Punjab School Education Board
Class XII – September Examination 2026-27
Subject: Biology (Theory)
- There shall be one theory paper comprising 20 questions. All questions are compulsory.
- Section A: Question No. 1 consists of 18 sub-parts carrying 1 mark each (16 MCQs and 2 Assertion-Reason questions).
- Section B: Question Nos. 2 to 11 are Very Short Answer Type carrying 2 marks each. Internal choice is provided in 4 questions.
- Section C: Question Nos. 12 to 17 are Short Answer Type carrying 3 marks each. Internal choice is provided in 2 questions.
- Section D: Question Nos. 18 to 20 are Long Answer Type carrying 5 marks each. All questions have internal choice.
- Use of calculators or log tables is not permitted. Draw neat and properly labelled diagrams wherever necessary.
Q.1 Select and write the most appropriate option out of the four given choices for each sub-part (i to xvi). Questions (xvii) and (xviii) are Assertion-Reasoning based. (18 × 1 = 18 Marks)
(i) Which one of the following is a post-fertilization event in angiosperms?
(ii) The major evolutionary advantage of cleistogamous flowers to a plant is:
(iii) In the human female reproductive tract, fertilization normally occurs in the:
(iv) Which outer cellular layer of the blastocyst directly participates in the formation of the placenta?
(v) 'Saheli', a widely used oral contraceptive pill developed in India, contains:
(vi) A child is born with blood group O. If the mother has blood group A and the father has blood group AB, which conclusion is genetically valid?
(vii) The physical association of two or more genes located on the same chromosome is termed:
(viii) A genetic condition in which a single gene controls and influences multiple phenotypic traits is known as:
(ix) If one strand of a double-stranded DNA molecule contains the sequence 3'-ATTCGTAC-5', the complementary sequence synthesized on the other strand will be:
(x) Hershey and Chase used radioactive isotopes of which pair of elements to verify that DNA is the genetic material?
(xi) The first hominid with a cranial capacity between 650–800 cc and tool-making ability was:
(xii) Genetic drift produces significant changes in allele frequencies primarily in:
(xiii) Which diagnostic test is widely used in laboratories to confirm typhoid fever?
(xiv) Morphine and related opiate narcotics are extracted from the latex of which plant?
(xv) Biochemical Oxygen Demand (BOD) serves as an index of water quality because it measures:
(xvi) Which of the following is a free-living, soil-dwelling bacterium that fixes atmospheric nitrogen?
(A) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(B) Both (A) and (R) are true, but (R) is NOT the correct explanation of (A).
(C) (A) is true, but (R) is false.
(D) (A) is false, but (R) is true.
(xvii) Assertion (A): The two complementary chains of a double-stranded DNA helix run antiparallel to each other.
Reason (R): One strand runs in the 5' → 3' direction with a free 5'-phosphate group, while the opposing strand runs in the 3' → 5' direction with an accessible 3'-hydroxyl group.
(xviii) Assertion (A): The primitive atmosphere of early Earth was completely reducing in nature without molecular oxygen.
Reason (R): Intense ultraviolet radiation instantly converted all free oxygen into an ozone shield during that era.
Answer the following Very Short Answer Type questions (Q. Nos. 2 to 11 carry 2 Marks each). (10 × 2 = 20 Marks)
Q.2 Compare geitonogamy and xenogamy based on their pollination mechanism and the genetic nature of the progeny produced. [2]
Q.3 Why is the human placenta described as a temporary endocrine tissue? Name any two hormones secreted exclusively or predominantly by it during pregnancy. [2]
Q.4 [Diagram-Based Question] The diagram given below represents sperm entry into an ovum during human fertilization.
[Figure: Sperm interaction with ovum layers]
(a) Identify the outer protective layer designated as (A) through which the sperm penetrates.
(b) State the role of acrosomal enzymes present at the tip of structure (B). [2]
A female has a regular menstrual cycle of 32 days. Calculate and predict her most likely day of ovulation, and justify your answer based on the fixed lifespan of the corpus luteum (luteal phase). [2]
Q.5 Explain the physiological mechanism by which Copper-T (Cu-T) prevents pregnancy when placed inside the human uterus. [2]
Q.6 A student wants to know whether a tall pea plant with violet flowers in the garden is homozygous or heterozygous dominant. Name the test cross method and represent it with a Punnett square. [2]
An individual is diagnosed with Klinefelter's syndrome having a chromosomal constitution of 44 + XXY. Mention any two distinct clinical symptoms observed in such an individual. [2]
Q.7 A sample of double-stranded DNA contains 2,000 nitrogenous bases in total, out of which 520 are adenine residues. Calculate the number of purines, pyrimidines, and guanine residues in this DNA sample. [2]
Q.8 Why are the wings of a butterfly and the wings of a pigeon called analogous organs? Name the evolutionary pattern responsible for this phenomenon. [2]
Q.9 In a human population that is in Hardy-Weinberg equilibrium, the frequency of a dominant allele (A) is 0.6. Calculate the frequency of the recessive allele (a) and the proportion of heterozygous individuals (Aa). [2]
What was the evolutionary significance of catching a live Coelacanth (lobe-finned fish) off the coast of South Africa in 1938? [2]
Q.10 Differentiate between benign tumours and malignant tumours based on their capsule formation and property of metastasis. [2]
Q.11 What are 'flocs' in a secondary sewage treatment plant? Explain their role in bringing down the BOD of effluent. [2]
Name the bacterium responsible for the large holes found in 'Swiss cheese' and explain how these holes are formed. [2]
Answer the following Short Answer Type questions (Q. Nos. 12 to 17 carry 3 Marks each). (6 × 3 = 18 Marks)
Q.12 Explain the two fusion events that constitute 'double fertilization' in angiosperms. Why is this event unique to flowering plants? [3]
Q.13 [Illustration-Based Question] The diagrammatic flow chart below shows the sequential maturation of ovarian follicles during the human ovarian cycle:
Corpus Luteum
(a) What is the fluid-filled cavity characteristic of the tertiary follicle called?
(b) Which anterior pituitary hormone surge triggers the rupture of the Graafian follicle?
(c) What is the endocrine function of the corpus luteum in maintaining the endometrium? [3]
Q.14 In a dihybrid cross involving Drosophila melanogaster, T.H. Morgan crossed yellow-bodied, white-eyed females with brown-bodied, red-eyed males. The F2 progeny showed 98.7% parental types and only 1.3% recombinant types.
(a) Explain why the phenotypic ratio deviated significantly from the expected Mendelian 9:3:3:1 ratio.
(b) Define the genetic mechanism responsible for this deviation. [3]
Q.15 In an experiment on E. coli, the mechanism of the lac operon was investigated:
(a) Identify the physiological inducer molecule in the lac operon.
(b) Explain what happens when the inducer binds to the repressor protein.
(c) List the three structural genes (z, y, a) and the specific enzymes they code for. [3]
Describe Frederick Griffith's transformation experiment on Streptococcus pneumoniae in mice. What conclusion was derived regarding the nature of the transforming principle? [3]
Q.16 Explain how natural selection operated on the peppered moth (Biston betularia) population in England before and after industrialization. What is this phenomenon termed? [3]
Q.17 Differentiate between active immunity and passive immunity based on the origin of antibodies and longevity of protection. Give one practical medical example for each. [3]
A patient exhibits high sustained fever (39°C–40°C), severe stomach pain, constipation, headache, and loss of appetite.
(a) Identify the bacterial pathogen causing this disease.
(b) Name the diagnostic test used to confirm this infection.
(c) How is this disease transmitted in human communities? [3]
Answer the following Long Answer Type questions (Q. Nos. 18 to 20 carry 5 Marks each). (3 × 5 = 15 Marks)
Q.18 [Draw, Label and Explain Type Question]
(a) Draw a neat and labelled diagram of the human female reproductive system.
(b) Explain the functions of the following structures: (i) Fimbriae, (ii) Endometrium, (iii) Myometrium.
(c) Describe the structure of a mature Graafian follicle. [2+2+1 = 5]
(a) Draw a schematic sectional view of a seminiferous tubule in a human testis and label any four parts.
(b) Explain the hormonal regulation of spermatogenesis in human males starting from the hypothalamus.
(c) State one difference between spermiogenesis and spermiation. [2+2+1 = 5]
Q.19
(a) Describe the Hershey-Chase blender experiment that proved that DNA is the genetic material.
(b) Why were 32P and 35S chosen for labelling nucleic acids and proteins respectively?
(c) What results were observed in the supernatant and bacterial pellet in both batches? [2+2+1 = 5]
(a) Explain the semi-conservative replication of DNA with the help of a labelled replication fork diagram.
(b) State the specific functions of: (i) DNA Helicase, (ii) DNA Polymerase III, (iii) DNA Ligase.
(c) Why is DNA replication continuous on one template strand and discontinuous on the other? [2+2+1 = 5]
Q.20 [Case Study / Application-Based Question]
A 35-year-old patient reports to a clinic with continuous fever, chronic diarrhoea, rapid weight loss, and recurrent opportunistic infections. An enzyme-linked immunosorbent assay (ELISA) confirms that the patient is infected with the Human Immunodeficiency Virus (HIV).
(a) Explain why HIV is classified as a "retrovirus" and name the key viral enzyme involved in synthesizing DNA from viral RNA.
(b) Which specific immune cells are progressively destroyed by HIV? Why does their destruction paralyze both cell-mediated and humoral immunity?
(c) Differentiate between being 'HIV-positive' and having 'full-blown AIDS'.
(d) Mention any two effective preventive strategies recommended to control HIV transmission. [1+2+1+1 = 5]
A 55-year-old chronic tobacco user is diagnosed with malignant lung cancer. Histological examination shows an abnormal mass of cells invading neighbouring tissues and blood vessels.
(a) Define contact inhibition and explain how normal cells differ from cancerous cells in this property.
(b) Explain the phenomenon of "metastasis" and why it is considered the most feared property of malignant tumours.
(c) Name three categories of carcinogens (physical, chemical, biological) with one example for each.
(d) Mention any two conventional therapies used in cancer management. [1+1.5+1.5+1 = 5]
