AIIMS 2009 Biology Question Paper with Answer and Solution

(C) The parasympathetic nervous system,specifically through the vagus nerve,releases acetylcholine which acts on the $SA$ node to decrease the heart rate.
When the parasympathetic nerve is cut,the inhibitory effect of the vagus nerve on the heart is removed.
As a result,the sympathetic nervous system's influence becomes dominant,leading to an increase in the heart rate.

(A) Cladistics is a method of classification that groups organisms based on shared derived characteristics (synapomorphies) that can be traced to a common ancestor.
It focuses exclusively on evolutionary and genetic relationships,often ignoring superficial morphological similarities or differences that may arise due to convergent evolution.
In contrast,phenetics classifies organisms based on overall morphological similarity,while classical systematics relies heavily on morphological traits.
Therefore,the system described is known as cladistics.

(B) Some $Basidiomycetes$ or club fungi (e.g., $Agaricus$ $campestris$) produce $basidiocarps$.
These aerial structures are also called mushrooms.
They occur in rings known as 'fairy rings' due to the outward radial growth of the mycelium.
$Basidiocarps$ of some fungi are edible.

(B) Chemosynthesis is a form of autotrophic nutrition where organisms synthesize organic molecules using energy derived from the oxidation of inorganic substances.
Chemoautotrophs do not contain chlorophyll pigments; chlorophyll is characteristic of photoautotrophs,which use solar energy to synthesize organic compounds.
Therefore,the Assertion is correct,but the Reason is incorrect.

(B) Selaginella bryopteris $(L.)$,popularly known as Sanjeevani,is a plant with medicinal properties.
Sanjeevani grows on the hills of tropical areas,particularly the Arawali mountain terrains from east to west in India.
Traditionally,this plant has been used to: $(i)$ provide relief from heat stroke and burning sensation during urination; $(ii)$ restore menstrual irregularities to normal and,when applied topically to pregnant women,aid in easy delivery; and $(iii)$ treat jaundice.

(D) Invertebrates,which are animals without a backbone,possess a higher proportion of undifferentiated cells (stem cells) compared to vertebrates.
These undifferentiated cells retain the potential to divide and differentiate into various cell types,which facilitates the regeneration of lost tissues or body parts.
In contrast,vertebrate cells are generally more specialized and have limited regenerative capacity.

(B) Earthworms possess a closed circulatory system,where blood circulates within defined,walled blood vessels.
Renal portal systems are characteristic of amphibians and reptiles,not humans. Humans possess a hepatic portal system that transports venous blood from the digestive system to the liver.
In cockroaches,the primary excretory organs are Malpighian tubules,whereas nephridia are the excretory structures found in annelids like earthworms.

(A) Chordates are a group of animals that include fish,amphibians,reptiles,birds,and mammals.
The defining characteristics of the chordate nervous system include a central nervous system consisting of a brain and a hollow,dorsal nerve cord.
Additionally,chordates possess highly developed sensory receptors to detect environmental stimuli.
In contrast,fused ganglia are characteristic of annelids and arthropods,tympana are found in insects like grasshoppers,and nerve nets are characteristic of cnidarians like Hydra.

(A) Sponges are multicellular organisms, but they exhibit a "cellular level" of body organization, meaning they do not possess true tissues, organs, or organ systems.
Although sponges lack tissues, there is a degree of physiological division of labour among the various types of cells (e.g., choanocytes, amoebocytes) present in their body.
These cells perform specific functions such as digestion, respiration, and reproduction, but they are not organized into permanent layers or masses to form tissues.
Therefore, both the Assertion and the Reason are correct, and the Reason provides a valid explanation for why the organization is considered "cellular level" despite the presence of functional specialization.

(D) The condition where anthers are fused together to form a tube while the filaments remain free is known as $syngenesious$ condition.
This characteristic feature is observed in the members of the family $Asteraceae$ (also known as $Compositae$).
For example,in the disc florets of the sunflower ($Helianthus$ $annuus$),the anthers are united to form a tube around the style.

(C) floral diagram provides information about the number of parts of a flower,their arrangement,and the relation they have with one another.
It indicates the aestivation,placentation,and cohesion or adhesion of floral parts.
However,it does not represent the position of the ovary on the thalamus (i.e.,whether the flower is hypogynous,perigynous,or epigynous),which is instead indicated by the floral formula.

(D) An aggregate fruit is formed from a single flower that has a multicarpellary,apocarpous gynoecium.
In an apocarpous ovary,the carpels are free from each other.
Each individual carpel develops into a small fruitlet,and the entire collection of these fruitlets,which are attached to a common receptacle,is known as an aggregate fruit or $etaerio$.

(D) Cilia are hair-like structures on a $Paramecium$ used for locomotion and capturing food.
An oral groove acts as a mouth, and food vacuoles are involved in the digestion of ingested food particles.
The absence of chloroplasts confirms that the organism cannot perform photosynthesis.
These features collectively indicate that the organism relies on preformed organic matter for energy, which is defined as $heterotrophic \text{ nutrition}$.

(C) The dissecting microscope allows for the viewing of $3D$ images at magnifications up to $50x$. It is commonly used during dissections to observe external morphological features.
An ultracentrifuge is used to separate components of a mixture based on density by spinning at high speeds.
Microdissection instruments are specialized tools used for manipulating microscopic structures,such as cell nuclei.
Electron microscopes provide extremely high magnification (up to $250,000x$) and are used to study internal cellular ultrastructure,not external features of an organism.

(D) The cross-sections of stem and root appear simple when viewed by the naked eye,but under a microscope,they can be differentiated by the $endarch$ condition in the stem and the $exarch$ condition in the root.
In the $endarch$ condition,the $protoxylem$ is present towards the center of the stem,while the $metaxylem$ is towards the periphery.
In the $exarch$ condition,the $protoxylem$ is present towards the periphery (pericycle) and the $metaxylem$ is towards the center of the root.

(B) Pinocytosis,often described as 'cell drinking' or the pinching in of the cell membrane,allows cells to engulf and internalize larger molecules or extracellular fluid that cannot pass through membrane pores.
Hydrolysis is the chemical breakdown of a compound due to reaction with water.
Cyclosis (or cytoplasmic streaming) is the directed flow of cytosol and organelles around the large central vacuole of a plant cell.
Synthesis is the process of building complex molecules from simpler ones within the cell.

(C) The Assertion is correct because the lipid composition of the two leaflets of the bilayer membrane is asymmetric. For example,in the erythrocyte membrane,lecithin (phosphatidylcholine) is primarily found on the outer leaflet,while cephalin (phosphatidylethanolamine) is more abundant on the inner leaflet.
The Reason is incorrect because oligosaccharides (forming the glycocalyx) are exclusively attached to the external surface of the plasma membrane,facing the extracellular environment. They are not present on the inner cytoplasmic surface of the biomembrane.

(D) Bivalents are pairs of homologous chromosomes formed during the $zygotene$ substage of $prophase-I$ of meiosis through a process called synapsis.
$DNA$ replication occurs during the $S$ phase (synthesis phase) of the interphase, which takes place just prior to the onset of meiosis (prophase $I$).

(B) The oxygen released during photosynthesis originates from water molecules. During the light-dependent reactions of photosynthesis,light energy is captured by chlorophyll in the chloroplasts. This energy facilitates the photolysis of water,which is the splitting of water molecules into protons,electrons,and oxygen gas.
The chemical process is represented as:
$4H_2O \rightleftharpoons 4H^+ + 4OH^-$
$4OH^- \xrightarrow{Mn^{2+}, Cl^-} 2H_2O + O_2 \uparrow + 4e^-$

(D) Dark reaction is also known as light-independent phase. Unlike light reaction,it does not require light as an essential factor. Thus,it can take place both in the presence or absence of light.
The term 'dark reaction' does not mean that it takes place only in the dark period or at night.
$CO_2$ fixation occurs in both $C_3$ and $C_4$ cycles.
In the $C_3$ cycle,$CO_2$ is fixed by the enzyme RuBisCO to a $5$-carbon compound $RuBP$,which is converted into $2$ molecules of $3$-carbon $PGA$.
In the $C_4$ cycle,the first product of $CO_2$ fixation (which takes place in the mesophyll) is a $4$-carbon compound called oxaloacetic acid $(OAA)$.
Since both the Assertion and the Reason are scientifically incorrect,the correct option is $D$.

(D) Glycolysis is the process of breakdown of glucose or similar hexose sugar into two molecules of pyruvic acid through a series of enzyme-mediated reactions,releasing energy $(ATP)$ and reducing power $(NADH_2)$.
It is the first step of respiration,which occurs inside the cytoplasm and is independent of $O_2$.
In glycolysis,two molecules of $ATP$ are consumed during the double phosphorylation of glucose to form fructose $1, 6$-diphosphate.
Four molecules of $ATP$ are produced in the conversion of $1, 3$-diphosphoglycerate to $3$-phosphoglycerate and phosphoenolpyruvate to pyruvate.
Additionally,two molecules of $NADH_2$ are formed during the oxidation of glyceraldehyde $3$-phosphate to $1, 3$-diphosphoglycerate.
Since each $NADH$ is equivalent to $3\, ATP$,the net gain in glycolysis is $8\, ATP$.
Therefore,both the Assertion and the Reason are incorrect.

(A) In the condition of obstructive jaundice,the entry of bile into the small intestine is prevented due to an obstruction in the bile duct.
Bile salts are essential for the emulsification of fats,which increases the surface area for lipase action.
Furthermore,bile salts are necessary for the formation of micelles,which are water-soluble aggregates that transport fatty acids,glycerides,and fat-soluble vitamins to the intestinal mucosa for absorption.
In the absence of bile,fats cannot be properly emulsified or absorbed.
Consequently,these unabsorbed fats pass through the digestive tract and are eliminated from the body in the faeces.

(B) An alveolus is a tiny air sac in the lung. It is the actual part of the lung where the exchange of atmospheric oxygen $(O_2)$ and carbon dioxide $(CO_2)$ occurs with the blood.
In the diagram,the exchange of gases ($O_2$ entering the blood and $CO_2$ leaving the blood) indicates that region $X$ is the alveolar space.
$A$ glomerulus is a network of capillaries in the kidneys for filtration,a villus is a projection in the small intestine for nutrient absorption,and the liver is involved in metabolic processes like bile production.

(C) The medulla oblongata,located at the base of the brain,acts as a respiratory rhythm center.
When the concentration of $CO_2$ in the blood plasma increases,it leads to a decrease in blood $pH$.
This change is detected by chemoreceptors,which send signals to the medulla oblongata.
In response,the medulla increases the breathing rate to expel excess $CO_2$ from the body.
Therefore,the concentration of $CO_2$ regulates the breathing rate by its effect on the medulla.

(A) Pulmonary ventilation is calculated as: $\text{Tidal Volume} \times \text{Respiratory Rate}$.
For $A$: $185 \, cc/breath \times 35 \, breaths/min = 6475 \, cc/min$.
For $B$: $259 \, cc/breath \times 25 \, breaths/min = 6475 \, cc/min$.
Since both values are equal, the pulmonary ventilation of $A$ and $B$ is the same.

(D) The lymphatic system plays a crucial role in maintaining fluid balance and supporting the immune system.
Lymph nodes act as filters for the lymph fluid,trapping and destroying pathogens such as bacteria and viruses.
When lymph nodes malfunction,they cannot effectively filter these harmful microorganisms,which can lead to the spread of infection.
Therefore,the primary function that would be interfered with is the filtering of bacteria from the lymph.

$(C)$ Endocrine glands are ductless glands that secrete their chemical messengers, known as hormones, directly into the blood.
The circulatory system (blood) then transports these hormones to their target organs or tissues to exert their physiological effects.
In contrast, digestive glands produce enzymes, which are typically transported through ducts to the site of action in the digestive tract, not primarily by the circulatory system.
Therefore, the correct sequence is: $(I)$ Endocrine glands, $(II)$ Hormones, $(III)$ Circulatory system. This corresponds to row $C$.

(A) Calcitonin (or thyrocalcitonin) is secreted by the parafollicular cells of the thyroid gland. It lowers blood calcium levels by inhibiting bone resorption and promoting calcium excretion in urine.
Parathormone $(PTH)$ is secreted by the chief cells of the parathyroid gland. It increases blood calcium levels by stimulating calcium absorption from the intestine,reabsorption from the renal tubules,and bone demineralization.
Since calcitonin lowers blood calcium and parathormone raises it,they have antagonistic effects.
Therefore,the Assertion is correct,the Reason is correct,and the Reason is the correct explanation of the Assertion.

(A) The plant hormone,$Auxin$,is distributed more on the side away from the unilateral illumination,causing cells to grow faster in the darker side. This differential growth causes the plant to bend toward the light,a phenomenon known as phototropism. If the distribution were equal,the plant would grow straight upwards. Neurotransmitters are chemicals secreted by multicellular animals and are used in transmitting impulses in the nervous system,not in plant growth responses.

(A) Lichens are economically important in various industries, including the chemical industry.
Litmus is a well-known acid-base indicator used extensively in chemical laboratories, which is extracted from lichens like $Rocella montaignei$.
Orcein is another important dye and biological stain derived from lichens like $Rocella tinctoria$.
Since these chemical products are derived from lichens, the reason correctly explains why lichens are important for chemical industries.

(A) $Trichodesmium erythreum$ is a cyanobacterium (blue-green alga). Although it is a blue-green alga, it possesses a pigment called phycoerythrin, which is red in colour. When these organisms multiply rapidly, they impart a red colour to the seawater, which is why the Red Sea is named as such.

(D) Xerophytic plants conserve water by storing it in parenchymatous cells specialized for this purpose.
These plants are specially adapted to survive in arid climates.
They are typically able to withstand long periods of drought and the drying effects of desert winds.
Some plants have adapted to arid lands by developing specialized water-storage tissues,which are modified parenchymatous cells.

(A) The correct answer is $A$.
Seminal plasma is the fluid component of semen,which is ejaculated from the penis during sexual climax.
It is primarily secreted by the seminal vesicles,prostate gland,and bulbourethral (Cowper's) glands.
Seminal plasma is rich in fructose,calcium,and various enzymes.
Fructose serves as the primary energy source for sperm,while calcium and enzymes help in sperm motility and activation.
Additionally,it provides a medium for sperm transport,lubricates the female reproductive tract,and neutralizes the acidic environment of the vagina to protect the sperm.

(C) Darwin's finches are a classic example of adaptive radiation.
Adaptive radiation is the process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats).
In the Galapagos Islands,Darwin observed that many varieties of finches evolved from a single ancestral species.
These finches adapted to different niches based on the availability of food resources such as insects,seeds,and cactus,leading to the diversification of their beak shapes and sizes.

(A) In $Pinus$,the proembryo consists of four tiers of cells.
$1$. The lowermost tier (farthest from the micropylar end) is the embryonal tier,which develops into the embryo.
$2$. The second tier from below is the suspensor tier,which elongates to push the embryo into the endosperm.
$3$. The third tier from below is known as the rosette tier.
$4$. The uppermost tier is the free-nuclear tier,consisting of $4$ nuclei.

(A) Based on the mode of secretion,glands are classified into three types: merocrine,apocrine,and holocrine.
Mammary glands,which are present in mammals to feed their young with milk,are classic examples of apocrine glands.
In apocrine glands,the secretion accumulates as secretory granules in the distal part (apical portion) of the cell.
During the process of secretion,this distal part of the cell breaks off and is released along with the secretory product.
Therefore,both the Assertion and the Reason are correct,and the Reason provides the correct explanation for the Assertion.

(B) Most vertebrate animals that live on land have an adaptation that allows $Internal \ fertilization$,whereby the male deposits sperm directly into the female body. This is advantageous because sperm require a liquid medium to swim,and the moist female reproductive tract provides this environment. In contrast,aquatic vertebrate animals live in water,which allows sperm to be easily released into the surrounding medium to swim toward the female gametes,making $External \ fertilization$ more common in aquatic environments.

(D) The Assertion is incorrect because Pollen Mother Cells $(PMCs)$ are diploid $(2n)$ sporophytic cells,not gametophytic cells. The gametophytic phase begins with the microspores.
The Reason is also incorrect because each $PMC$ undergoes meiosis to produce $4$ haploid microspores (pollens),not $2$.

(B) In males,one primary spermatocyte undergoes meiosis to produce $4$ functional,viable sperm cells.
In females,one primary oocyte undergoes meiosis,but due to unequal cytoplasmic division,it produces only $1$ functional,viable ovum and $2$ or $3$ non-functional polar bodies that eventually degenerate.
Therefore,the ratio of gametes produced from one male primary sex cell to one female primary sex cell is $4:1$.

(A) In females,the Graafian follicle transforms into the corpus luteum after ovulation.
The cells of the corpus luteum are known as luteal cells,which contain yellow granules called lutein.
These cells secrete the hormone progesterone,which is essential for maintaining pregnancy if fertilization occurs.
In the absence of fertilization,the corpus luteum degenerates into the corpus albicans,leading to a significant decrease in the level of progesterone.

(C) The $Clitoris$ is a female reproductive organ that is homologous to the $penis$ in males,meaning they share a common embryonic origin.
However,the $Clitoris$ is not a 'remnant' of the $penis$; rather,both develop from the same genital tubercle.
The Assertion is correct because the $Clitoris$ is a distinct organ,not a vestigial or remnant structure of the male $penis$.
The Reason is also correct because the $Clitoris$ is composed of erectile tissue and possesses a rich blood supply,similar to the $penis$,which allows it to become engorged during sexual arousal.
Therefore,both statements are correct,but the Reason does not explain why the $Clitoris$ is not a remnant of the $penis$.

(D) The Assertion is incorrect because $Hyaluronidase$ is an enzyme found in the acrosome of the sperm,not the ovum. It helps the sperm penetrate the layers surrounding the ovum during fertilization.
The Reason is also incorrect. Mammalian eggs are $alecithal$ (or $microlecithal$ in some contexts,but not $telolecithal$). $Telolecithal$ eggs,such as those of birds and reptiles,have a large amount of yolk concentrated at one pole. Mammalian eggs contain negligible amounts of yolk due to their placental development.
Therefore,both the Assertion and the Reason are incorrect.

(C) $GIFT$ stands for Gamete Intra Fallopian Transfer.
It is an assisted reproductive technology used for women who cannot produce ova but can provide a suitable environment for fertilization and further development.
In this procedure,an ovum is collected from a donor and transferred into the fallopian tube of the recipient female.
Fertilization and subsequent embryo development occur naturally within the fallopian tube of the recipient.

(C) $Cu-T$ and $Cu-7$ are copper-releasing intrauterine devices $(IUDs)$.
They do not suppress sperm motility; instead,they release copper ions $(Cu^{2+})$ which suppress sperm motility and fertilizing capacity of sperms.
However,the assertion states they do not suppress sperm motility,which is scientifically incorrect because copper ions are known to reduce sperm motility.
Furthermore,the reason is incorrect because these devices do not release any hormones.
Therefore,both the Assertion and the Reason are incorrect.

(C) In birds,sex is determined by a morphologically dissimilar pair of chromosomes called sex chromosomes.
$Z$ and $W$ are the two sex chromosomes found in birds.
$A$ male bird has a $ZZ$ (homogametic) arrangement,while a female bird has a $ZW$ (heterogametic) arrangement of chromosomes.
Therefore,the female bird has the $ZW$ chromosome arrangement.

(B) Gene pool is defined as the sum total of all genes and their alleles present in a population at a given time.
It represents the complete set of genetic information available to the individuals of a population.
These genes are passed from one generation to the next through gametes.
The frequency of these alleles within the gene pool determines the genetic structure of the population.

(D) The inheritance of blood groups is determined by multiple alleles. For a child to have blood group '$O$' (genotype $ii$),both parents must contribute an 'i' allele.
Since both parents have blood group '$A$',their genotypes must be $I^A I^A$ or $I^A i$.
If a child has blood group '$O$' $(ii)$,it implies that both parents must carry the recessive 'i' allele.
Therefore,both parents must be heterozygotic for blood group '$A$',with the genotype $I^A i$.
When both parents are $I^A i$,the possible genotypes for the offspring are $I^A I^A$ (blood group $A$),$I^A i$ (blood group $A$),and $ii$ (blood group $O$).

(B) The figure represents a $DNA$ replication fork. $DNA$ replication always occurs in the $5' \rightarrow 3'$ direction.
In the provided diagram,the polarity of the template strands is incorrectly labeled relative to the replication fork.
Specifically,the template strand with $3'$ end at the fork should be the one where the leading strand is synthesized continuously in the $5' \rightarrow 3'$ direction.
Since the strands are antiparallel,the orientation of the $3'$ and $5'$ ends at the replication fork must be consistent with the direction of synthesis for both the leading and lagging strands.
Therefore,the polarity indicated in the diagram is incorrect.

(D) Comparative anatomy is the study of similarities and differences in the structures of different organisms. Homologous structures,which share a common evolutionary origin but may have different functions,are a key focus of this field.
Comparative cytology involves the examination of similarities in the cellular structures of different organisms.
Biochemistry compares $DNA$ sequences and proteins to determine evolutionary relationships.
Geology is the study of the Earth,its physical structure,and the processes acting upon it.

(B) During his voyage on the ship $H.M.S.$ Beagle,Charles Darwin visited the Galapagos Islands.
He observed a variety of small black birds,now known as Darwin's finches.
He realized that there were many varieties of finches in the same island.
All the varieties,he conjectured,evolved on the island itself from the original seed-eating features.
From the original insectivorous features,many other forms with altered beaks arose,enabling them to become insectivorous and vegetarian finches.
This process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats) is called adaptive radiation.
Therefore,Darwin's finches are a classic example of adaptive radiation.

(C) The primitive atmosphere was composed of light atoms like hydrogen,which were highly reactive. These hydrogen atoms combined with all available oxygen atoms to form water $(H_2O)$,leaving no free oxygen in the atmosphere. Therefore,the primitive atmosphere was reducing in nature.
The formation of the ozone layer $(O_3)$ is a result of the modern oxidizing atmosphere,which contains abundant free oxygen produced by photosynthetic organisms. In the primitive atmosphere,there was no free oxygen available to form ozone. Thus,the Reason is incorrect.

(B) The fossil of Java Ape-man was discovered from Pleistocene rocks in central Java. The fossil of Peking man was discovered from the limestone caves of Choukoutien near Peking, while that of Heidelberg man was discovered in mid-Pleistocene deposits. All these three fossils are classified under the category of $Homo \text{ } erectus$.
$Homo \text{ } erectus$ appeared about $1.7$ million years ago during the middle Pleistocene epoch. It is widely accepted that $H. \text{ } erectus$ evolved from $Homo \text{ } habilis$.
$H. \text{ } erectus$ individuals were about $1.5-1.8$ meters tall and possessed an erect posture. Their skulls were flatter than those of modern humans. They had protruding jaws, prominent brow ridges, small canines, and large molar teeth. They crafted more elaborate tools from stones and bones, hunted large animals, and likely possessed the knowledge of using fire.

(A) The $Y$-shaped antibody molecule is composed of $4$ polypeptide chains: two identical light chains and two identical heavy chains,held together by disulphide bonds.
The antigen-binding site (paratope) is formed at the $N$-terminal end of the antibody molecule,where the variable region of a heavy chain and the variable region of a light chain come together.
Therefore,the antigen-binding site involves both the light chain and the heavy chain.

(D) $Cannabis \text{ } sativa$ (Hemp) is an annual herb belonging to the family $Cannabaceae$. It is primarily known as a source of fiber.
$Cannabis \text{ } sativa$ is not an antidepressant. Antidepressants are psychiatric medications used to treat mood disorders like major depression.
Hashish and Marijuana are indeed drugs derived from this plant. Marijuana is prepared from the dried flowers and leaves, while Hashish is prepared from the resinous extract.
Both substances are hallucinogens that affect the central nervous system and can cause acute panic and anxiety reactions, rather than acting as antidepressants.
Therefore, the Assertion is incorrect, but the Reason is correct.

(A) Developed in $1984$ by Kary Mullis,$PCR$ is now a common and often indispensable technique used in medical and biological research labs for a variety of applications.
These include $DNA$ cloning for sequencing,$DNA$-based phylogeny,or functional analysis of genes; the diagnosis of hereditary diseases; the identification of genetic fingerprints (used in forensic sciences and paternity testing); and the detection and diagnosis of infectious diseases.
In $1993$,Mullis won the Nobel Prize in Chemistry for his work on $PCR$.

(C) Restriction enzymes,a type of endonuclease,function by inspecting the length of a $DNA$ sequence.
Once they find a specific recognition sequence,they bind to it and cut each of the two strands of the double helix at specific points,leaving single-stranded portions at the ends.
This results in overhanging stretches called sticky ends.
These are named so because they form hydrogen bonds with their complementary counterparts; i.e.,they can join similar complementary ends of $DNA$ fragments from other sources with the help of the enzyme $DNA$ ligase.
Therefore,the stickiness of the ends facilitates the action of the enzyme $DNA$ ligase,not $DNA$ polymerase.
Thus,the Assertion is correct,but the Reason is incorrect.

(B) Carrying capacity is defined as the maximum number of individuals of a population that can be supported by the available resources in a given habitat.
Beyond this limit,the environment cannot sustain additional individuals,and therefore,no further population growth is possible.
When a population reaches its carrying capacity,the resources become limited,often leading to a state where mortality exceeds or equals natality,preventing further increase.

(C) For an ecosystem to be self-sustaining,it must maintain a balance where essential nutrients and materials such as oxygen,carbon dioxide,water,and nitrogen are continuously recycled between the biotic components (organisms) and the abiotic environment. This process of nutrient cycling ensures that resources are not depleted,allowing the ecosystem to function independently.

(B) The graph illustrates a classic case of competitive exclusion. As population $B$ increases,population $A$ decreases over time. This suggests that both populations are competing for the same limited resource (e.g.,grass). Since population $B$ is better adapted or more efficient at utilizing this resource,it outcompetes population $A$,leading to a decline in the number of individuals in population $A$.

(C) scorpion is a $predator$ because it stalks,kills,and eats its prey.
Since it feeds on a spider (another animal),it is classified as a $carnivore$.
Because it ingests organic food rather than producing it,it is a $consumer$.
In contrast,a $producer$ is an $autotroph$ that synthesizes its own food from inorganic substances,a $decomposer$ breaks down dead organic matter,and a $herbivore$ feeds exclusively on plants.

(A) Ecological succession is the process by which the structure of a biological community evolves over time.
Primary succession occurs in areas where no living organisms have previously existed,such as bare rock or newly cooled lava.
Secondary succession occurs in areas where a pre-existing community has been destroyed or removed,such as after a forest fire,flood,or deforestation.
These areas still contain soil,organic debris,and biological propagules (seeds,spores,etc.) from the previous community.
The assertion is correct because secondary succession indeed occurs in recently denuded areas.
The reason is also correct because the process of secondary succession is initiated precisely because an area has been 'bared' (cleared) of its previous vegetation or community due to disturbances like fire or human activity.
Therefore,the reason correctly explains why secondary succession occurs in such areas.

(A) Excess of nitrates in drinking water is harmful to human health and can be fatal for infants. Excessive use of fertilizers often leads to the accumulation of nitrates in water sources. In infants, excess nitrate reacts with haemoglobin to form non-functional methaemoglobin, which impairs oxygen transport in the blood. This condition is clinically termed as methaemoglobinemia or $blue \text{ } baby \text{ } syndrome$. This disease can damage the respiratory and vascular systems and may lead to severe suffocation.