CBSE Class 12 2025 Biology 57-1-3​ Question Paper Set-3 :Download Solutions with Answer Key

Step 1: In an ecological pyramid of biomass, typically the biomass decreases at each higher trophic level.

Step 2: In this case, the biomass of primary producers is 10 g, primary consumers 60 g, and secondary consumers 120 g.

Step 3: Since biomass increases at higher levels, the pyramid will be inverted.

Thus, the data represents an inverted pyramid of biomass.
Quick Tip: If biomass increases from producers to consumers, it forms an inverted pyramid of biomass — typical in aquatic ecosystems.

Step 1: Flowers with a single ovule per ovary and those arranged in inflorescences are often adapted for wind pollination.

Step 2: Wind-pollinated flowers typically have numerous small, inconspicuous flowers packed together, and produce abundant pollen grains.

Step 3: These adaptations favor successful pollination by wind rather than by insects or animals.

Hence, such flowers are generally wind-pollinated.
Quick Tip: In wind pollination, flowers are small, light, numerous, and often packed in inflorescences with a single ovule for efficient pollen transfer.

Step 1: The labelled parts of the dicot embryo during its development are as follows:

P — Zygote

Q — Suspensor

R — Cotyledon

S — Radicle

Step 2: Among the given options, option (D) correctly matches these labels.
Quick Tip: In dicot embryo development: Zygote forms the suspensor, followed by formation of cotyledons and radicle in sequence.

Step 1: Human evolution followed a distinct sequence among hominids.

Step 2: Australopithecines are the earliest hominids, followed by \textit{Homo erectus, then Neanderthals, and finally modern \textit{Homo sapiens.

Step 3: Option (B) correctly follows this evolutionary order.
Quick Tip: Remember: \textit{Australopithecines are considered the ancestors of Homo erectus, who then gave rise to Neanderthals and Homo sapiens.

Step 1: Colour blindness is an X-linked recessive disorder.

Step 2: The man (whose father was colour-blind) is normal but a carrier of the normal X chromosome.

Step 3: The woman’s mother was colour-blind, so the woman must be a carrier (heterozygous).

Step 4: Crossing these, 50% of male children will inherit the defective X chromosome from the mother, resulting in colour-blindness.
Quick Tip: For X-linked recessive disorders, males need only one defective X chromosome to express the trait.

Step 1: In dicots like Pea and Groundnut, the endosperm is used up during seed development.

Step 2: These seeds are non-endospermic or ex-albuminous as their cotyledons store food.

Step 3: Thus, no endosperm remains at maturity in these seeds.
Quick Tip: In dicots like Pea and Groundnut, the cotyledons store nutrients and the endosperm is completely absorbed during development.

Step 1: GEAC is an Indian regulatory body under the Ministry of Environment, Forest and Climate Change.

Step 2: Its full form is Genetic Engineering Approval Committee.

Step 3: It monitors the approval of GMOs and related research.
Quick Tip: GEAC monitors and regulates the release of genetically engineered organisms in India.

Step 1: Tissue/organ rejection is primarily due to recognition of foreign antigens on transplanted cells.

Step 2: This activates T-lymphocytes of the recipient, initiating a cell-mediated immune response.

Step 3: Antibodies play a lesser role in acute rejection.
Quick Tip: Organ transplant rejection is mainly driven by T-cell mediated immunity, not by antibodies.

Step 1: In PCR, DNA polymerase amplifies the target gene exponentially.

Step 2: With 30-35 cycles, amplification can reach about 1 billion copies.
Quick Tip: PCR can amplify DNA up to a billion times within hours.

Step 1: RNAi involves the silencing of specific mRNA to prevent the synthesis of its corresponding protein.

Step 2: Double-stranded RNA (dsRNA) triggers degradation of nematode mRNA in host plants.

Step 3: Thus, nematode proteins essential for survival are not produced.
Quick Tip: RNA interference (RNAi) works by degrading target mRNA and blocking translation.

Step 1: In transcription, mRNA is complementary to the template strand and identical (with U in place of T) to the coding strand.

Step 2: Here, the coding strand sequence is copied with U replacing T.

Step 3: Thus, the mRNA sequence is 5' - AAUGCUAGGCAC - 3'.
Quick Tip: Remember: mRNA sequence is same as coding strand (except U instead of T).

Step 1: Backcrossing YyRr with yyrr (double recessive) produces four types of gametes from YyRr: YR, Yr, yR, yr.

Step 2: Crossing these with yyrr gives four possible offspring types, each in equal proportion.

Step 3: Thus, the ratio is 1:1:1:1 for both phenotype and genotype.
Quick Tip: Backcross with a double recessive always gives a 1:1:1:1 phenotypic and genotypic ratio for dihybrid heterozygotes.

Step 1: Mammary glands are specialized to produce milk for feeding offspring.

Step 2: Embryologically, mammary glands are modified sweat glands.

Step 3: So, both Assertion and Reason are true and Reason correctly explains Assertion.
Quick Tip: Mammary glands are modified sweat glands and function to nourish offspring with milk.

Step 1: Scaffold proteins are indeed non-histone chromosomal proteins.

Step 2: However, histones—not scaffold proteins—are rich in lysine and arginine.

Step 3: So, Assertion is true but Reason is false.
Quick Tip: Histones are basic proteins rich in lysine and arginine, not scaffold proteins.

Step 1: When genes are linked (on same chromosome), parental combinations predominate because crossing over between linked genes is rare.

Step 2: Reason incorrectly attributes higher parental combinations to crossing over, which actually increases recombinants.

Step 3: Thus, Assertion is true but Reason is false.
Quick Tip: Linked genes reduce recombination; crossing over increases recombinants, not parental types.

Step 1: Bioreactors ensure optimal conditions (temperature, pH, nutrients, oxygen) for microbial or cell culture growth.

Step 2: Stirred-tank bioreactors are the most commonly used type for uniform mixing and aeration.

Step 3: So both Assertion and Reason are true and Reason explains Assertion.
Quick Tip: Stirred-tank bioreactors are widely used for industrial fermentation and enzyme production.

Step 1: Affected individuals are both male and female, suggesting the trait is autosomal.

Step 2: The trait appears in the offspring of unaffected parents, indicating it is recessive.

Step 3: In recessive autosomal inheritance:

Affected individual: \texttt{aa
Unaffected carrier: \texttt{Aa
Unaffected normal: \texttt{AA

Step 4: Since 'c' is unaffected but passes the trait to offspring, 'c' must be \texttt{Aa.

Step 5: 'd' is affected, hence genotype is \texttt{aa. Quick Tip: In autosomal recessive inheritance, the trait skips generations and affects both sexes equally.

Step 1: In Cross I, the mother produces an abnormal gamete (22 + O) instead of a normal haploid (22 + X). When this fuses with a normal sperm (22 + X), it results in an offspring labeled ‘1’.

Step 2: In Cross II, nondisjunction during gamete formation leads to gametes with 23 + X and 21 + X chromosomes instead of the normal 22 + X or 22 + Y. Fusion of these abnormal gametes results in offspring labeled ‘2’.

Step 3:

Abnormality ‘1’ is caused by gamete formation error in the mother.
Abnormality ‘2’ is caused by nondisjunction in the father’s meiosis.

Step 4: These abnormalities typically result in chromosomal disorders due to aneuploidy (addition or loss of chromosomes). Quick Tip: Nondisjunction during meiosis leads to abnormal chromosome numbers in gametes, causing genetic disorders like monosomy or trisomy in offspring.

Step 1: Morphine and heroin are both opiate drugs derived from the latex of the poppy plant \textit{Papaver somniferum.

Step 2: Heroin is a chemically modified form of morphine (diacetylmorphine).

Step 3: Both act on the central nervous system and are used for pain relief, but are highly addictive.

Step 4: Their excessive use leads to drowsiness, slowed heart rate, impaired respiration, and may cause coma or death. Quick Tip: Morphine is a natural opiate, while heroin is its synthetic derivative—both depress the central nervous system and are highly addictive.

Step 1:
(1) Alcoholic drink produced with distillation: \textit{Whisky, rum, or brandy.

(2) Alcoholic drink produced without distillation: \textit{Wine and beer.

Step 2: Cyanobacteria like \textit{Anabaena, \textit{Nostoc, and \textit{Oscillatoria fix atmospheric nitrogen and enrich the soil.

Step 3: They are commonly used in paddy fields as bio-fertilizers, improving soil fertility naturally and reducing the need for chemical fertilizers. Quick Tip: Cyanobacteria naturally fix nitrogen, enhancing soil fertility—while wine and beer ferment without distillation, whisky and rum require it.

Step 1: Primary productivity refers to the rate at which producers (plants, algae) convert solar energy into organic material.

Step 2: Among the given, a shallow polluted lake has the highest primary productivity due to abundant nutrient availability causing algal blooms.

Step 3: A young forest shows moderate productivity due to rapid growth, while an old natural forest has stable but lower net primary productivity. Quick Tip: Eutrophic (nutrient-rich) shallow water bodies often exhibit the highest productivity due to algal blooms.

Step 1: Gross Primary Productivity (GPP): Total amount of organic matter produced by producers through photosynthesis.

Step 2: Net Primary Productivity (NPP): The organic matter remaining after deducting the energy used by producers for respiration (R) from GPP.

Step 3: Relation — \( NPP = GPP - R \) Quick Tip: NPP represents the energy available to consumers in the ecosystem.

Step 1: ADA (Adenosine Deaminase) deficiency leads to severe combined immunodeficiency (SCID), where lymphocytes are non-functional.

Step 2: Periodic infusion of genetically engineered lymphocytes is needed because these modified cells have a limited lifespan and cannot multiply to maintain the therapeutic effect.

Step 3: Since these engineered cells are not incorporated into the patient's genome permanently, repeated infusions are necessary.

Step 4: A permanent cure can be achieved by gene therapy where a functional ADA gene is introduced into embryonic stem cells or early bone marrow cells of the patient, allowing lifelong expression. Quick Tip: Gene therapy offers a long-term solution for ADA deficiency by incorporating the correct gene into the patient's genome—eliminating the need for repeated infusions.

Step 1: Microinjection: The recombinant DNA is directly injected into the nucleus of an animal cell using a fine micro-needle under a microscope.

Step 2: Gene Gun or Biolistics: High-velocity micro-particles coated with DNA are bombarded into plant cells to deliver the recombinant DNA directly.

Step 3: These physical methods bypass the need for a vector, allowing direct DNA transfer into host cells. Quick Tip: Microinjection and gene gun are physical, vector-free methods for direct DNA delivery—commonly used in animal and plant biotechnology.

Step 1: Convergent Evolution: It is the process where unrelated organisms independently evolve similar traits as a result of having to adapt to similar environments or ecological niches.

Step 2: Example: The wings of birds, bats, and insects perform the same function but have different structural origins.

Step 3: Divergent Evolution: It is the process where related organisms evolve different traits, leading to the formation of new species due to different environmental pressures.

Step 4: Example: Darwin's finches evolving different types of beaks to adapt to various kinds of food sources in the Galápagos Islands. Quick Tip: Convergent evolution results in analogous structures, while divergent evolution leads to homologous structures.

Step 1: (a) The suitable test is amniocentesis, where a small amount of amniotic fluid containing fetal cells is withdrawn and analyzed for chromosomal abnormalities like Down's syndrome and genetic disorders like cystic fibrosis.

Step 2: (b) If the fetus is diagnosed with severe abnormalities that threaten the mother's health, one option is medical termination of pregnancy (MTP). However, as Radha is in her second month (8 weeks), MTP can be safely performed under medical supervision.

Step 3: (c) MTP is illegal if performed:

Without the consent of the pregnant woman.
Beyond 24 weeks of pregnancy (as per the latest Indian law, unless permitted by a medical board in special cases).
For reasons such as sex selection or without medical justification. Quick Tip: Amniocentesis helps detect genetic disorders early; MTP laws ensure protection of both maternal health and ethical boundaries.

Step 1: (a) The palindromic sequence is:

5'-GAATTC-3'

3'-CTTAAG-5'

Step 2: (b) The restriction endonuclease that recognizes this sequence is EcoRI.

Step 3: (c) Sticky ends are produced when restriction enzymes like EcoRI cut the DNA at specific sites within the palindromic sequence, leaving overhanging single-stranded regions.

Step 4: These sticky ends can form hydrogen bonds with complementary sequences on other DNA fragments, facilitating the ligation process during recombinant DNA formation. Quick Tip: Sticky ends produced by restriction enzymes like EcoRI enable easy and specific joining of DNA fragments in genetic engineering.

Step 1: (a) The infective stage of Plasmodium stored in the female \textit{Anopheles mosquito is the sporozoite.

Step 2: (b) Fertilization and development of the parasite take place inside the female Anopheles mosquito (the definitive host).

Step 3: (c) In the diagram:

P is Ookinete
Q is Sporozoites


Step 4: (d)

Asexual phase occurs in the human (intermediate host).
Sexual phase occurs in the female Anopheles mosquito (definitive host). Quick Tip: The sporozoite stage of \textit{Plasmodium is transmitted from mosquito to human and initiates the infection. Sexual reproduction always occurs in the definitive host—here, the mosquito.

Step 1: (a) The structure shown in the figure is a nucleosome.

Step 2: (b) In the diagram:

P is Histone octamer
Q is DNA strand


Step 3: (c) Histone proteins are basic in nature due to the presence of positively charged amino acids like lysine and arginine.

Step 4: (d) Difference between Euchromatin and Heterochromatin:

Euchromatin: Lightly stained, transcriptionally active, loosely packed.
Heterochromatin: Darkly stained, transcriptionally inactive, tightly packed. Quick Tip: Nucleosomes help in compacting DNA and play a role in gene regulation. Histones are positively charged, allowing tight binding with negatively charged DNA.

Step 1: (A) Columbia has more species diversity than Greenland because it lies near the equator where tropical regions support higher biodiversity due to favourable climatic conditions, consistent temperature, abundant rainfall, and diverse habitats.

Step 2: (B) Paul Ehrlich proposed the Rivet Popper Hypothesis, which compares an ecosystem to an airplane where all parts (species) are like rivets. Removal of a few rivets (species) might not affect flight (ecosystem stability) immediately, but beyond a point, loss of key species can cause ecosystem collapse. Quick Tip: Tropical regions like Columbia are biodiversity hotspots. Ehrlich’s hypothesis warns against unchecked species loss, comparing it to removing bolts from a flying plane.

Step 1: (A) Difference between Recombinant DNA and cDNA:

Recombinant DNA: Artificially created by joining DNA fragments from different sources.
cDNA (complementary DNA): Synthesized from an mRNA template using reverse transcriptase enzyme.


Step 2: (B) Bacterial cell membrane competency can be increased by:

Treating cells with a cold calcium chloride (\(CaCl_2\)) solution.
Followed by a brief heat shock which creates pores in the bacterial membrane, allowing the uptake of recombinant DNA. Quick Tip: Recombinant DNA joins genes from different sources, while cDNA is made from mRNA. Calcium chloride treatment and heat shock boost bacterial DNA uptake.

Step 1: (a) The Monarch butterfly acquires the distasteful chemical by feeding on a poisonous plant called Milkweed during its caterpillar stage, which makes it unpalatable to predators.

Step 2: (b) \[ N_{t+1 = N_t + (Natality + Immigration) - (Mortality + Emigration) \]
Given: \[ 800 = N_t + (200+200) - (150+100) \] \[ 800 = N_t + 400 - 250 \] \[ 800 = N_t + 150 \] \[ N_t = 800 - 150 = 650 \]
Since natality and immigration are higher than mortality and emigration, it indicates a triangular age pyramid (expanding population).

Step 3: (c)
For 200 carrot grass plants: Number of individuals per unit area (density) is used.

For a single banyan tree: Biomass or canopy cover area is a better parameter than simple count.


OR



Step 4: (d)
Two methods to measure tiger population density:

Pugmark analysis
Camera trapping Quick Tip: Population density can be expressed by number, biomass, or area cover. Monarch butterflies acquire toxins from Milkweed, and increasing population trends form triangular age pyramids.

Step 1: (a) The hormone responsible for the peak in Case 1 and Case 2 is Luteinizing Hormone (LH).

Function: LH triggers ovulation and the formation of the corpus luteum.

Step 2: (b)

Ovary: Maturation of ovarian follicles takes place.
Uterus: Endometrial lining of uterus proliferates under the influence of estrogen.


Step 3: (c)
Name of Hormone Q: Progesterone

Function: Maintains the endometrial lining and prepares uterus for implantation.


OR

(d) Structure that remains functional: Corpus luteum

Formation: Formed from the ruptured Graafian follicle after ovulation under the influence of LH. Quick Tip: The LH surge induces ovulation. If fertilization occurs, HCG from the embryo maintains the corpus luteum and supports progesterone secretion to sustain pregnancy.

N/A

Step 1: Isolate DNA from both blood samples R and S.

Step 2: Cut the DNA using specific restriction endonucleases at specific sites.

Step 3: Separate the resulting DNA fragments by gel electrophoresis.

Step 4: Transfer the separated DNA fragments onto a nylon or nitrocellulose membrane by Southern blotting.

Step 5: Hybridize the membrane with radioactive or fluorescent DNA probes specific for known variable number tandem repeat (VNTR) sequences.

Step 6: Develop the autoradiograph or imaging and compare the banding pattern of samples R and S.

Step 7: Matching banding patterns confirm if both samples originate from the same individual. Quick Tip: A test cross helps reveal an unknown dominant genotype, and haplodiploidy controls sex in honey-bees. DNA fingerprinting uniquely identifies individuals using VNTR patterns.

Option (A):

[(i)] MALT stands for Mucosa-Associated Lymphoid Tissue. It is located in the lining of respiratory, digestive and urogenital tracts.
[(ii)] Cytokine barriers involve antiviral proteins like interferons that protect non-infected cells from viral infection.
[(iii)] ELISA (Enzyme Linked Immunosorbent Assay) is used for AIDS diagnosis. It works on the principle of antigen-antibody interaction.
[(iv)] Bone marrow produces lymphocytes. Thymus is the site for T-cell maturation. Both are primary lymphoid organs and ensure immune cell development.



Option (B):

[(i)] Table with filled values:


\begin{tabular{|p{2cm|p{3cm|p{3cm|p{3cm|p{4cm|
\hline
(a) & Butyric acid & Clostridium butylicum & Anaerobic bacteria & Important applications in food, chemical \& pharma industry

\hline
(b) & Statins & Monascus purpureus & Fungus & Inhibit cholesterol biosynthesis pathway

\hline
(c) & Cyclosporin A & Trichoderma polysporum & Fungus & Immunosuppressive agent in organ transplant

\hline
\end{tabular


[(ii)] Baculoviruses are used as biological control agents as they are species-specific, affect only insect pests (especially arthropods), and do not harm plants, mammals, or beneficial insects. Quick Tip: MALT forms a major part of our immune defense in mucosal regions. Microbes like \textit{Monascus and Trichoderma produce bioactive compounds with crucial industrial applications.

Step 1: (A)
In a mature male gametophyte of an angiosperm, the two cells enclosed are:

Generative cell: Divides to form two male gametes.
Tube cell: Forms the pollen tube for sperm delivery.


Step 2: (B)
(i) The technical terms are:

1 – Autogamy

2 – Geitonogamy

3 – Xenogamy

(ii)
(a) Water lily – Pollination occurs through insects (entomophily).

(b) \textit{Vallisneria – Pollination occurs through water (hydrophily).

(iii) Advantages of xenogamy:

Introduces genetic variation.
Helps plants adapt to changing environments. Quick Tip: Xenogamy leads to maximum genetic diversity, improving adaptability and survival of plant species.

(i) Fate of sperms:

Sperm P: Does not reach the ovum; blocked by surrounding layers.
Sperm Q: Actively moving toward the ovum, may achieve fertilization.
Sperm R: Unable to penetrate; will degenerate eventually.


(ii) Role of Zona pellucida:
It is a glycoprotein membrane that ensures species-specific sperm binding and prevents polyspermy after the first sperm fuses with the ovum.

(iii) Changes after sperm entry:

Completion of ovum’s second meiotic division.
Formation of second polar body.
Restoration of diploid number through fusion of male and female pronuclei.
Zona reaction prevents entry of other sperms.


(iv) Role of acrosome and middle piece:

Acrosome: Releases hydrolytic enzymes to digest corona radiata and zona pellucida.
Middle piece: Contains mitochondria that provide energy for sperm motility, aiding fertilization. Quick Tip: The zona pellucida plays a critical role in preventing multiple sperm from fertilizing an ovum, thus ensuring proper embryonic development.