BSEH Class 12th Solved Biology 2025

Haryana Board Solved Biology Paper March 2025 | Set: A

All answers are based on standard NCERT Biology concepts. Diagrams (if required) are described in text.

SECTION – A (Objective Type Questions)

1. What is the ploidy of endosperm in flowering plants?

• (A) 2n
• (B) 3n
• (C) 4n
• (D) 6n

(B) 3n
Explanation: Endosperm is formed by fusion of one male gamete (n) with two polar nuclei (2n), resulting in triploid (3n) endosperm.

2. What happens to the BOD of a water body with removal of effluents from it?

• (A) Increases
• (B) Decreases
• (C) Remains the same
• (D) Increases and then stagnates

(B) Decreases
Explanation: BOD (Biochemical Oxygen Demand) measures organic pollution. Removing effluents reduces organic load → lowers BOD.

3. The distance between the two strands of a DNA molecule ………… from one end to another.

• (A) increases
• (B) remains the same
• (C) decreases
• (D) none of these

(B) remains the same
Explanation: DNA is a uniform double helix with constant diameter (~2 nm). The distance between strands is fixed by hydrogen bonds.

4. The black walnut plant secretes juglone... What type of population interaction does it denote?

• (A) Predation
• (B) Competition
• (C) Mutualism
• (D) Amensalism

(D) Amensalism
Explanation: One organism (walnut) harms another (pepper) without benefiting or being harmed → Amensalism (-/0).

5. Ginkgo biloba is known as a living fossil... It is the example of which of the following?

• (A) Speciation
• (B) Fossilization
• (C) Adaptive radiation
• (D) Survival of the fittest

(D) Survival of the fittest
Explanation: It has survived unchanged for millions of years due to resistance and tolerance → Darwin’s natural selection.

6. The cause of sickle cell anemia is:

• (A) Point mutation
• (B) Natural selection
• (C) Adaptive radiation
• (D) Both (B) and (C)

(A) Point mutation
Explanation: Substitution of valine for glutamic acid at 6th position of β-globin chain due to single base mutation in HBB gene.

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

• (A) beer
• (B) wine
• (C) fruit juice
• (D) wheat bread

(C) fruit juice
Explanation: Saccharomyces causes fermentation → spoils fruit juice by producing alcohol and CO₂. It is desired in beer, wine, and bread.

8. An individual infected with a fungal disease shows dry scaly lesions... Which disease?

• (A) Ringworm
• (B) Filariasis
• (C) Amoebiasis
• (D) Ascariasis

(A) Ringworm
Explanation: Caused by dermatophytes (e.g., Trichophyton). Shows circular, scaly, itchy lesions.

9. Which will lead to a decline in carbon sink?

• (A) Deforestation
• (B) Population growth
• (C) Burning of fossil fuels
• (D) Increase in industrial activity

(A) Deforestation
Explanation: Forests absorb CO₂. Cutting trees reduces carbon sequestration → decline in sink.

10. What kind of charge is present on DNA molecule?

Negative charge
Explanation: Due to phosphate groups (PO₄³⁻) in the sugar-phosphate backbone.

11. In PCR at Step-2, denaturation of DNA takes place. Name the factor responsible.

High temperature (94–95°C)
Explanation: Heat breaks hydrogen bonds between base pairs → separates double-stranded DNA into single strands.

12. At what organism level in this food chain the concentration of BFR's will be highest?
Phytoplanktons → Zooplanktons → Herring → Mackerel → Humans

Humans
Explanation: Biomagnification: Persistent pollutants (BFRs) accumulate at higher trophic levels. Top consumer (humans) has maximum concentration.

13. …………. are called molecular scissors.

Restriction enzymes (or Restriction endonucleases)
Explanation: They cut DNA at specific recognition sites.

14. Energy is always highest at the first trophic level and reduces as we move up the ………… .

food chain (or trophic levels)
Explanation: Only ~10% energy transfers per level (10% law).

15. The non-coding sequences are called ………… .

Introns
Explanation: Introns are removed during RNA splicing; exons are coding sequences.

16. Assertion (A): DNA and RNA are two types of nucleic acids...
Reason (R): RNA acts as genetic material in most organisms.

(C) Assertion (A) is true, but Reason (R) is false.
Explanation: DNA is genetic material in most organisms (except some viruses like TMV, HIV).

17. Assertion (A): Health does not simply mean 'absence of disease'...
Reason (R): Health is defined as complete physical, mental and social well-being.

(A) Both true and R explains A.
Explanation: WHO definition of health.

18. Assertion (A): Tropics have greater biological diversity.
Reason (R): Most solar energy available in tropics...

(A) Both true and R explains A.
Explanation: High sunlight → high photosynthesis → high productivity → supports more species.

SECTION – B (Very Short Answer Type – 2 Marks)

19. What is Emasculation?

Removal of anthers from bisexual flowers of female parent before anthesis to prevent self-pollination in artificial hybridization.

20. Differentiate between Transcription and Translation.

Transcription	Translation
DNA → RNA	mRNA → Protein
Occurs in nucleus	Occurs in cytoplasm (ribosomes)
RNA polymerase	Ribosomes, tRNA, amino acids
No codon-anticodon	Codon-anticodon pairing

21. Define Semi-conservative DNA replication scheme.

Each new DNA molecule consists of one parental (old) strand and one newly synthesized strand. Proven by Meselson-Stahl experiment using ¹⁵N.

22. What is allergy? Give one example.

Hypersensitivity of immune system to harmless environmental substances (allergens).
Example: Pollen allergy (hay fever) → sneezing, watery eyes.

OR (Competency) Nikita snorted a flower and suffered sneezing, running nose...

Cause: Pollen allergy (Type I hypersensitivity). Pollen acts as allergen → triggers IgE → mast cell degranulation → histamine release → symptoms.
Cure: Antihistamines (e.g., cetirizine), avoid allergen, nasal corticosteroids.

23. What are auto-immune diseases? Give one example.

Immune system attacks self-tissues due to loss of self-tolerance.
Example: Rheumatoid arthritis (attacks synovial joints).

24. What is Totipotency?

Ability of a single cell to divide and produce all differentiated cells of an organism → forms complete plant (e.g., carrot callus → whole plant).

OR What is Explants?

Any living plant tissue (leaf, stem, root) removed and used for tissue culture.

25. Differentiate between Immigration and Emigration.

Immigration: Entry of individuals into a population from outside.
Emigration: Exit of individuals from a population to outside.

SECTION – C (Short Answer Type – 3 Marks)

26. How ploidy changes in spermatogenesis, spermiogenesis, and spermiation?

• Spermatogenesis: 2n (spermatogonium) → 2n (primary spermatocyte) → n (secondary spermatocyte) → n (spermatid)
• Spermiogenesis: n (round spermatid) → n (mature spermatozoon) [No change in ploidy, only morphological change]
• Spermiation: n spermatozoa released from Sertoli cells into lumen [Ploidy remains n]

Diagram: Flow: 2n → meiosis I → n → meiosis II → n → differentiation → n → release.

27. What is adaptive radiation? Explain with one example.

Evolution of different species from a common ancestor in diverse environments → occupy different ecological niches.
Example: Darwin’s finches (Galápagos): One ancestral species → 14 species with varied beaks (seed-eating, insect-eating, cactus-feeding).

28. What are the barriers of innate immunity?

1. Physical: Skin, mucous membranes
2. Physiological: Acidic pH in stomach, lysozyme in tears/saliva
3. Cellular: Phagocytes (neutrophils, macrophages), NK cells
4. Cytokine: Interferons

OR Differentiate between innate and acquired immunity.

Innate	Acquired
Present at birth	Develops after exposure
Non-specific	Antigen-specific
No memory	Immunological memory
Fast response	Slower initial response

29. Explain briefly the phenomenon of PCR.

Polymerase Chain Reaction (PCR): In vitro DNA amplification.
Steps:

1. Denaturation (94°C): dsDNA → ssDNA
2. Annealing (50–60°C): Primers bind to ssDNA
3. Extension (72°C): Taq polymerase adds dNTPs → new strand

Result: Exponential amplification (2ⁿ copies after n cycles).

30. What are the causes of bio-diversity losses?

The Evil Quartet:

1. Habitat loss & fragmentation (deforestation, urbanization)
2. Over-exploitation (hunting, poaching)
3. Alien species invasions (e.g., Lantana, Eichhornia)
4. Co-extinctions (host-parasite loss)

OR What do you understand by Ecological Pyramids? Mention types with examples.

Graphical representation of trophic structure and function.
Types:

1. Pyramid of Number: No. of individuals (e.g., upright in grassland)
2. Pyramid of Biomass: Total dry weight (e.g., inverted in aquatic)
3. Pyramid of Energy: Energy flow (always upright)

Section D - Case-Based Questions (31-32)

31. Case Study - 1 (Reproductive Health & Family Planning)

Passage: India was amongst the first countries... 'Reproductive and Child Health Care (RCH) programmes'. Creating awareness... building up a reproductively healthy society are the major tasks...

(a) What are family planning programmes?
1 mark

Family planning programmes are government-initiated action plans and schemes aimed at achieving total reproductive health as a social goal. They include contraception, maternal and child health care, population control, and awareness about reproductive issues.

(b) In which year the 'family planning' programmes were initiated?
1 mark

The 'family planning' programmes were initiated in 1951.

(c) What are the major tasks of the family planning programmes?
2 marks

The major tasks are:

1. Creating awareness among people about various reproduction-related aspects (e.g., contraception, STDs, infertility).
2. Providing facilities and support (medical infrastructure, contraceptives, counseling, safe delivery, postnatal care) to build a reproductively healthy society.

Now operates under RCH programmes with periodic evaluation.

Total: 4 marks | Key: Focus on awareness + facilities

32. Case Study - 2 (Origin of Universe & Earth)

Passage: The universe is old - almost 20 billion years... earth was supposed to have been formed about 4.5 billion years back.

(a) How old is the universe?
1 mark

The universe is approximately 20 billion years old (or 20,000 million years).

(b) When was earth formed?
1 mark

Earth was formed about 4.5 billion years ago (or 450 crore years) in the solar system of the Milky Way galaxy.

(c) What does the universe comprise of?
2 marks

The universe comprises:

• Huge clusters of galaxies
• Galaxies containing:
  • Stars
  • Clouds of gas and dust

According to Big Bang theory, it began from a singular explosion, expanded, cooled, formed hydrogen and helium, then galaxies under gravity.

Total: 4 marks | Key: Include Big Bang context for 2-mark answer

Marking Scheme (Section D)

Question	Part (a)	Part (b)	Part (c)	Total
31	1	1	2	4
32	1	1	2	4

Total Section D: 8 marks

Section E - Long Answer Type Questions (33–35)

33. What is DNA replication? Explain the process of DNA replication in prokaryotes with a labeled diagram.

5 marks

Definition (1 mark): DNA replication is the biological process of producing two identical copies of the DNA molecule from one original DNA molecule. It is semi-conservative, i.e., each new DNA contains one parental and one newly synthesized strand.

Process in Prokaryotes (3 marks):

1. Initiation: Replication begins at a specific site called origin of replication (ori). The enzyme helicase unwinds the double helix, forming a replication fork. Single-strand binding proteins (SSBs) stabilize the unwound strands.
2. Elongation:
   • Primase synthesizes a short RNA primer.
   • DNA polymerase III adds nucleotides in 5' → 3' direction.
   • On the leading strand, synthesis is continuous.
   • On the lagging strand, synthesis occurs in short Okazaki fragments.
   • DNA polymerase I removes RNA primer and fills gaps with DNA.
   • DNA ligase joins Okazaki fragments by forming phosphodiester bonds.
3. Termination: Replication completes when the entire circular DNA is duplicated, forming two daughter DNA molecules.

[Diagram: Replication fork showing leading and lagging strands, helicase, primase, DNA polymerase, ligase, Okazaki fragments – properly labeled]

(Draw a Y-shaped replication fork with arrows indicating direction of synthesis.)

Key Enzymes (1 mark): Helicase, Topoisomerase, Primase, DNA Polymerase III, DNA Polymerase I, Ligase.

Marking: Definition: 1 | Steps: 3 (1 each for initiation, elongation, termination) | Diagram: 1

34. (a) What is an ecological pyramid? Describe the pyramid of energy with a suitable example.

    (b) Why is the pyramid of energy always upright?

5 marks

(a) Ecological Pyramid (2 marks):

An ecological pyramid is a graphical representation showing the relationship between different trophic levels in an ecosystem in terms of number, biomass, or energy.

Pyramid of Energy (2 marks):

• It represents the amount of energy at each trophic level per unit area/time (kcal/m²/year).
• Always upright because energy decreases at successive trophic levels due to 10% law (only ~10% energy transferred).
• Example: In a grassland ecosystem:
  Producers (Grass): 10,000 kcal/m²/year →
  Herbivores (Rabbit): 1,000 kcal →
  Carnivores (Fox): 100 kcal →
  Top Carnivore (Hawk): 10 kcal

[Diagram: Upright pyramid with decreasing bars labeled Producers → Herbivores → Carnivores → Top Carnivore]

(b) Reason (1 mark): The pyramid of energy is always upright because energy flow is unidirectional and non-recyclable. At each trophic level, a large amount of energy is lost as heat (respiration, metabolism), so less energy is available for the next level.

Marking: Definition: 1 | Energy Pyramid + Example: 2 | Diagram: 1 | Reason: 1

35. Explain the process of double fertilization in angiosperms. Draw a labeled diagram of a mature embryo sac and show the path of pollen tube entry and fertilization.

5 marks

Double Fertilization (3 marks):

It is a unique process in angiosperms where two male gametes participate in fertilization:

1. Syngamy: One male gamete fuses with the egg cell → forms diploid zygote (2n) → develops into embryo.
2. Triple Fusion: Second male gamete fuses with two polar nuclei → forms triploid primary endosperm nucleus (3n) → develops into endosperm (nutritive tissue).

Steps:

• Pollen grain lands on stigma → germinates → forms pollen tube.
• Pollen tube grows through style → enters ovule via micropyle.
• Two male gametes are released into the synergid.
• One fuses with egg, other with central cell.

[Diagram: L.S. of ovule showing mature embryo sac (8-nucleate, 7-celled): Egg apparatus (egg + 2 synergids), 3 antipodals, central cell with 2 polar nuclei. Pollen tube entering through micropyle, releasing two male gametes. Arrows showing syngamy and triple fusion.]

Significance (1 mark): Ensures seed development with both embryo and nutritive endosperm.

Diagram Labels (1 mark): Micropyle, Integuments, Nucellus, Egg cell, Synergids, Polar nuclei, Antipodals, Pollen tube, Male gametes.

Marking: Explanation: 3 | Diagram: 1.5 | Labels: 0.5 | Significance: 1

Section E - Marking Scheme

Question	Content	Diagram	Total
33	4	1	5
34	4	1	5
35	3	2	5

Total Section E: 15 marks

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Prepared as per NCERT & HBSE Syllabus