Class 12th Physics Answer Key | ICSE Exam 2025-26 Board Term 1

1 (i). Zara is suffering from cough and has bluish lips. Her mucus is mixed with blood. What is the biological name of the causative organism responsible for Zara’s condition?

Correct Answer: Mycobacterium tuberculosis

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1 (ii). Observe the relation between the first two words and complete the analogy.
Dengue : Aedes :: Plague : ?

Correct Answer: Rat flea (Xenopsylla cheopis)

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1 (iii). If the number of chromosomes in the cells of the calyx is 16, how many chromosomes will be formed in the endosperm after double fertilisation?

Correct Answer: 24

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1 (iv). A scientist isolates a second restriction endonuclease from the strain DC3 of Helicobacter pyogenes. What would the scientist name it using the standard naming technique of restriction enzyme?

Correct Answer: H _pyDC3II

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1 (v). Meenu is suffering from a genetic disorder in which phenylpyruvic acid and its metabolites accumulate in blood. It causes impairment of nervous tissues. Which enzyme deficiency has caused this disorder in Meenu?

Correct Answer: Phenylalanine hydroxylase

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1 (vi). There were 770 frogs in a pond. 70 of them died within a month. Calculate the death rate of the population of frogs in the pond.

Correct Answer: 70/770 = 0.091 (approximately) or 9.09%

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v1 (vii). A farmer notices that the leaves of his cabbage plants are curling up because of heavy infestation of aphids. Suggest one ecofriendly method to control their spread in the cabbage plants.

Correct Answer: Use biological control such as releasing ladybird beetles (predators of aphids).

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1 (viii). In a particular plant with bilobed dithecous anther, there were 50 Pollen Mother Cells. Only 25% could develop their pollen tubes. Calculate the number of pollen grains that would proceed for fertilisation.

Correct Answer: 50 pollen grains

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1 (ix). Choose the components that are required for the proper functioning of DNA polymerase enzyme.
• (P) DNA template
• (Q) A DNA primer
• (R) An RNA primer
• (S) Four different dNTPs

(a) Only (P), (Q) and (R)
(b) Only (P), (R) and (S)
(c) Only (Q), (R) and (S)
(d) Only (P), (Q) and (S)

Correct Answer: (b) Only (P), (R) and (S)

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1 (x). The left hind limb of a person was swollen like an elephant’s limb. Which one of the following was MOST likely the cause of swelling?

(a) Accumulation of uric acid
(b) Blockage of blood vessels
(c) Blockage of lymph vessels
(d) Hypertrophy of skeletal muscles

Correct Answer: (c) Blockage of lymph vessels

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1 (xi). Given below are two statements marked Assertion and Reason. Read the two statements carefully and choose the correct option.
Assertion: RNAi technique is applied to plants and animals to protect them from pest infestation.
Reason: This technique inhibits the expression of certain genes in pests.

(a) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.
(b) Both Assertion and Reason are true but Reason is not the correct explanation for Assertion.
(c) Assertion is true and Reason is false.
(d) Both Assertion and Reason are false.

Correct Answer: (a) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.

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1 (xii). Given below are two statements marked Assertion and Reason. Read the two statements carefully and choose the correct option.
Assertion: Individuals with Klinefelter syndrome lose their secondary sexual characters and develop gynaecomastia.
Reason: The presence of the XYY chromosomal pattern leads to underdeveloped testes and reduced testosterone levels in affected males.

(a) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.
(b) Both Assertion and Reason are true but Reason is not the correct explanation for Assertion.
(c) Assertion is true and Reason is false.
(d) Both Assertion and Reason are false.

Correct Answer: (c) Assertion is true and Reason is false.

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1 (xiii). In a garden pea plant, the flowers are in axial (A) position. Find out the proportion of flowers in terminal position (a), if a cross is made between two heterozygous plants.

Correct Answer: 1/4 or 25%

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1 (xiv). Name the autoimmune disorder that affects the skeletal muscles in humans

Correct Answer: Myasthenia gravis

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1 (xv). Answer the following: (a) Expand the abbreviation IUI.
(b) The enzyme polynucleotide phosphorylase helps the in-vitro synthesis of RNA.
Name the scientist who discovered this enzyme.

Correct Answer: (a) Intrauterine Insemination (b) Severo Ochoa

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1 (xvi). Shama complained of severe itching in certain parts of her body where patches as shown below appeared on her skin. Write the biological name of the causative agent responsible for her condition.

Correct Answer: Trichophytona

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1 (xvii). Name the chemical messenger that activates Natural Killer T-cells and macrophages.

SCorrect Answer: Interferon-γ (IFN-γ)

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1 (xviii). Give a reason for each of the following:
(a) Kanika was suffering from SCID and needed periodic infusion of transformed lymphocytes.
(b) In HIV infected persons, the immune system is compromised.

Solution:
Concept: Certain diseases affect the immune system and reduce the body’s ability to fight infections.
(a) SCID (Severe Combined Immunodeficiency)
Step 1: Understand the disorder.
SCID is a genetic disorder in which the body fails to produce functional T-lymphocytes and B-lymphocytes.
Step 2: Due to this deficiency, the immune system becomes extremely weak and cannot defend the body against infections.
Step 3: Therefore, patients require periodic infusion of genetically transformed lymphocytes to restore immune function.
Reason: Because SCID patients lack functional lymphocytes, transformed lymphocytes are infused to help build immune defense.

(b) HIV infection
Step 1: The Human Immunodeficiency Virus (HIV) specifically infects and destroys CD4+ helper T-lymphocytes.
Step 2: These cells play a crucial role in coordinating immune responses.
Step 3: As the number of helper T-cells decreases, the immune system becomes weak and unable to fight infections. Reason: In HIV infection, destruction of CD4+ T-cells leads to weakening of the immune system.

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Question 2. The diagram below shows DNA banding patterns obtained after DNA samples collected from a crime scene were subjected to gel electrophoresis. Samples from the crime scene are denoted by C and three suspects are represented by S1, S2, S3.
(i) Which suspect’s DNA sample matches the sample collected from the crime scene? (ii) Mention the principle on which DNA profiling is based.

Correct Answer: (i) S2 (ii) DNA profiling is based on DNA polymorphism in VNTR (Variable Number Tandem Repeats).

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Question 3. In a group discussion on pollination, Shubham argued that Cleistogamy was a type of Xenogamy whereas Ameena was of the opinion that it was a kind of Autogamy. Whose argument is correct? Justify.

Correct Answer: Ameena is correct.

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Question 4. (i) The amount of energy at the fourth trophic level in the food chain given below is 4 J.
Grass → Zebra → Crocodile → Vulture
What will be the amount of energy available at the sunlight and transducer level?

Correct Answer: Energy at producer (transducer) level = 4000 J Energy at sunlight level=4,00,000 J

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(ii) Roselin had an aquarium at her home with 10 fish in it. In a year, a fish gave birth to 7 fish. During this period, 3 adult fish had died. Roselin’s father purchased 2 more pairs of fish from the market for the aquarium. Later, Roselin gifted 4 fish from the aquarium to her friend. Find out the growth rate of fish in Roselin’s aquarium.

Correct Answer: Growth rate = 4 fish per year

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Question 5. Anthony is a scientist who has been provided with raw materials to obtain a specific biological product in large quantities. His laboratory has two large vessels for this purpose – one with a sparger and the other vessel with a stirrer. Which vessel will be more advantageous to Anthony? Why?

Correct Answer: The vessel with a sparger (bioreactor) will be more advantageous.

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Question 6. The data given below shows a significant decline in the population of migratory birds in a wetland in Southern India over the last three years.

Year	Population of Birds	Aquatic Plants (%)	Aquatic Area (ha)
2023	10,000	85	350
2024	8,000	60	280
2025	5,000	45	220

Based on the above data, interpret two reasons for change in the population of migratory birds.

Solution:

Concept: Migratory birds depend heavily on wetlands for food, shelter, and breeding. Any decline in habitat quality or availability of food resources can lead to a decrease in their population.
Step 1: Observe the trend in aquatic plants.
The percentage of aquatic plants has decreased from:
85% → 60% → 45%
Aquatic plants provide food and shelter for many aquatic organisms that serve as food for migratory birds. Reduction in aquatic vegetation leads to a decline in food availability.
Reason 1: Decrease in aquatic plant cover has reduced food resources and habitat for migratory birds.
Step 2: The aquatic area of the wetland has also decreased:
350 ha → 280 ha → 220 ha
This indicates shrinkage of the wetland habitat.
Reason 2: Reduction in wetland area has decreased the available habitat for migratory birds.
Thus, the decline in bird population is mainly due to:
• Decrease in aquatic plant cover
• Reduction in wetland (aquatic) area

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Question 7. Paramecium caudatum and Paramecium aurelia were growing in separate culture media in a laboratory. When both the species were transferred to the same culture medium for growth, it was observed that Paramecium caudatum got eliminated from the habitat after some time. Identify and explain the type of ecological interaction between the aforementioned species of Paramecia on the basis of Gause’s principle.

Correct Answer: Interspecific competition (Competitive exclusion principle).

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Question 8. In an experiment to produce recombinant cells, a plasmid without selectable marker was chosen as the vector for cloning a gene.
(i) Predict the outcome of this experiment.
(ii) Name an artificial chromosome that can be used as a vector to transfer the recombinant DNA in Penicillium notatum.

Correct Answer: (i) Recombinant cells cannot be easily identified or selected. (ii) Yeast Artificial Chromosome (YAC).

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Question 9. (i) How is mature insulin different from pro-insulin secreted by pancreas in humans?
(ii) Name the company that produced genetically engineered insulin for the first time.
(iii) Why is functional insulin produced by r-DNA technology better than the ones produced earlier?

Correct Answer: • (i) Mature insulin contains only A and B chains, while pro-insulin contains an additional C-peptide.
• (ii) Eli Lilly
• (iii) Recombinant insulin is highly pure, safe, and does not cause allergic reactions like earlier animal-derived insulin.

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Question 10. A biologist surveyed islands of different sizes and consolidated the results in the form of the data given below. Study the data carefully and answer the questions that follow.

Name of Islands	Island Area (sq. km)	No. of mammalian species
P	1	5
Q	10	18
R	100	60
S	1000	150

(i) What pattern do you observe in the above data?
(ii) Name the scientist who proposed the concept of relationship between area and species.
(iii) Write the mathematical expression for this relationship.

Correct Answer:
• (i) As the island area increases, the number of species also increases.
• (ii) Alexander von Humboldt • (iii) S = CAZ

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Question 11. (i) Draw a flowchart to show the process of sex determination in honeybees.
(ii) If there are 16 chromosomes in the somatic cells of a male honeybee, how many chromosomes would be present in spermatozoa?

Correct Answer: • (i) Sex determination in honeybees follows the haplodiploid mechanism. • (ii) 16 chromosomes

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Question 12. Study the chemical structure of an addictive drug shown below and answer the questions that follow.

(i) Mention the scientific name of the source plant of the above drug.
(ii) State any one effect of this drug on human body.
(iii) What is meant by withdrawal symptom?

Correct Answer:
• (i) Cannabis sativa
• (ii) It affects the central nervous system and may cause hallucinations or impaired coordination.
• (iii) Withdrawal symptoms are unpleasant physical or psychological effects experienced when a person stops taking an addictive drug.

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Question 13. A researcher compares two embryos, X and Y.
• Embryo X shows equal division of the zygote, forming two cells that are almost identical in size.
• Embryo Y shows an unequal first division and produces a small apical cell and a large basal cell.
(i) Which embryo is likely to develop into a normal dicot embryo?
(ii) What is the fate of:
(a) Apical cell?
(b) Basal cell?

Correct Answer:
• (i) Embryo Y
• (ii)
– (a) Apical cell forms the embryo proper.
– (b) Basal cell forms the suspensor.

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Question 14. (i) The graph given below shows the level of antibodies in the blood after exposure to a pathogen.

(a) What types of responses of the immune system do the peaks A and B represent?
(b) Name the type of antibody that is at the highest level in the 7th week.

Correct Answer: • (a) Peak A – Primary immune response; Peak B – Secondary immune response.
• (b) IgG

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(ii) Plasmodium is a digenetic parasite, completing its life cycle in humans and mosquitoes. In humans, it completes different parts of its life cycle in the liver and RBCs. Draw a flow chart to outline its erythrocytic cycle in humans.

Solution: Concept: The erythrocytic cycle of Plasmodium occurs in the red blood cells of humans. During this stage, the parasite multiplies inside RBCs and causes their rupture, leading to the characteristic symptoms of malaria.
Flow chart of erythrocytic cycle:
Merozoites released from liver → Enter RBCs
→ Trophozoite stage (feeding stage)
→ Schizont stage (multiple nuclear divisions)
→ Formation of many merozoites
→ Rupture of RBC
→ Release of merozoites and toxins
→ Invasion of new RBCs
This cycle repeats and leads to the periodic fever observed in malaria.

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Question 15. The two structures shown below are formed in the process of spermatogenesis. Study them carefully and answer the questions that follow.

(i) Identify the process which converts Structure A to Structure B. (ii) State any two modifications observable in Structure B.

Correct Answer: • (i) Spermiogenesis • (ii) – Formation of a flagellum (tail) for motility. – Development of an acrosome on the head of the sperm.

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Question 16. As a part of his research, Arun conducted a study on the number of locusts in a forest. He compiled his findings in the form of the following data.

Day	Number of locusts
1	5
2	10
5	25
10	50
15	105
20	170
25	280
30	360
35	200

(i) Construct a growth curve based on the above data. What type of a growth curve is obtained?
(ii) Write a mathematical expression for the growth curve obtained

Correct Answer: • (i) Sigmoid (S-shaped) growth curve / Logistic growth curve. • (ii) dN/dt = rN ( K − N/K )

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Question 17. Study the diagram given below and answer the questions that follow.

(i) Identify the parts marked ‘E’ and ‘F’.
(ii) Explain the post-transcriptional modifications that occur in part ‘F’.

Correct Answer: • (i) E – Promoter region ; F – Primary RNA transcript (hnRNA / pre-mRNA)
• (ii) Post-transcriptional modifications include capping, tailing and splicing

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Question 18. (i) Outline three major steps in the process of artificial hybridisation.
(ii) Discuss the role of pollen–pistil interaction in ensuring successful hybrid formation.
OR
(iii) Draw a flowchart to represent the process of oogenesis.
(iv) Differentiate between oogenesis and spermatogenesis on the following basis:
(a) Number of products
(b) Gonadal hormones

Solution:
(i) Major steps in artificial hybridisation
Concept: Artificial hybridisation is a plant breeding technique in which pollen from a selected male parent is manually transferred to the stigma of a chosen female parent to obtain desired hybrids.
Steps:

• Emasculation: Removal of anthers from a bisexual flower before they release pollen to prevent self-pollination.
• Bagging: The emasculated flower is covered with a suitable bag to prevent contamination from unwanted pollen.
• Artificial Pollination: Pollen grains from the desired male parent are dusted onto the stigma of the female flower and the flower is again bagged.
(ii) Role of pollen–pistil interaction Concept: Pollen–pistil interaction refers to the series of events that occur from the time pollen lands on the stigma until fertilisation occurs.
• The stigma recognises whether the pollen is compatible or incompatible.
• Compatible pollen grains germinate on the stigma and produce pollen tubes.
• The pollen tube grows through the style and reaches the ovule.
• This interaction ensures that only the correct pollen fertilises the ovule, leading to successful hybrid formation.
OR (iii) Flowchart of oogenesis
(iv) Difference between oogenesis and spermatogenesis

Basis	Oogenesis	Spermatogenesis
Number of products	One ovum and three polar bodies	Four functional sperms
Gonadal hormones	Estrogen and progesterone	Testosterone

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Last Updated on : March 26, 2026