w). The allele frequency for white wool (W) is 0.19.
a)� What is the percentage of heterozygous individuals in the population?
b)� What is the percentage of homozygous recessive individuals in the population?

10. Complete the given table-
Name of the Causative Symptoms
disease organism
Pneumonia Streptococcus a) -----------------------------------
Typhoid b) ----------------- High fever, weakness, headache,
stomach ache
c)-------------- Rhinovirus Nasal congestion, and
discharge,sorethroat,cough,headac
he
Ascariasis Ascaris d)------------------------------------

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o Write the scientific name of the nematode that infests the tobacco plantemand. the part that it

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infects.
How is Agrobacterium used to protect tobacco plants from this attack?

12. A forest can accommodate a maximum of 500 deer (carrying capacity). If the initial population 2
size is 100 deer and the maximum per capita growth rateis 0.05 per year, calculate the rate of
change of the population size of deer for given time period.
SECTION-C
13. Given below is a diagrammatic representation of the experimental setup used by SL Miller for 3
his experiment.

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a) Write the names of different gases present in the flask A.
b) State the type of organic molecule he collected in the water at B.
c) Write the conclusion he arrived at.

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14. The diagram shows the life cycle of a pathogenic protozoan 3

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a) Name the parasitic stage that is being transferred from host 'X' to host "Y".

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b) Write the changes the parasite undergoes in the liver.

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c) Write the changes the parasite undergoes when it enters the RBC.

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15. c. Given s em 3

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a) Identify A, B, C and D in the given diagram.
b) What is insertionalinactivation?

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16. Two children, A and B aged 4 and 5 years respectively visited a hospital with a similar genetic 3
disorder in which immunodeficiency is observed. The girl A was provided enzyme-replacement
therapy and was advised to revisit periodically for further treatment. The girl, B was, however,
given a therapy that did not require revisit for further treatment.
a) Name the ailments the two girls were suffering from?
b) Why did the treatment provided to girl A require repeated visits?
c) How was the girl B cured permanently?

17. Study the 3 representative figures of age pyramid relating to human population given below and 3
answer the following question:

a)� Mention the names given to the 3 kinds of age profiles (i), (ii) and (iii).
b)� What does age pyramid reflects about the population?
c)� How do such age-profile studies help policy-makers get concerned about our growing
population and prepare for future planning? (say for example: for the year 2025)
SECTION-D
18. A 17-year-old student, Rohan, was rushed to the emergency room in an unconscious state. His 4
friends admitted that he had injected a substance commonly known as 'smack' and immediately
felt a rush of euphoria followed by severe respiratory depression. The hospital staff noted his
pinpoint pupils and dangerously slow breathing. A syringe was found near him.
a) Identify the chemical name of smack.
b) What is the plant source of the drug?
c) Explain the mode of action of this drug on the Central Nervous System (CNS) and justify the
symptom of respiratory depression.
OR
What is the chemical structure of morphine? Mention the process by which smack isobtained?
SECTION-E
19. Explain the role of the following in Recombinant DNA technology: 5
a) Restriction Endonuclease , b) DNA Ligase , c)Taq Polymerase, d) Chitinase and e)
Lysozyme.
OR
A schematic representation of Polymerase Chain Reaction (PCR) is given below. Answer the
questions that follow.

a) Name the process ‘a’.
b) Identify ‘b’.
c) Identify ‘c’ and mention its importance in PCR.
d) What is the importance of PCR in biotechnology?
e) What is the significance of primer in PCR?
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