AIPMT 1992 Biology Question Paper with Answer and Solution

(D) taxon is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit.
In biological classification,taxonomic categories such as Kingdom,Phylum,Class,Order,Family,Genus,and Species represent different levels of the taxonomic hierarchy.
Each of these categories represents a rank and is commonly referred to as a taxon (plural: taxa).
Since Genera,Family,and Class are all levels within the taxonomic hierarchy,they are all considered taxa.

(D) In biological classification,a $Taxon$ (plural: $Taxa$) represents a group of organisms of any rank that share similar traits or characteristics. It is a fundamental unit of classification used in taxonomy to categorize living beings at various levels,such as species,genus,family,order,class,phylum,or kingdom.

(A) The term "phylum" was introduced by the German biologist Ernst Haeckel in $1866$.
It was used to describe the primary divisions of the animal kingdom based on body plan and evolutionary relationships.

(C) The taxonomic hierarchy is a sequence of categories in decreasing order of similarity. The standard hierarchical arrangement is: Kingdom $\rightarrow$ Division (in plants) or Phylum (in animals) $\rightarrow$ Class $\rightarrow$ Order $\rightarrow$ Family $\rightarrow$ Tribe $\rightarrow$ Genus $\rightarrow$ Species. Option $C$ represents the correct sequence for plants,starting from the highest rank (Division) to the lowest rank (Species).

(A) According to the $NCERT$ textbook,the total number of species described and known to science is between $1.7$ to $1.8$ million. Out of these,the number of animal species is approximately $1.2$ million,while the number of plant species is about $0.5$ million. Therefore,the total number of species in the animal kingdom is about $1$ million.

(D) $Rhizopus$ and $Mucor$ belong to the class Zygomycetes within the kingdom Fungi.
In fungi,the reserve food material is typically stored in the form of glycogen and oil droplets.
Unlike plants,fungi do not store food as starch.
Therefore,the correct storage products in these organisms are glycogen and oil.

(C) Lichens are highly sensitive to air pollution,particularly to higher $SO_2$ concentrations in the atmosphere.
This $SO_2$ causes plasmolysis of algal cells and converts the chlorophyll of algae into pheophytin by removing the magnesium $(Mg)$ ion.
This process results in chlorosis and the eventual death of the lichen.

(B) In $Chlamydomonas$,the life cycle is haplontic. In this type of life cycle,the dominant phase is the free-living haploid gametophyte. The sporophytic generation is represented only by the single-celled zygote,which undergoes meiosis to produce haploid spores. Therefore,the zygote is the only diploid stage in its life cycle.

(A) $Ascon$ type is the simplest type of canal system found in $Porifera$.
In this type,the body wall is thin,and the water enters directly into the central cavity,known as the $spongocoel$,through numerous pores called $ostia$.
From the $spongocoel$,the water exits through a single large opening called the $osculum$.

(D) $Taenia$ is a hermaphrodite (monoecious) organism. In the strobila of $Taenia$,the proglottides are arranged in a sequence of development. The mature proglottides,which are found in the middle region of the body,contain fully developed sets of both male and female reproductive organs.

(B) $Anopheles$ and $Culex$ mosquitoes can be easily distinguished by their resting or sitting posture.
$Anopheles$ mosquitoes sit at an angle to the surface,appearing as if they are standing on their heads.
In contrast,$Culex$ mosquitoes sit parallel to the surface,keeping their bodies horizontal.

(A) Anadromous fishes are those that spend most of their adult lives in the sea but migrate to freshwater rivers or streams to spawn (breed).
Examples include $Salmon$ and $Lamprey$.
Therefore,the correct movement is from sea to freshwater.

(A) Cartilaginous fishes (Class $Chondrichthyes$) are characterized by a cartilaginous endoskeleton. $A$ key distinguishing feature of these fishes is the absence of an $operculum$ (gill cover) over their gill slits. Instead,they possess $5-7$ pairs of separate gill slits that open directly to the exterior. While they do possess placoid scales,pelvic fins,and gill slits,the operculum is absent.

(A) The common Indian bullfrog is scientifically known as $Rana \ tigrina$.
It belongs to the class $Amphibia$ and is widely distributed across the Indian subcontinent.
$Rana \ esculenta$ is the edible frog,$Rana \ silvatica$ is the wood frog,and $Rana \ cyanophlyctis$ is the skittering frog.

(A) The correct answer is $Ornithorhynchus$ (also known as the Platypus).
Male platypuses have spurs on their hind limbs that are connected to venom glands,which they use for defense or during mating competition.
$Echidna$ is a monotreme but is not venomous.
$Guinea$ $pig$ is a rodent and is not venomous.
$Snake$ is a reptile,not a mammal.

(D) The correct answer is $D$.
$1$. Epipetalous stamens are those where the stamens are attached to the petals.
$2$. Syngenesious stamens are those where the anthers are fused together while the filaments remain free.
$3$. These conditions are characteristic features of the family $Asteraceae$ (also known as $Compositae$).
$4$. In $Solanaceae$, stamens are epipetalous but not syngenesious.
$5$. In $Brassicaceae$, the condition is tetradynamous.
$6$. In $Fabaceae$, the condition is usually diadelphous.

(A) The family $Cruciferae$ (also known as $Brassicaceae$) is characterized by a bicarpellary,syncarpous,superior ovary that is unilocular but becomes bilocular due to the formation of a false septum called a $replum$. The characteristic fruit of this family is a $siliqua$ or $silicula$.
In contrast,the family $Malvaceae$ typically possesses a multicarpellary,syncarpous,and multilocular ovary. The fruit in $Malvaceae$ is usually a $capsule$ or $schizocarp$.
Therefore,$Cruciferae$ differs from $Malvaceae$ primarily in its specific ovary structure and the presence of $siliqua$ fruit.

(B) In a $Bicollateral$ vascular bundle,the phloem is present on both the outer and inner sides of the xylem. This type of arrangement is characteristic of the members of the $Cucurbitaceae$ family.

(A) Desmosomes are specialized adhesive protein complexes that occur at cell-cell junctions.
They are primarily found in epithelial tissues,where they provide mechanical strength by anchoring the intermediate filaments of the cytoskeleton to the plasma membrane,thereby holding adjacent cells together.
Therefore,the correct option is $A$.

(D) The areolar tissue is a type of loose connective tissue that acts as a support framework for epithelium.
It contains various types of cells,including fibroblasts,macrophages,mast cells,plasma cells,lymphocytes,and adipose cells.
Since all the options listed ($A$,$B$,and $C$) are present in the areolar tissue,the correct answer is $D$.

(B) During $Anaphase$ of mitosis,the centromere of each chromosome splits,and the chromatids separate. Each separated chromatid is now considered an independent chromosome. Therefore,each chromosome at this stage consists of $1$ chromatid.
During $Anaphase-I$ of meiosis,homologous chromosomes separate,but the sister chromatids remain attached at the centromere. Thus,each chromosome still consists of $2$ chromatids.
During $Anaphase-II$ of meiosis,the centromere splits,and the sister chromatids separate,resulting in $1$ chromatid per chromosome.
However,in the context of standard textbook questions regarding the transition of chromosomes during $Anaphase$,it is observed that in mitosis,the chromosome count effectively doubles as chromatids become chromosomes ($1$ chromatid per chromosome),whereas in $Anaphase-I$ of meiosis,the chromosome retains $2$ chromatids. Thus,the correct answer is $1$ in mitosis and $2$ in meiosis.

(B) According to the starch-sugar interconversion theory proposed by $Sayre$ $(1926)$,the opening and closing of stomata are regulated by the interconversion of starch and soluble sugars.
During the day,the $pH$ of the guard cells increases due to the consumption of $CO_2$ in photosynthesis.
This high $pH$ triggers the conversion of starch into organic acids (like malic acid) and glucose$-1-$phosphate.
This increases the osmotic concentration of the guard cells,leading to the endosmosis of water,which causes the guard cells to become turgid and the stomata to open.
Therefore,the conversion of starch to organic acid is essential for stomatal opening.

(C) In the evening,photosynthesis in leaves stops. Carbon dioxide concentration increases inside the leaf,which results in a decrease in $pH$. Glucose is phosphorylated to form glucose $6$-phosphate,which is then converted into glucose $1$-phosphate,from which starch is synthesized.
Starch is insoluble and does not exert any osmotic potential. Consequently,guard cells lose water to the nearby epidermal cells through osmosis.
As a result,the turgor pressure of the guard cells decreases,causing the stomatal pore to close.

(C) Plants synthesize carbohydrates in the form of glucose during photosynthesis. However,glucose is highly reactive and not suitable for long-distance transport. Therefore,it is converted into a non-reducing sugar,$Sucrose$,which is chemically stable and less reactive. $Sucrose$ is then loaded into the sieve tube elements of the phloem and transported to various sink organs of the plant.

(A) The transport of sugar (like glucose) across the cell membrane often occurs via secondary active transport,specifically through $Na^+$-glucose symporters.
In this mechanism,$Na^+$ ions move down their electrochemical gradient into the cell,and this energy is used to transport sugar molecules into the cell in the same direction.
Therefore,$Na^+$ flows in the direction of the sugar.

(D) In both photosynthesis and respiration,the synthesis of $ATP$ occurs via a process known as chemiosmosis or oxidative/photophosphorylation.
This process relies on an electron transport chain $(ETC)$.
As electrons move through a series of electron carriers (such as cytochromes),they lose energy.
This released energy is used to pump protons $(H^+)$ across a membrane,creating a proton gradient.
The dissipation of this gradient through $ATP$ synthase drives the phosphorylation of $ADP$ to $ATP$.
Therefore,the energy for $ATP$ synthesis is derived from the movement of electrons.

(D) Plastids are organelles found in plant cells that share a common developmental origin from proplastids.
Because they originate from the same precursor,they possess a similar basic structure.
Furthermore,plastids exhibit plasticity,meaning one type of plastid (e.g.,a chloroplast) can differentiate into another type (e.g.,a chromoplast or leucoplast) based on the specific physiological requirements of the plant cell.

(B) Photosystem-$II$ is a photosynthetic pigment system,along with some electron carriers,that is located in the appressed part of the grana thylakoids. Therefore,it is found in the grana of the chloroplast.

(A) Chlorophyll $a$ is the primary photosynthetic pigment present in all oxygenic photoautotrophs.
Oxygenic photosynthesis is performed by plants,algae,and cyanobacteria.
Non-oxygenic photosynthetic bacteria (such as green and purple sulfur bacteria) use bacteriochlorophyll instead of chlorophyll $a$.
Therefore,chlorophyll $a$ is found in all oxygen-releasing photosynthetic organisms.

(A) During aerobic respiration,the complete oxidation of one molecule of glucose $(C_6H_{12}O_6)$ results in the production of a net total of $38$ $ATP$ molecules in prokaryotes,while in most eukaryotic cells,the net yield is $36$ $ATP$ molecules due to the cost of transporting $NADH$ into the mitochondria via the shuttle system.
Standard $NCERT$ textbooks typically refer to the net gain of $38$ $ATP$ molecules as the theoretical maximum yield for the complete oxidation of one glucose molecule.
Therefore,the process involves the phosphorylation of $38$ $ADP$ molecules into $38$ $ATP$ molecules.

(B) Oxidative phosphorylation is the process in which $ATP$ is synthesized from $ADP$ and inorganic phosphate $(Pi)$ using the energy released during the oxidation of reduced coenzymes ($NADH$ and $FADH_2$) via the electron transport system $(ETS)$.
This process occurs in the inner mitochondrial membrane during aerobic respiration.
Therefore,it is the formation of $ATP$ in respiration.

(B) In the electron transport system $(ETS)$,the terminal cytochrome is $Cyt$ $a_3$.
$Cyt$ $a_3$ contains both iron and copper centers.
It receives electrons from $Cyt$ $a$ and transfers them to oxygen,which is the final electron acceptor.
Because it is the final component in the chain that passes electrons to oxygen,it is known as the terminal cytochrome.

(D) Artificial ripening is a process used to accelerate the maturation of climacteric fruits,which continue to ripen after being harvested. Fruits like $Mango$,$Banana$,and $Grapes$ are climacteric and respond to ethylene treatment. However,$Pomegranate$ and $Coconut$ are non-climacteric fruits. Non-climacteric fruits do not produce significant amounts of ethylene and do not ripen further once harvested; therefore,artificial ripening treatments are ineffective and useless for them.

(C) Abscisic acid $(ABA)$ acts as a stress hormone in plants.
When plants experience water stress,$ABA$ is synthesized in the leaves.
It inhibits the $H^+/K^+$ exchange mechanism in the guard cells and promotes the efflux of potassium ions $(K^+)$ and malate.
This reduction in osmotically active solutes causes water to move out of the guard cells,making them flaccid.
As a result,the stomatal pore closes to prevent further transpiration.

(C) In short day plants $(SDP)$,the dark period is critical and must be continuous for flowering to occur.
If this dark period is interrupted even by a brief exposure to light,the plant will not flower.
Therefore,flowering in $(SDP)$ is induced by a photoperiod shorter than the critical value and an uninterrupted long night.

(B) The correct answer is $B$. Vernalization is the process of inducing flowering in plants by exposing them to low temperatures for a specific period. This practice allows plants that typically require a cold season to flower during the summer. During this process,a hypothetical hormone-like substance called 'vernalin' is produced,which is responsible for triggering the flowering response.

(C) Chief cells (also known as Zymogen cells or Peptic cells) are primarily located in the basal region of the gastric glands in the stomach. These cells are responsible for the secretion of proenzymes like pepsinogen and prorennin,which are essential components of gastric juice.

(C) Brunner's glands are specialized tubular glands located in the $Submucosa$ of the $Duodenum$.
These glands secrete an alkaline fluid containing mucus,which helps to neutralize the acidic chyme coming from the stomach and protects the intestinal wall from damage.

(A) The gall bladder stores and concentrates bile juice produced by the liver.
Bile juice contains bile salts which are essential for the emulsification of fats.
Emulsification breaks down large fat globules into smaller droplets,increasing the surface area for the action of the enzyme lipase.
If the gall bladder is surgically removed (cholecystectomy),the bile is no longer stored or concentrated; instead,it flows directly into the small intestine.
While the liver continues to produce bile,the lack of concentrated bile storage leads to a reduced efficiency in the digestion and absorption of dietary fats,particularly after a high-fat meal.

(D) In humans,cellulose is not digested at all. This is because the human alimentary canal lacks the enzyme cellulase,which is required to break down the complex polysaccharide cellulose. Furthermore,humans do not possess the symbiotic micro-organisms in their digestive tract that are capable of fermenting cellulose,unlike ruminant animals.

(C) The term "vitamin" was coined by Casimir Funk in $1912$. He derived it from the words "vita" (life) and "amine" (as he initially believed all vitamins were amines). Therefore, option $C$ is the correct answer.

(A) Vitamin $A$ (retinol) is essential for the formation of rhodopsin,a pigment in the rod cells of the retina that is responsible for vision in low light.
Deficiency of vitamin $A$ leads to nyctalopia (night blindness),which is the inability to see clearly in dim light.
Severe or prolonged deficiency leads to xerophthalmia,a condition characterized by dryness of the conjunctiva and cornea,which can eventually lead to blindness.

(D) Serotonin $(5-HT)$ is a neurotransmitter that also acts as a potent vaso-constrictor. When released from platelets during blood vessel injury,it causes the smooth muscles of the blood vessels to contract,thereby reducing blood flow to the site of injury. Therefore,the correct option is $D$.

(C) During ventricular systole,the pressure increases in the ventricles. This contraction forces the deoxygenated blood from the right ventricle into the pulmonary artery,which carries it to the lungs for oxygenation. Simultaneously,oxygenated blood from the left ventricle is forced into the systemic aorta.

(A) In haemoglobin,the iron atom is present in the ferrous $(Fe^{2+})$ state.
This ferrous iron is essential for the reversible binding of oxygen to form oxyhaemoglobin.
If the iron is oxidized to the ferric $(Fe^{3+})$ state,the haemoglobin becomes methaemoglobin,which is unable to bind oxygen effectively.

(D) The correct answer is $D$.
Nephrons are the structural and functional units of the kidney.
Each human kidney contains approximately $1$ million $(10^6)$ nephrons.

(B) The correct answer is $B$.
Urine is a transparent,light yellow liquid with a slightly acidic $pH$ (average $pH \ 6.0$).
The yellow colour of urine is caused by the pigment urochrome.
Urochrome is a breakdown product of haemoglobin derived from worn-out red blood corpuscles $(RBCs)$.

(C) The glomerular filtration rate $(GFR)$ is the amount of filtrate formed by the kidneys per minute.
In a healthy adult,the $GFR$ is approximately $125 \text{ ml/minute}$,which is commonly rounded to $120 \text{ ml/minute}$.
This results in a total of about $180 \text{ liters}$ of filtrate formed per day.

(B) The protein present in the bone matrix is known as $Ossein$.
$Ossein$ is a collagenous protein that provides structural support and flexibility to the bone tissue.
It plays a crucial role in the process of bone formation and mineralization.

(D) The correct answer is $D$. The $Cerebrum$ is the largest part of the human brain and is responsible for higher-order functions such as memory,intelligence,consciousness,reasoning,and voluntary actions. If a person loses their memory due to an accident,it indicates that the $Cerebrum$ has been injured.

(A) The life cycle of $Plasmodium$ is digenetic,involving two hosts: humans and female $Anopheles$ mosquitoes.
In the human host,the parasite undergoes asexual reproduction (schizogony and gametogony).
In the female $Anopheles$ mosquito,the parasite undergoes the sexual cycle,which includes fertilization and the formation of the zygote (ookinete) and sporozoites.
Therefore,the sexual cycle occurs within the female $Anopheles$ mosquito.

(A) Saffron is a spice derived from the flower of $Crocus \text{ } sativus$.
Specifically, the dried dark red stigmas and the top portion of the style are collected and used as the spice known as saffron.
These parts are carefully hand-picked from the flower, making saffron one of the most expensive spices in the world.

(D) $Sertoli$ cells are specialized cells found within the seminiferous tubules of the testes.
They are also known as nurse cells because they provide structural and metabolic support to the developing germ cells (spermatids) during spermatogenesis.
Unlike germinal cells which undergo meiosis to form sperm,$Sertoli$ cells are somatic in nature and do not produce gametes themselves.

(B) Teratology is the branch of embryology that deals with the study of abnormalities of physiological development,specifically congenital malformations and birth defects in the embryo or fetus.

(D) The human female is homogametic,meaning she produces gametes with the sex chromosome constitution $22 + X$. When an egg is released from the ovary,it is a haploid cell containing $22$ autosomes and one $X$ chromosome. Therefore,the correct option is $(d)$.

(B) Epiboly is a morphogenetic movement during gastrulation in which a sheet of cells (micromeres) spreads over the surface of the embryo.
In this process,the micromeres divide rapidly and migrate downwards to cover the larger cells (megameres) of the vegetal hemisphere,except for a small area known as the yolk plug.
Therefore,option $B$ is the correct description of epiboly.

(C) The blood group $O$ is represented by the genotype $I^O I^O$.
Since the child inherits one allele from each parent,the child must have received one $I^O$ allele from the father and one $I^O$ allele from the mother.
Given that the father has blood group $B$,his possible genotypes are $I^B I^B$ (homozygous) or $I^B I^O$ (heterozygous).
Since the father must contribute an $I^O$ allele to the child to result in an $O$ blood group child,the father must be heterozygous for blood group $B$,which is $I^B I^O$.

(C) According to the British geneticist Mary Lyon $(1961)$,one of the two $X$-chromosomes of a normal female becomes heterochromatic and appears as a Barr body. This phenomenon is known as Lyonization or $X$-inactivation. Therefore,Barr bodies are associated with the female sex chromosome.

(D) gene is defined as dominant if it expresses its phenotypic effect in both the homozygous $(AA)$ and heterozygous $(Aa)$ conditions. In a heterozygous state,the dominant allele masks the expression of the recessive allele.

(B) Chicken pox is a highly contagious viral infection caused by the $Varicella-zoster$ virus $(VZV)$.
It is characterized by an itchy,blister-like rash on the skin.
The virus spreads easily through respiratory droplets or direct contact with the fluid from the blisters.

(C) $Glossina \ palpalis$ (tse-tse fly) acts as a vector for the transmission of $Trypanosoma \ gambiense$.
$Trypanosoma \ gambiense$ is the causative agent of Gambian sleeping sickness,also known as Gambian fever.
Therefore,the correct option is $C$.

(D) The housefly $(Musca \text{ domestica})$ acts as a mechanical vector for several pathogens, including those causing $Leprosy$, $Dysentery$, and $Typhoid$. However, $Sleeping \text{ sickness}$ (African trypanosomiasis) is caused by the protozoan $Trypanosoma \text{ brucei}$ and is transmitted by the $Tse-tse \text{ fly}$ $(Glossina \text{ species})$, not the housefly. Therefore, the correct option is $D$.

(C) Amoebiasis,also known as amoebic dysentery,is an infection of the intestinal tract caused by the protozoan parasite $Entamoeba \ histolytica$.
This parasite is transmitted through the fecal-oral route,typically by consuming contaminated food or water.
It infects the large intestine,leading to symptoms such as abdominal pain,diarrhea,and stools with mucus and blood.

(A) The rearing of silkworms for the production of silk is known as $Sericulture$.
$Pisciculture$ refers to the breeding,rearing,and transplantation of fish.
$Apiculture$ is the maintenance of honey bee colonies,commonly in hives,by humans.
$Horticulture$ is the branch of agriculture that deals with the art,science,technology,and business of growing plants.

(A) Petroleum is a fossil fuel formed over millions of years from the remains of ancient marine organisms. Because its rate of consumption is significantly faster than its rate of formation, it cannot be replenished on a human timescale. Therefore, it is classified as a $Non-renewable$ source of energy.

(C) Minerals and metals are generally considered non-renewable because their formation takes millions of years. However,they can be recycled. While some minerals and metals have very long recycling cycles,others can be recovered and reused,making them technically renewable through human intervention and recycling processes. Therefore,they are categorized as both renewable and non-renewable depending on the context of recycling and geological time scales.

(D) Soil fertility is defined as the capacity of the soil to provide essential nutrients in adequate amounts and in proper proportions for the growth of plants.
While soil structure,water-holding capacity,and organic matter content are important physical and chemical properties,the primary measure of soil fertility is its ability to supply nutrients to support plant growth and sustain life.

(C) Water is considered a renewable resource because it is continuously recycled through the hydrological cycle (water cycle). Through processes like evaporation,condensation,and precipitation,water is naturally replenished in the environment,allowing it to be used repeatedly over time.

(B) The correct answer is $B$.
$CO_2$ is a naturally occurring gas in the atmosphere and is essential for the process of photosynthesis in plants.
It is not considered a pollutant in normal concentrations as it is non-toxic to humans and animals.
In contrast,$CO$,$SO_2$,and hydrocarbons are harmful substances that degrade air quality and are classified as atmospheric pollutants.

(C) The correct answer is $C$. In India,wheat is one of the most important food crops and covers a significant portion of the total cultivated land,approximately $29$ million hectares (often cited as $29$ million acres in older texts). Among the given options,wheat occupies the largest area under cultivation.

(B) The common name "Indian Rye" is often used to refer to $Brassica \, juncea$, which is widely known as Indian mustard or Rai in India. It belongs to the family $Brassicaceae$ and is an important oilseed crop.

(B) Turpentine is a volatile essential oil obtained by the distillation of resin collected from the wood of various species of coniferous trees,primarily those in the genus $Pinus$ (e.g.,$Pinus \, longifolia$).
$Pinus$ belongs to the family $Pinaceae$,which is a group of gymnosperms.
Therefore,the correct source of turpentine is the wood of gymnosperms.

(B) $Essential$ oils are aromatic in nature and they evaporate upon contact with air. These oils are primarily used in the preparation of perfumes,flavoring agents,and medicines.

(B) $Linum$ $usitatissimum$ (Flax) is a plant that provides both fibre (from the stem) and oil (from the seeds).
$Brassica$ (Mustard) is primarily an oil-yielding plant.
$Gossypium$ (Cotton) is primarily a fibre-yielding plant.
Therefore,$Brassica$ and $Linum$ $usitatissimum$ are the correct pair as they are both associated with oil production,and $Linum$ additionally provides fibre.

(D) $MET$ stands for Magnetoencephalography. It is a non-invasive neuroimaging technique for mapping brain activity by recording magnetic fields produced by electrical currents occurring naturally in the brain. It is primarily used to investigate brain health and diagnose various brain diseases such as epilepsy,tumors,and functional brain disorders. Therefore,both $(a)$ and $(b)$ are correct.

(B) Petroleum is a fossil fuel formed over millions of years from the remains of ancient marine organisms.
Because it takes such a vast amount of time to form and is being consumed at a rate much faster than it can be replenished,it is classified as a non-renewable resource.
Once the existing reserves are depleted,they cannot be replaced within a human timescale.

(A) The life cycle of the malaria parasite $Plasmodium$ involves two hosts: humans and the female $Anopheles$ mosquito.
In the human host,the parasite undergoes asexual reproduction (schizogony).
In the female $Anopheles$ mosquito,the parasite undergoes sexual reproduction (gametogony and sporogony).
Therefore,the stage that occurs within the mosquito is the sexual cycle.

(B) Adult mosquitoes of the genera $Culex$ and $Anopheles$ can be distinguished primarily by their resting position on a surface.
$Anopheles$ mosquitoes rest at an angle,with their body held at an acute angle to the surface,often appearing as if they are standing on their head.
In contrast,$Culex$ mosquitoes rest with their body parallel to the surface,appearing hunched or horizontal.

(C) Meiosis is a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell. It is specifically associated with sexual reproduction. In flowering plants,meiosis occurs in the microspore mother cells $(MMC)$ within the anther to produce microspores (pollen grains). Apical and lateral meristems undergo mitosis for growth,and the anther wall cells also undergo mitosis for development and protection. Therefore,the best site to observe meiosis is the microspore mother cells.

(A) The human eye lens is formed during embryonic development from the surface ectoderm. Specifically,the lens placode,which is a thickening of the surface ectoderm,invaginates to form the lens vesicle,which eventually differentiates into the mature lens. Therefore,the correct germ layer is the ectoderm.

(A) Cleavage is the series of mitotic divisions of the zygote.
$1$. $Holoblastic$ cleavage refers to complete cleavage where the entire egg is divided.
$2$. $Meroblastic$ cleavage refers to incomplete or partial cleavage,which occurs in eggs with a large amount of yolk (e.g.,birds,reptiles).
Therefore,incomplete cleavage is synonymous with $Meroblastic$ cleavage.

(A) Epistasis is a genetic interaction where the expression of one gene is modified or masked by the presence of one or more other genes.
In this phenomenon,the gene that suppresses the effect of another gene is called the epistatic gene,and the gene whose effect is suppressed is called the hypostatic gene.
Unlike dominance,which involves the interaction between alleles of the same gene,epistasis involves the interaction between different gene loci.

(A) According to the law of dominance,a dominant allele is one that expresses its phenotypic effect in both the homozygous $(AA)$ and heterozygous $(Aa)$ conditions.
In the heterozygous condition,the dominant allele masks the expression of the recessive allele.
Therefore,the correct option is $A$.

(A) According to Mendel's Law of Segregation,the two alleles of a gene pair segregate from each other during the formation of gametes.
In meiosis,this segregation of homologous chromosomes occurs during $Anaphase-I$.
During $Anaphase-I$,the homologous chromosomes separate and move towards opposite poles,which ensures that each gamete receives only one of the two alleles.
Therefore,the correct phase for the segregation of Mendelian factors is $Anaphase-I$.

(D) An organism is said to be $Homozygous$ for a trait when it carries two identical alleles for that specific gene (e.g., $TT$ or $tt$).
In contrast, an organism is $Heterozygous$ if it carries two different alleles for a gene (e.g., $Tt$).
Therefore, the correct term for an organism with two identical alleles is $Homozygous$.

(C) $Neurospora$ में,जब अर्धसूत्रीविभाजन (Meiosis) के दौरान जीन लोकस और सेंट्रोमियर के बीच क्रॉसिंग ओवर होता है,तो इसे द्वितीय विभाजन पृथक्करण (Second division segregation) कहा जाता है। सामान्यतः,यदि क्रॉसिंग ओवर नहीं होता है,तो एस्कोस्पोर्स की व्यवस्था $4a : 4a$ (प्रथम विभाजन पृथक्करण) होती है। लेकिन जब $2a : 4a : 2a$ जैसी व्यवस्था दिखाई देती है,तो यह स्पष्ट करता है कि जीन का पृथक्करण अर्धसूत्रीविभाजन के दूसरे चरण में हुआ है,जो क्रॉसिंग ओवर के कारण संभव होता है।

(B) Cytoplasmic inheritance,also known as extranuclear inheritance,refers to the transmission of genes present in cytoplasmic organelles like mitochondria or chloroplasts.
Since the zygote receives the majority of its cytoplasm from the egg cell (ovum) during fertilization,traits controlled by these organellar genes are inherited exclusively from the female parent.
This phenomenon is also known as maternal inheritance.

(D) $Down's$ syndrome is a chromosomal disorder caused by the presence of an additional copy of chromosome number $21$ (trisomy of $21$).
This condition arises due to the failure of homologous chromosomes or sister chromatids to separate properly during meiosis,a phenomenon known as non-disjunction.
As a result,one gamete receives an extra chromosome,and upon fertilization,the zygote develops with $47$ chromosomes instead of the normal $46$.

(A) Color blindness is an $X$-linked recessive disorder.
Let the genotype of the color-blind mother be $X^cX^c$ and the normal father be $XY$.
When these parents cross:
- The mother produces gametes $X^c$ and $X^c$.
- The father produces gametes $X$ and $Y$.
- The offspring genotypes will be:
- $X^cX$ (Carrier daughter)
- $X^cY$ (Color-blind son)
Therefore,all sons will be color-blind and all daughters will be normal carriers.

(C) Turner syndrome is caused by the absence of one of the $X$ chromosomes,i.e.,$45$ chromosomes with $XO$ genotype $(44 + XO)$.
Such individuals are sterile females with rudimentary ovaries and lack other secondary sexual characters.
Down syndrome is caused by trisomy of chromosome $21$.
Klinefelter syndrome is caused by the presence of an additional copy of the $X$ chromosome,resulting in a genotype of $47$ $(44 + XXY)$.
Edward syndrome is caused by trisomy of chromosome $18$.

(D) In $DNA$,the four nitrogenous bases are $Adenine$ $(A)$,$Guanine$ $(G)$,$Cytosine$ $(C)$,and $Thymine$ $(T)$.
In $RNA$,$Thymine$ is replaced by $Uracil$ $(U)$.
Therefore,$RNA$ contains $Adenine$,$Guanine$,$Cytosine$,and $Uracil$.

(A) The experiment described is the Meselson-Stahl experiment,which proves the semi-conservative nature of $DNA$ replication.
$1$. Initially,the $E. coli$ $DNA$ is labeled with the heavy isotope $N^{15}$.
$2$. When these bacteria are transferred to a medium containing the lighter isotope $N^{14}$,the first generation of $DNA$ molecules will consist of one strand of $N^{15}$ (parental) and one strand of $N^{14}$ (newly synthesized).
$3$. This hybrid $DNA$ $(N^{15}N^{14})$ has an intermediate density compared to pure $N^{15}$ $DNA$ and pure $N^{14}$ $DNA$.
$4$. Therefore,the $DNA$ strands have a different density compared to the original parental $DNA$ $(N^{15}N^{15})$ and are not similar to the parental $DNA$.

(D) Har Gobind Khorana developed a chemical method to synthesize $RNA$ molecules with defined combinations of bases (homopolymers and copolymers).
By using these synthetic $RNA$ molecules in a cell-free protein-synthesizing system,he was able to decipher the genetic code.
Specifically,he demonstrated that the triplet codons $UUU$ (which codes for Phenylalanine) and $AUG$ (which codes for Methionine) were among the first to be deciphered using his experimental approach.

(A) The experimental evidence for the semi-conservative nature of $DNA$ replication was provided by $Matthew$ $Meselson$ and $Franklin$ $Stahl$ in $1958$.
They grew $Escherichia$ $coli$ ($E.$ $coli$) in a medium containing $^{15}N$ (a heavy isotope of nitrogen) as the only nitrogen source for many generations.
This resulted in the incorporation of $^{15}N$ into the newly synthesized $DNA$ (as well as other nitrogen-containing compounds).
This heavy $DNA$ molecule could be distinguished from the normal $DNA$ by centrifugation in a $Cesium$ $Chloride$ $(CsCl)$ density gradient.

(B) According to the Oparin-Haldane hypothesis and the chemical evolution theory,the first life forms on Earth were simple,anaerobic,and heterotrophic organisms. These organisms obtained their energy by consuming organic molecules present in the 'primordial soup' of the primitive ocean. Since they relied on external organic sources for energy and carbon,they are classified as chemoheterotrophs. Autotrophs evolved much later after the depletion of organic molecules.

(B) River water carries sediments such as silt,clay,and sand from its upper reaches and deposits them along its banks and in its delta region. This type of soil is known as $Alluvial$ $soil$. It is highly fertile and is formed by the depositional action of rivers.

(B) The topsoil,also known as the $A$-horizon,is the uppermost layer of soil.
It appears darker in color primarily because it is rich in organic matter,known as humus,which is formed by the decomposition of plant and animal remains.
This organic matter provides essential nutrients and improves the soil structure,making it highly fertile.
In contrast,the deeper layers (subsoil) contain less organic matter and more mineral particles,resulting in a lighter color.

(C) Automobiles are a major source of air pollution.
Among the heavy metals released,$Lead$ $(Pb)$ is considered one of the most harmful pollutants.
It is added to gasoline as an anti-knocking agent.
When fuel is burned,$Lead$ is released into the atmosphere,which can cause serious health issues,including neurological damage,especially in children.
Therefore,the correct option is $C$.