AIIMS 1997 Biology Question Paper with Answer and Solution

(B) $Plasmogamy$ is the first stage of sexual reproduction in fungi,in which the cytoplasm of two motile or non-motile gametes fuses with each other.
In this process,the nuclei of the sex cells come close to each other but do not fuse immediately.
Thus,the resulting cell contains two nuclei per cell,which is known as a $dikaryon$ or $binucleate$ stage.

(D) Yeast are facultative aerobes,meaning they are primarily anaerobic but can also survive and respire under aerobic conditions. Therefore,they exhibit both aerobic and anaerobic respiration depending on the availability of oxygen.

(B) The sago obtained from the stem of $Cycas$ is a type of starch.
It is recommended for patients with stomach disorders because it is easily digestible and contains a lower percentage of starch (approximately $31\%$) compared to other sources,making it gentle on the digestive system.

(D) The corpora allata are endocrine glands located in the head of insects,typically attached to the brain.
These glands are responsible for secreting the juvenile hormone.
The juvenile hormone plays a crucial role in regulating insect development by preventing metamorphosis into the adult stage during early larval stages,thereby prolonging the larval period.

(B) The correct answer is $(b)$.
In $Lathyrus$ $aphaca$ (Wild pea),the entire leaf is modified into slender,wiry,and coiled structures known as leaf tendrils.
These tendrils help the plant in climbing by providing support to the weak stem.

(B) Lodicules are modified tepals $(Perianth)$.
In the family Poaceae, to which $Triticum$ $aestivum$ (wheat) belongs, the perianth is represented by $2$ small, hyaline, membranous, antero-lateral scales known as lodicules.
These structures are responsible for the opening of the floret by absorbing water and swelling at the time of anthesis.

(D) Submerged hydrophytes are plants that grow entirely underwater.
Since they are completely submerged,they lack functional stomata because stomata would be ineffective for gas exchange in an aquatic environment.
Instead,these plants absorb dissolved gases like $CO_2$ and $O_2$ directly from the surrounding water through their general body surface via diffusion.
Therefore,the correct answer is the general surface.

(D) The Assertion is incorrect because many cold-blooded animals (ectotherms) do possess fat layers or fat bodies for energy storage.
The Reason is correct because cold-blooded animals,such as certain reptiles and amphibians,rely on stored fat reserves to sustain metabolic activities during periods of dormancy like hibernation or aestivation.
Therefore,the Assertion is incorrect,but the Reason is correct.

(A) Acraniata (also known as Protochordata) is a group of chordates that lack a distinct cranium (brain box) and a vertebral column.
These organisms are exclusively marine and lack a well-defined head region,which is why they are often referred to as 'acraniates'.
Since the absence of a cranium is directly linked to the lack of a developed head region in these primitive chordates,the Reason correctly explains the Assertion.

(C) Plants do lack specialized excretory organs because their metabolic rate is low and they produce fewer nitrogenous wastes compared to animals.
However,the Reason provided is incorrect because plants do not lead a 'passive life' in a way that explains the absence of excretory organs.
Plants manage waste products through various mechanisms: carbon dioxide produced during respiration is released through stomata,and other metabolic wastes are stored in vacuoles,leaves,bark,or secreted as alkaloids,gums,and resins.

(C) The Assertion is correct because submerged hydrophytes do not have stomata as they are not required for gaseous exchange in an aquatic environment.
The Reason is incorrect because,in submerged plants,respiration and gaseous exchange occur through the general body surface via diffusion,not specifically through air chambers. Air chambers (aerenchyma) are primarily involved in buoyancy and gas storage,not the primary mechanism for respiration.

(A) Leguminous plants are known as nitrogen fixers because they establish a symbiotic relationship with nitrogen-fixing bacteria.
These bacteria,such as $Rhizobium$,reside in the root nodules of leguminous plants.
The bacteria fix atmospheric nitrogen into ammonia,which the plant can utilize for growth.
Therefore,the presence of $Rhizobium$ in the root nodules is the direct reason why these plants are capable of nitrogen fixation.
Thus,both the Assertion and the Reason are correct,and the Reason is the correct explanation of the Assertion.

(D) Eugenics is the branch of biology that deals with the improvement of the human race by controlling heredity through selective breeding.
Euthenics refers to the improvement of human well-being by modifying the environment.
Euphenics refers to the improvement of the human phenotype through medical or genetic intervention.
Obstetrics is the field of medicine focused on pregnancy,childbirth,and the postpartum period.

(B) Psychiatry is the branch of medicine focused on the diagnosis,treatment,and prevention of mental,emotional,and behavioral disorders.
Neurology deals with disorders of the nervous system.
Psychology is the scientific study of the mind and behavior,which does not primarily involve medical treatment of disorders.
Neuropsychiatry is a branch of medicine that deals with mental disorders attributable to diseases of the nervous system.

(D) The presence of a $Barr$ body is determined by the formula $N - 1$,where $N$ is the number of $X$ chromosomes. $A$ normal male $(XY)$ has $0$ $Barr$ bodies,and a normal female $(XX)$ has $1$ $Barr$ body.
An extra $Barr$ body in an embryo indicates the presence of an additional $X$ chromosome.
$Klinefelter$ syndrome is characterized by the genotype $XXY$.
In this case,the number of $Barr$ bodies is $2 - 1 = 1$. However,if the question implies an 'extra' $Barr$ body compared to a normal male embryo (which has $0$),or if it refers to a condition like $XXX$ (Triple $X$ syndrome) or $XXY$ ($Klinefelter$ syndrome),$Klinefelter$ syndrome is the most standard clinical association for an extra $X$ chromosome in a male-developing embryo.

(A) The first mammal to be cloned from an adult somatic cell was a sheep named $Dolly$.
This historic scientific achievement was accomplished by $Ian \ Wilmut$ and his colleagues at the $Roslin \ Institute$ in $Scotland$ in $1996$.
Therefore,the correct option is $A$.

(B) . Eugenics is the branch of science that deals with the improvement of the human race genetically.
This aspect of human betterment aims to improve the human germplasm by encouraging the inheritance of desirable characteristics so that defective traits may be eliminated.

(C) $Gambusia$ is a carnivorous fish. It feeds on the larvae of $Anopheles$ mosquitoes. This fish is widely used in the biological control of mosquitoes to prevent the spread of diseases like malaria.

(A) $Bacillus$ $thuringiensis$ (often abbreviated as $Bt$) is a soil-dwelling bacterium that produces insecticidal crystal proteins ($Cry$ proteins).
These proteins are toxic to specific insect larvae, such as those of lepidopterans, coleopterans, and dipterans, making it a widely used microbial biocontrol agent in agriculture.

(C) The correct answer is $C$.
Chaparral is a biome characterized by broad-leaved evergreen shrubs and small trees with hard,thick leaves.
These plants often contain resins,which makes them highly flammable,yet they have evolved mechanisms to be fire-resistant or to recover quickly after fires.
Additionally,both the flora and fauna in this biome are well-adapted to survive frequent and prolonged periods of drought.

(B) The $CO_2$ concentration in the atmosphere was approximately $270$ ppm around the year $1750$ and rose to about $368$ ppm by the year $2000$. This significant increase is primarily attributed to large-scale deforestation (for grazing land,cropland,or urban development),changes in land use,and the extensive combustion of fossil fuels.

(A) Chaparral vegetation is a type of biome characterized by dense,evergreen,sclerophyllous (hard-leaved) shrubs and small trees.
These plants are often adapted to Mediterranean climates with hot,dry summers and mild,wet winters.
Many of these species are fire-resistant or have adaptations to recover quickly after fire,often containing resinous compounds that make them flammable but also resilient.

(D) The plant species $Centella$,$Podophyllum$,and $Gloriosa$ are all extensively used for medicinal purposes. Due to human activities and over-exploitation,these plants are on the verge of extinction in their natural habitats. Therefore,the correct answer is 'All of the above'.

(B) Stilbesterol is a synthetic estrogen. In dairy farming,it is sometimes used to induce lactation or increase milk yield in cows by stimulating the mammary glands. However,its use is strictly regulated or banned in many countries due to potential health risks to consumers.

(A) Positron Emission Tomography $(PET)$ scanning is a functional imaging technique that uses radioactive tracers to map metabolic activity in the brain.
Unlike $CT$ or $MRI$ (which primarily provide structural information),$PET$ scans can detect changes in blood flow and glucose metabolism associated with specific neural activities.
Therefore,scientists use $PET$ scanning to identify the active regions of the human visual cortex involved in colour processing and perception.

(A) Plant cuticles are protective waxy coverings produced by the epidermal cells of leaves,young shoots,and other aerial plant organs.
Beyond acting as a permeability barrier for water and other molecules,the micro- and nanostructure of the cuticle provides specialized surface properties that prevent the contamination of plant tissues by external water,dirt,and microorganisms.
The waxy layer of the cuticle also functions in defense by forming a physical barrier that resists penetration by virus particles,bacterial cells,and the spores or growing filaments of fungi.
Therefore,a thicker cuticle provides enhanced resistance against pathogens,making the Assertion and Reason both correct,and the Reason serves as the correct explanation for the Assertion.

(A) Pollen mother cells $(PMC)$ are diploid $(2n)$ cells that undergo meiosis to produce microspores or pollen grains.
Meiosis is a reductional division,which reduces the chromosome number to half $(n)$.
If the $PMC$ has $42$ chromosomes $(2n = 42)$,then the pollen grains,being haploid,will have $n = 42/2 = 21$ chromosomes.
Therefore,both the Assertion and the Reason are correct,and the Reason is the correct explanation of the Assertion.

(B) Leydig cells,also known as interstitial cells,are located in the interstitial spaces between the seminiferous tubules of the testes in mammals.
These cells are responsible for the synthesis and secretion of testicular hormones called androgens,primarily testosterone.
Testosterone plays a crucial role in the development of secondary sexual characteristics in males and the regulation of spermatogenesis.
Since the rabbit is a mammal,Leydig cells are present in its testes.

(C) During the process of fertilization,the sperm head (containing the nucleus) enters the egg cytoplasm,while the tail and middle piece (containing mitochondria) are usually left outside.
Therefore,the Assertion is correct.
However,the Reason is incorrect because the egg has mechanisms to prevent polyspermy (the entry of more than one sperm). Once the first sperm enters the egg,the zona pellucida undergoes changes to block the entry of other sperm,ensuring that only one sperm fertilizes the egg.

(A) During the process of cleavage,the zygote undergoes repeated mitotic divisions to form a solid ball of $8-16$ cells called a morula.
In this stage,the cells divide without any increase in the overall size of the embryo because the cytoplasm does not grow between divisions.
The embryo remains enclosed within the $Zona \ pellucida$ throughout the cleavage process until it reaches the blastocyst stage.
Therefore,both the Assertion and the Reason are correct,and the Reason explains why the embryo does not increase in size during cleavage.