CBSE Class 12 2025 Biology 57-5-2 Question Paper Set-2 :Download Solutions with Answer Key

The region of chromatin that is actively transcribed is euchromatin. Euchromatin is a loosely packed form of chromatin, making it accessible to the transcriptional machinery, such as RNA polymerase, allowing gene expression to occur. In contrast, heterochromatin is tightly packed and transcriptionally inactive, while scaffold and solenoid chromatin are not standard terms for describing transcriptionally active regions. Quick Tip: Euchromatin is the loosely packed, transcriptionally active form of chromatin, while heterochromatin is tightly packed and inactive.

Statements (i), (ii), (iv), and (v) are true. The ovaries are 2--4 cm long and are connected to the pelvic wall by ligaments (often referred to as tendons in some contexts), covered by a thin epithelium enclosing the stroma, are the primary female sex organs producing ova, and are located on each side of the lower abdomen. However, statement (iii) is false because the stroma is divided into an outer cortex and an inner medulla, not the other way around. Thus, option (A) with (i), (ii), and (iv) contains all true statements. Quick Tip: The ovarian stroma has an outer cortex (where follicles develop) and an inner medulla (containing blood vessels). Ovaries produce ova and are 2--4 cm long.

The Human Genome Project (HGP) used a whole-genome shotgun sequencing approach to sequence both coding and non-coding regions, but this is not listed among the options. Expressed Sequence Tags (ESTs) focus on sequencing expressed genes (mRNA), not the whole genome. Bioinformatics is a field, not a sequencing method. Sequence Annotation involves analyzing sequences after sequencing, and DNA Polymorphism refers to genetic variations, not a sequencing approach. None of the given options correctly describe the HGP's sequencing method. Quick Tip: The HGP used whole-genome shotgun sequencing to sequence the entire genome, followed by annotation to identify coding and non-coding regions.

The pedigree shows an autosomal dominant trait (affected individuals in every generation, both sexes affected). The affected male parent is heterozygous (Aa), and the unaffected female parent is aa. The indicated female child is affected, so she must have inherited the dominant A allele from the father, making her genotype Aa. Option (A) AA is not possible as the mother is aa, (C) XY refers to sex chromosomes, not the trait, and (D) aa would mean she’s unaffected, which she isn’t. Quick Tip: In autosomal dominant traits, affected individuals are typically heterozygous (Aa), and the trait appears in every generation, affecting both sexes.

The thymus is a lobed organ located near the heart and beneath the breastbone (sternum) in the mediastinum. It plays a key role in the immune system by aiding T-cell development. The liver and spleen are in the abdomen, and the lungs, while near the heart, are not a single lobed organ beneath the breastbone. Quick Tip: The thymus, located in the mediastinum beneath the sternum, is crucial for immune function and T-cell maturation.

Unisexuality of flowers (having either male or female parts, not both) prevents self-pollination. Geitonogamy is self-pollination between flowers on the same plant, which unisexuality prevents. Xenogamy (cross-pollination between different plants) can still occur. Autogamy (self-pollination within the same flower) is also prevented, but the question focuses on the process most directly affected, which is geitonogamy. Quick Tip: Unisexual flowers prevent geitonogamy (self-pollination on the same plant) but allow xenogamy (cross-pollination between plants).

Convergent evolution occurs when unrelated species develop similar traits due to similar environmental pressures. Placental mammals and Australian marsupials show convergence, e.g., the marsupial mole and placental mole. However, none of the options are strong examples: (A) Anteater and Lemur (lemurs are primates, not similar), (B) Numbat and Bobcat (insectivore vs. carnivore), (C) Tasmanian tiger and Anteater (carnivore vs. insectivore), (D) Lemur and Spotted cuscus (both arboreal, but not a strong convergence example). A classic pair like the marsupial wolf (Tasmanian tiger) and placental wolf isn’t listed. Quick Tip: Convergent evolution in placental mammals and marsupials includes pairs like the marsupial mole and placental mole, adapting similarly to their environments.

Lysozyme is an enzyme that breaks down the peptidoglycan layer of bacterial cell walls, making it effective for isolating DNA from bacterial cells. Cellulase degrades plant cell walls, protease breaks down proteins, and ribonuclease degrades RNA, none of which are suitable for this purpose. Quick Tip: Lysozyme targets the peptidoglycan in bacterial cell walls, facilitating DNA extraction by lysing the cells.

Dominance occurs when one allele masks the other in a heterozygote (a-ii). Codominance is when both alleles are expressed fully in a heterozygote (b-iii). Pleiotropy refers to one gene affecting multiple traits (c-iv). Polygenic inheritance involves multiple genes influencing a single trait (d-i). Quick Tip: Codominance and dominance differ in allele expression: codominance shows both alleles, while dominance shows only one.

The lac-y gene encodes lactose permease in the lac operon. UAG is a stop codon, so its insertion in lac-y would truncate lactose permease production. However, the lac-z gene (encoding \(\beta\)-galactosidase) and lac-a gene (encoding transacetylase) are downstream, but \(\beta\)-galactosidase can still be produced if the lac-z gene is intact, which is typically the case in such mutations. Quick Tip: The lac operon includes three genes: lac-z (\(\beta\)-galactosidase), lac-y (lactose permease), and lac-a (transacetylase). A mutation in one gene may not affect the others.

Aspergillus sp., particularly Aspergillus niger, is widely used for the industrial production of citric acid due to its efficiency in fermenting sugars into citric acid. Streptococcus, Clostridium, and Trichoderma are not typically used for this purpose. Quick Tip: Aspergillus niger is a key microbe in citric acid production, often using molasses or glucose as a substrate in fermentation.

In DNA, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). If thymine is 20%, adenine must also be 20%. Together, A+T = 40%, so G+C = 60%. Since G and C are equal, G = 30% and C = 30%. Quick Tip: In DNA, the base pairing rule (A=T, G=C) ensures that the percentages of complementary bases are equal.

Step 1: Gene pairs on the same chromosome can be tightly or loosely linked based on their recombination frequency — Assertion is true.

Step 2: Recombination frequency is used for gene mapping, but it applies to genes on the same chromosome, not different chromosomes — Reason is true but misphrased.

Step 3: The Reason does not explain the Assertion, as it incorrectly references genes on different chromosomes — so option (B) is correct. Quick Tip: Gene mapping uses recombination frequency between genes on the same chromosome, not different chromosomes.

Step 1: Cu-T, Cu-7, and LNG-20 are widely used IUDs, though LNG-20 is hormone-based, not copper-releasing — Assertion is partially true but accepted in this context.

Step 2: Copper ions in IUDs do suppress sperm motility and fertilizing capacity — Reason is true.

Step 3: The mechanism of copper ions explains why copper-releasing IUDs like Cu-T and Cu-7 are effective — so option (A) is correct. Quick Tip: Copper IUDs release Cu-ions, creating a hostile environment for sperm, while LNG-20 primarily releases hormones.

Step 1: Totipotency refers to the ability of a plant cell to regenerate into an entire plant, not just a part — Assertion is false.

Step 2: Sterile conditions and nutrient media are indeed required for in vitro culture of explants — Reason is true.

Step 3: Since Assertion is false but Reason is true, option (D) is correct. Quick Tip: Totipotency means a cell can form a whole plant, not just a part, and \textit{in vitro conditions support this process.

Step 1: Lymphocytes, including natural killer (NK) cells, do arise from bone marrow and are present in blood and lymph — Assertion is true.

Step 2: Some lymphocytes migrate to the thymus to develop into mature T-cells — Reason is true.

Step 3: However, the Reason explains T-cell development, not the role of lymphocytes as NK cells — so option (B) is correct. Quick Tip: Lymphocytes include B-cells, T-cells, and NK cells; only T-cells mature in the thymus, while NK cells develop elsewhere.

Step 1:
(a) The enzyme used to make multiple copies of foreign DNA is DNA polymerase, specifically Taq polymerase during the PCR (Polymerase Chain Reaction) process.

Step 2:
(b) EcoR I creates sticky ends (overhanging sequences) in both plasmid and foreign DNA.

These sticky ends are complementary, allowing the foreign DNA to bind with the plasmid DNA.
DNA ligase then joins the sugar-phosphate backbones, forming a stable recombinant DNA molecule.


Step 3:
(c) The most commonly used host is Escherichia coli (E. coli). Quick Tip: PCR uses DNA polymerase to amplify DNA. EcoR I generates sticky ends, and DNA ligase seals the DNA strands. E. coli is the standard host in rDNA technology.

(A)
Step 1: Amniocentesis is a prenatal diagnostic technique used to detect chromosomal abnormalities in the fetus.

Step 2: A small amount of amniotic fluid is withdrawn from the amniotic sac surrounding the fetus using a syringe under ultrasound guidance.
Step 3: Misuse in India:

The technique is often misused for sex determination.
This leads to female foeticide, contributing to an imbalanced sex ratio.


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OR


(B)
Step 1: Two common venereal diseases (VDs) in females are:

Gonorrhoea
Syphilis

Step 2: If left untreated, complications may include:

Pelvic inflammatory disease (PID), which can lead to infertility.
Ectopic pregnancy or damage to reproductive organs. Quick Tip: Amniocentesis is for fetal diagnosis but misused for sex determination. VDs like gonorrhoea and syphilis can cause infertility and reproductive damage.

(A)
Step 1: Mutualism is an interaction where both species benefit from each other.

Step 2: In the case of fig tree and wasp:

The fig tree is pollinated only by its specific species of wasp.
The wasp lays eggs inside the fig fruits.
The fig provides shelter and nourishment for the larvae.
Both species are dependent on each other for survival and reproduction.

Step 3: This is a classic example of co-evolved mutualism.

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OR


(B)
Step 1: The ecological pyramid of number represents the number of organisms at each trophic level. In this case, the pyramid is upright.


Step 2: Representation of the pyramid:

\begin{tabular{|c|c|
\hline
Trophic Level & Organisms and Number

\hline
Trophic Level 3 (Secondary Consumers) & Birds – 32

\hline
Trophic Level 2 (Primary Consumers) & Insects – 20

\hline
Trophic Level 1 (Producers) & One Banyan Tree – 1

\hline
\end{tabular


Step 3:

The number of organisms decreases from producers to top consumers.
A single banyan tree supports many insects, which in turn are preyed upon by more birds.

Thus, this pyramid shows how energy flow supports different populations. Quick Tip: Mutualism involves reciprocal benefit. The fig-wasp relationship is highly specific. Ecological pyramids of number can be upright or inverted depending on the ecosystem.

(a)
Step 1: The correct diagram is (ii).

Step 2: DNA replication occurs in the 5′ to 3′ direction. At the replication fork, one strand (leading strand) is synthesized continuously, while the other (lagging strand) is synthesized discontinuously in short fragments.

Step 3: In diagram (ii), both forks show correct orientation: the leading strand is synthesized towards the fork and the lagging strand away from it, consistent with the antiparallel nature of DNA and enzymatic action of DNA polymerase.

\medskip

(b)
Step 1: The enzyme used is DNA ligase.

Step 2: DNA ligase joins Okazaki fragments on the lagging strand by catalyzing the formation of phosphodiester bonds. Quick Tip: In DNA replication, DNA polymerase adds nucleotides only in the 5′ to 3′ direction. DNA ligase seals nicks between Okazaki fragments on the lagging strand.

(A)
Step 1: An antibody is a Y-shaped glycoprotein made by B-lymphocytes in response to an antigen.

Step 2: Structure of an antibody molecule:

Composed of four polypeptide chains — two identical light (L) chains and two identical heavy (H) chains.
Chains are held together by disulfide bonds.
Each chain has a variable region and a constant region.
The variable region forms the antigen-binding site — specific to each antigen.
The constant region determines the antibody class (IgA, IgG, etc.).

Step 3: The specificity of the antibody is due to the antigen-binding sites present on the variable regions of the light and heavy chains.

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OR


(B)
Step 1: Public awareness and counselling are essential to prevent HIV spread. In addition to these, the following measures are taken:

Ensuring safe blood transfusion by proper screening.
Use of disposable syringes and needles.
Promoting safe sex practices (e.g., use of condoms).
Providing antiretroviral therapy (ART) to infected individuals.

Step 2: Organizations like NACO and WHO also promote voluntary testing and confidentiality to reduce stigma and improve early diagnosis. Quick Tip: Antibodies are specific to antigens and made of two heavy and two light chains. HIV prevention includes awareness, safe sex, safe blood, and ART programs.

Habitat loss and fragmentation reduce the available area for wildlife, restricting food, shelter and breeding grounds.
Fragmentation divides populations into small, isolated groups, leading to inbreeding and reduced genetic diversity.
An example is the clearing of Tropical Rain Forests for agriculture and settlements, which leads to species extinction.
Fragmentation causes edge effects, making populations more vulnerable to predators and environmental changes. Quick Tip: Remember — fragmentation reduces population size and gene flow, increasing extinction risks.

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Three alleles are involved in blood grouping: \(I^A\), \(I^B\), and \(i\).
This inheritance is called codominance because both \(I^A\) and \(I^B\) alleles express themselves equally in the phenotype.
Yes, they can have a child with ‘O’ blood group if both the parents are heterozygous, i.e., the man has genotype \(I^A i\) and the woman has genotype \(I^B i\).

Cross:

\begin{tabular{c|c c
& \(I^B\) & \(i\)

\hline
\(I^A\) & \(I^A I^B\) (AB) & \(I^A i\) (A)

\(i\) & \(I^B i\) (B) & \(ii\) (O)

\end{tabular Quick Tip: Blood group inheritance involves multiple alleles and codominance. Use Punnett squares to predict possible genotypes.

Step 1: Parturition is the process of childbirth, initiated by a complex neuroendocrine mechanism.


Step 2:

Signals from the fully developed foetus and placenta trigger mild uterine contractions.
These contractions are sensed by the maternal hypothalamus, which signals the posterior pituitary to release oxytocin.
Oxytocin causes stronger uterine contractions (positive feedback mechanism).
These contractions further stimulate oxytocin release, intensifying the process.
Eventually, this leads to expulsion of the baby through the birth canal. Quick Tip: Oxytocin plays a major role in parturition through a positive feedback loop, leading to stronger and more frequent uterine contractions.

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In the given type, stigma becomes receptive when anthers are not shedding pollen and vice versa. This prevents self-pollination and promotes cross-pollination, which enhances genetic variation and adaptability.
Yes, the plant can show geitonogamy because the flowers are on the same plant. Though genetically it resembles self-pollination, it functionally involves different flowers. Quick Tip: Dichogamy and herkogamy are common mechanisms to promote cross-pollination in hermaphrodite plants.

Thalassemia is a blood-related autosomal Mendelian disorder.
It is called a Mendelian disorder because it follows the inheritance patterns first proposed by Gregor Mendel, involving a single gene mutation.
Thalassemia is caused by mutations in the gene responsible for haemoglobin chain production.
The disorder is transmitted from parents to offspring when both parents are carriers (heterozygous), with a 25% chance of the child being affected if both defective alleles are inherited. Quick Tip: Remember — Thalassemia follows autosomal recessive inheritance and requires both parents to be carriers for an affected offspring.

The full form of BOD is Biochemical Oxygen Demand. Quick Tip: Remember — BOD stands for Biochemical Oxygen Demand, an important indicator of water pollution.

Biochemical Oxygen Demand (BOD) is the amount of dissolved oxygen required by aerobic microorganisms to break down organic matter in a given volume of water over a specified period.
It indicates the level of organic pollution in water.
Higher BOD values mean more organic matter is present, which requires more oxygen for decomposition, indicating higher pollution.
It is a crucial parameter in assessing the health and quality of water bodies. Quick Tip: Remember — Higher BOD means more organic waste in water, lowering its quality.

Genetically modified (GM) plants show increased resistance to pests, reducing pesticide use.
They offer higher yield and improved nutritional content.
GM plants can tolerate abiotic stresses like drought, salinity, and extreme temperatures. Quick Tip: Remember — GM plants improve productivity, pest resistance, and stress tolerance.

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People in the adolescent age group (15–16 years) are more vulnerable to drug abuse due to ongoing brain development and susceptibility to peer pressure.

Coca cultivation indirectly accelerates “Narco-deforestation” by laundering drug profits into land speculation, increasing deforestation rates and endangering the ecological balance of the Amazon rainforest.



[(i)] Cannabinoids are mainly obtained from the resin of the flowering tops and leaves of the \textit{Cannabis plant.
Negative effect: It can lead to impaired cognitive development, memory loss and addiction in adolescents.

OR

[(ii)] The scientific name of the plant is \textit{Erythroxylum coca.
Excessive use of cocaine can cause hallucinations, severe mental instability and cardiac failure. Quick Tip: Remember — adolescents are highly vulnerable to drug abuse, and coca cultivation indirectly worsens deforestation by fuelling illegal land speculation.

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The correct option is (iii) — Greater the time of separation, greater the number of nucleotide substitutions. This reflects the accumulation of genetic changes over evolutionary time.



[(i)] Human and Kangaroo: Divergent evolution, as both groups evolved from a common ancestor but underwent different evolutionary changes over time.
[(ii)] Human and Rodent: Divergent evolution, as they also share a common ancestor but diverged due to different selective pressures and environments.




[(ii)] Divergent evolution is the process by which two or more related species become more dissimilar over time, usually in response to different environmental pressures or ecological niches. Quick Tip: Greater nucleotide substitutions in conserved genes like Cytochrome-C typically indicate a longer evolutionary separation between species.

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[(i)] Structure of Monocotyledonous Embryo:

It consists of a single large cotyledon called scutellum placed laterally.
There’s a short axis with a plumule (embryonic shoot) and radicle (embryonic root) enclosed within protective sheaths called coleoptile and coleorhiza, respectively.


[(ii)] Multiple embryos in citrus fruits:

Multiple embryos arise when nucellar cells surrounding the embryo sac start dividing and develop into embryos.
This phenomenon is known as Polyembryony.





[(i)] Male Accessory Ducts:

Includes rete testis, vasa efferentia, epididymis, and vas deferens.
They transport, store, and aid maturation of sperm.


[(ii)] Role of FSH in Spermatogenesis:

FSH acts on Sertoli cells to stimulate secretion of factors aiding spermatogenesis.
Supports nourishment and maturation of developing sperms. Quick Tip: Polyembryony is a form of asexual reproduction in plants where multiple embryos develop from a single fertilized ovule.

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[(i)] The protein produced by Bacillus thuringiensis is in inactive form (protoxin) inside the bacteria. It becomes active only in the alkaline gut of insects.

[(ii)] The gene coding for this protein has been incorporated into cotton plants. This makes Bt cotton produce the insecticidal protein, which kills bollworm pests on consumption.




[(i)] In the diagram of pBR322 vector:

‘a’ is amp\(^{r\) gene (ampicillin resistance)
‘b’ is tet\(^{r}\) gene (tetracycline resistance)


[(ii)] The \(\beta\)-galactosidase gene produces a visible colour change when active. This allows for blue-white screening, a simpler and faster method to identify recombinant colonies.

[(iii)] Uses of Cloning Vector:

For producing multiple copies of a specific gene.
For transferring desirable genes into host organisms. Quick Tip: The Bt toxin gene works only inside insect gut as it requires an alkaline pH for activation, thus remaining harmless to the bacteria itself.

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[(i)] When unlimited resources are available, populations exhibit exponential growth. The population size increases rapidly without any environmental resistance.

[(ii)] The equation for exponential growth is:
\[ \frac{dN}{dt} = rN \]
where:

\( N \) = population size
\( r \) = intrinsic rate of natural increase
\( \frac{dN}{dt} \) = change in population size per unit time


[(iii)] The growth curve is called the J-shaped curve. It shows rapid increase initially and continues rising steeply without a plateau.






[(i)] Alexander von Humboldt observed that within a region, species richness increases with increasing area but only up to a certain limit.

[(ii)] The Species-Area relationship equation is:
\[ \log S = \log C + Z \log A \]
where:

\( S \) = species richness
\( A \) = area
\( C \) and \( Z \) = constants


[(iii)] Graphical Representation:
\end{itemsize Quick Tip: Exponential (J-shaped) growth occurs only under ideal conditions. In nature, due to limited resources, most populations follow logistic (S-shaped) growth patterns.