NEET 2016 Biology Question Paper with Answer and Solution

(B) : Synapsis is the process of pairing of homologous chromosomes. It occurs specifically during the zygotene stage of prophase $I$ of meiosis. This phenomenon is not observed during mitosis,as mitosis involves the division of somatic cells without the pairing of homologous chromosomes.

(C) : Crossing over is a process of exchange of genetic material or chromatid segments between two homologous chromosomes. It is initiated during the pachytene stage of meiosis $I$.

(B) Phloem sap is the fluid transported through the phloem tissue,which primarily consists of sucrose and other organic solutes.
Chemically,phloem sap is known to be alkaline in nature,typically having a $pH$ ranging from $7.5$ to $8.5$.
In contrast,xylem sap is generally acidic.
Therefore,an alkaline test result indicates that the collected fluid is phloem sap.

(C) The correct answer is $C$.
Calcium $(Ca^{2+})$ is an essential mineral element required by plants.
It is specifically required for the formation of the middle lamella in cell walls and is crucial for the normal functioning of the root apical meristem.
It plays a vital role in cell division and cell elongation at the root tip,making it essential for root tip growth.

(D) None of the options is correct. Macronutrients are essential elements which are present in easily detectable quantities,$1-10 \ mg$ per gram of dry weight. The macronutrients include carbon,hydrogen,oxygen,nitrogen,phosphorus,sulphur,potassium,calcium,and magnesium.
Micronutrients,or trace elements,are needed in very small amounts (equal to or less than $0.1 \ mg/g$ of dry matter). These include iron,manganese,copper,molybdenum,zinc,boron,chlorine,and nickel.

(C) : Photorespiration is the light-dependent process of oxygenation of ribulose bisphosphate $(RuBP)$ and the release of carbon dioxide by the photosynthetic organs of a plant.
It leads to the oxidation of a considerable amount of photosynthetic products into $CO_2$ and $H_2O$ without the production of useful energy.
Photorespiration occurs significantly in $C_3$ plants because,at high temperatures and high oxygen concentrations,the enzyme $RuBP$ carboxylase acts as $RuBP$ oxygenase.
Photorespiration is essentially absent in $C_4$ plants due to their specialized anatomy (Kranz anatomy) which maintains a high $CO_2$ concentration around the enzyme.
Peroxisomes and mitochondria are required for completing this process.

(D) The correct answer is $D$. Transpiration (loss of water vapour) and photosynthesis (uptake of $CO_2$) occur through the stomata. These two processes can occur simultaneously because the diffusion of gases is independent of each other. The rate of diffusion depends on the diffusion coefficient of the specific gas and the concentration gradient. Since water vapour and $CO_2$ have different diffusion coefficients,they can diffuse in opposite directions through the same stomatal pore at the same time.

(D) During the light-dependent reactions of photosynthesis,protons $(H^+)$ are pumped from the stroma into the lumen of the thylakoids. This process is driven by the electron transport chain and the photolysis of water. As a result,a high concentration of protons accumulates within the thylakoid lumen,creating a proton gradient across the thylakoid membrane. This gradient is essential for the synthesis of $ATP$ via the enzyme $ATP$ synthase. Therefore,the highest concentration of protons is found in the lumen of the thylakoids.

(D) : Emerson et al. $(1957)$ found that if light of shorter wavelengths was provided at the same time as the longer red wavelengths,photosynthesis was even faster than the sum of the two rates with either colour alone. This synergism or enhancement became known as the Emerson enhancement effect. The two separate groups of pigments or photosystems cooperate in photosynthesis and that such long red wavelengths are absorbed only by one photosystem,called photosystem $I$ $(PS\ I)$. The second photosystem,photosystem $II$ $(PS\ II)$,absorbs wavelengths shorter than $690 \ nm$,and for maximum photosynthesis,wavelengths absorbed by both systems must function together. The two photosystems normally cooperate to cause photosynthesis at all wavelengths shorter than $690 \ nm$,because both photosystems absorb those wavelengths. The importance of Emerson's work is that it suggested the presence of two distinct photosystems.

(D) The correct answer is $(d) C_4$ plants.
$C_4$ plants are specifically adapted to minimize photorespiration because they possess a specialized anatomy called Kranz anatomy,which separates the initial $CO_2$ fixation from the Calvin cycle.
These plants show high rates of photosynthesis even at high temperatures.
They exhibit improved water use efficiency compared to $C_3$ plants because they can fix $CO_2$ even when stomata are partially closed.
Additionally,$C_4$ plants have a higher nitrogen-use efficiency because they require less Rubisco enzyme to fix the same amount of carbon compared to $C_3$ plants.

(B) : Anthocyanins are water-soluble pigments that are commonly found in membrane-bound vacuoles. They are responsible for the red,purple,and blue colors of many fruits and flower petals.

(D) The correct answer is $(d)$. Carbohydrates are usually first converted into glucose before they are used for respiration.
Fats are broken down into glycerol and fatty acids. Fatty acids are degraded to acetyl CoA to enter the respiratory pathway,while glycerol is converted to $3-$phosphoglyceraldehyde $(PGAL)$.
Proteins are degraded by proteases into individual amino acids. After deamination,these amino acids enter the respiratory pathway at different stages,such as pyruvate,acetyl CoA,or intermediates of the Krebs' cycle.
Thus,acetyl CoA acts as a common metabolic intermediate for the breakdown of all three major biomolecules (carbohydrates,proteins,and fats),making the respiratory pathway an amphibolic pathway.

(D) Oxidative phosphorylation is the synthesis of energy-rich $ATP$ molecules using the energy liberated during the oxidation of reduced coenzymes $(NADH, FADH_2)$ produced during respiration.
The enzyme required for this synthesis is called $ATP$ synthase.
It is located in the $F_1$ headpiece of the $F_0-F_1$ complex (elementary particles) present in the inner mitochondrial membrane.
The $F_1$ particle is capable of $ATP$ synthesis.
$ATP$ synthase becomes active in $ATP$ formation only when there is a proton gradient with a higher concentration of $H^+$ ions on the $F_0$ side compared to the $F_1$ side.
This concentration difference creates an electric potential across the mitochondrial membrane.
The proton gradient and membrane electric potential together form the proton motive force $(PMF)$.
The flow of protons through the $F_0$ channel induces the $F_1$ particle to function as $ATP$ synthase.
The energy of the proton gradient is used to attach a phosphate group to $ADP$ via a high-energy bond,producing $ATP$.

(B) The correct answer is $B$. Cytokinin and auxin are two essential plant hormones added to the tissue culture medium in specific proportions to regulate morphogenesis.
These hormones induce cell division and the differentiation of the callus.
Specifically,a low auxin-to-cytokinin ratio promotes the formation of shoots,while a high auxin-to-cytokinin ratio promotes the development of roots in the callus.

(D) : Phytochrome is a chromoprotein,a plant pigment that can detect the presence or absence of light. It is involved in regulating many processes linked to day length (photoperiod),such as seed germination and the initiation of flowering.
It consists of a light-detecting portion,called a chromophore,linked to a protein moiety.
It exists in two interconvertible forms,$Pr$ and $Pfr$,which possess different physical and chemical properties.

(A) $Avena$ curvature test is a bioassay based on the experiments of Went $(1928)$,which can measure auxin concentration up to $300 \ mg/litre$.
Indole-$3$-Acetic Acid $(IAA)$ is a universal natural auxin,and the $Avena$ curvature test serves as an accurate bioassay for this specific plant hormone.

(C) The correct answer is $C$.
$1$. Cholecystokinin $(CCK)$ is a hormone secreted by the $I$-cells of the duodenum and jejunum. It stimulates the pancreas to secrete and release digestive enzymes into the pancreatic juice and also causes the contraction of the gall bladder to release bile.
$2$. Secretin is a hormone secreted by the $S$-cells of the duodenum in response to low $pH$ (acidic chyme). It primarily stimulates the pancreatic ducts to secrete a fluid rich in bicarbonate ions $(HCO_3^-)$,which helps neutralize the acidic chyme in the duodenum.

(B) The correct answer is $B$. The $Sphincter$ \text{ of } $Oddi$ is a muscular valve that regulates the flow of digestive juices, specifically bile and pancreatic juice, from the hepatopancreatic duct into the duodenum. It ensures that these secretions enter the small intestine only when required for digestion.

(D) : Parietal cells,also known as oxyntic cells,are large cells present on the side walls of the gastric glands.
They lie against the basement membrane.
They secrete hydrochloric acid $(HCl)$ and Castle's intrinsic factor,which is essential for the absorption of vitamin $B_{12}$ in the ileum.

(B) The partial pressure of oxygen $(pO_2)$ in the alveoli is approximately $104 \ mm \ Hg$.
In contrast,the $pO_2$ in deoxygenated blood is $40 \ mm \ Hg$ and in oxygenated blood is $95 \ mm \ Hg$.
Since the partial pressure of oxygen in the alveoli is higher than that in the blood,oxygen diffuses from the alveoli into the blood.
Therefore,the correct option is $B$.

(B) : Intrapleural pressure is the pressure within the pleural cavity.
Intrapleural pressure is always negative,which acts like a suction to keep the lungs inflated and prevent them from collapsing.
The negative intrapleural pressure is due to three main factors: surface tension of the alveolar fluid,elasticity of the lungs,and elasticity of the thoracic wall.
Normally,there is a difference between intrapleural and intrapulmonary pressure,which is called transpulmonary pressure.
This transpulmonary pressure creates the suction to keep the lungs inflated.
If there is no pressure difference,there is no suction and the lungs will collapse.

(A) The correct answer is $A$.
Reduction in $pH$ of blood (increase in $H^+$ ion concentration) causes the oxygen-haemoglobin dissociation curve to shift to the right,a phenomenon known as the Bohr effect.
This shift indicates a decrease in the affinity of haemoglobin for oxygen,facilitating the dissociation of oxygen from haemoglobin at the tissue level where $pH$ is lower.

(C) : Emphysema is a chronic obstructive pulmonary disease $(COPD)$ caused mainly due to cigarette smoking.
It is a condition characterized by the abnormal distension of the bronchioles or alveolar sacs of the lungs.
This leads to the irreversible damage and loss of elasticity of the alveolar walls,which reduces the surface area available for gas exchange.

(D) : Asthma is an allergic condition in which the tissue surrounding the bronchioles of the lungs swell up and compress the bronchioles,thus causing difficulty in breathing.
This allergy mainly involves $IgE$ antibodies and the release of chemicals like histamine and serotonin from the mast cells in the lungs.

(D) : Thrombocytes are also known as blood platelets. They are minute,disc-shaped cell fragments found in mammalian blood. They are formed as fragments of larger cells called megakaryocytes in the red bone marrow and lack a nucleus. They play a crucial role in blood clotting by releasing substances like thromboxane $A_2$,serotonin,and other clotting factors. These chemicals initiate a chain of events that form a plug at the site of injury,preventing excessive blood loss. $A$ significant reduction in their number leads to clotting disorders,resulting in excessive bleeding from the body.

(C) $Serum$ is the fluid that remains after blood has clotted.
When blood is allowed to clot, the clotting factors, such as $fibrinogen$, are consumed in the process of forming the clot.
Therefore, $serum$ is essentially blood plasma from which the clotting factors have been removed.
Thus, the correct answer is $C$.

(A) The blood pressure in the pulmonary artery is higher than that in the pulmonary vein.
$1$. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs.
$2$. The pulmonary vein carries oxygenated blood from the lungs to the left atrium.
$3$. Since the pulmonary artery is closer to the pumping action of the heart (right ventricle) and the pulmonary vein is a venous vessel returning blood to the heart,the pressure in the artery is significantly higher than in the vein.

(A) The correct answer is $A$.
Urea is synthesized in the liver through the ornithine cycle (urea cycle) as a byproduct of amino acid metabolism.
Since the liver is the site of urea production,the blood leaving the liver via the hepatic vein contains the highest concentration of urea in the body.
In contrast,the renal vein carries blood away from the kidneys,which have already filtered the urea out of the blood,resulting in a lower concentration of urea compared to the hepatic vein.

(B) : From the Bowman's capsule,glomerular filtrate enters the proximal convoluted tubule $(PCT)$.
Absorption of selected materials takes place from the filtrate into the blood of the peritubular capillaries or vasa recta,which is termed tubular reabsorption.
Reabsorption involves both passive and active transport across the tubular epithelium.
About $65$ percent of the glomerular filtrate is normally reabsorbed in the $PCT$ before reaching the loop of Henle.
Glucose,amino acids,vitamins,hormones,sodium $(Na^+)$,potassium $(K^+)$,chlorides,phosphates,bicarbonates,much of the water,and some urea are absorbed here.
Sodium and potassium are reabsorbed primarily by active transport.

(A) During muscle contraction,the $Ca^{2+}$ ions play a critical role.
When a signal from the motor neuron reaches the muscle fiber,$Ca^{2+}$ ions are released into the sarcoplasm.
These $Ca^{2+}$ ions bind to the $Troponin$ subunit on the actin filaments.
This binding causes a conformational change in the $Troponin-Tropomyosin$ complex,which shifts the $Tropomyosin$ away from the active sites on the $F-actin$ filaments.
By unmasking these active sites,the $Myosin$ heads can bind to them to form cross-bridges,leading to muscle contraction.

(C) Osteoporosis is a condition characterized by a reduction in bone mineral density,which makes bones brittle and prone to fractures.
Generalised osteoporosis is common in the elderly and in women after menopause.
After menopause,the levels of the hormone estrogen in the blood plasma are significantly reduced.
Estrogen plays a crucial role in regulating bone cells,specifically osteoblasts (which build bone) and inhibiting excessive activity of osteoclasts (which break down bone).
When estrogen levels drop,the balance of bone remodeling is disrupted,leading to bone loss and osteoporosis.

(A) : Tetanus refers to a sustained state of muscle contraction resulting from the summation of a series of rapid muscular contractions (twitches) that are induced by repeated stimulation of the muscle. When the frequency of stimuli is high enough that the muscle cannot relax between them,the contractions fuse into a smooth,sustained contraction.

(C) The correct statement is $C$.
Photoreceptors in the human eye are unique because they are the only type of sensory cells that are relatively depolarised (about $-35 \ mV$) when at rest (i.e.,in the dark),and they become hyperpolarised (to about $-70 \ mV$) in response to an adequate light stimulus.
Nociceptors respond to potentially damaging stimuli that result in pain,not pressure.
Meissner's corpuscles are a type of mechanoreceptor responsible for touch sensitivity,not thermoreception.
Receptors generally produce graded potentials known as receptor potentials.

(D) The correct answer is $D$.
In the human eye,the rods contain a photosensitive pigment known as rhodopsin.
Rhodopsin is a conjugated protein composed of two parts: a protein component called opsin and a light-absorbing pigment called retinal.
Retinal is an aldehyde derivative of vitamin $A$.
When light strikes the rhodopsin,it causes the dissociation of retinal from opsin,which triggers the nerve impulse.

(B) : Exophthalmic goitre or Graves' disease is a thyroid enlargement (goitre) in which the thyroid gland secretes an excessive amount of thyroid hormones.
It is characterized by exophthalmia (protrusion of eyeballs due to fluid accumulation behind them),weight loss,a slight rise in body temperature,excitability,rapid heartbeat,nervousness,and restlessness.

(A) $Insulin$ is a peptide hormone secreted by the $\beta$-cells of the islets of $Langerhans$ in the pancreas.
It acts mainly on hepatocytes (liver cells) and adipocytes (fat cells).
It enhances cellular glucose uptake and utilisation, thereby lowering blood glucose levels.

(B) : The posterior lobe of the pituitary gland does not synthesize any hormones.
Its hormones,specifically $Oxytocin$ and $Vasopressin$ $(ADH)$,are synthesized by the neurosecretory cells of the hypothalamus.
The posterior pituitary only stores and releases these hormones into the bloodstream.
Therefore,it is not considered a 'true' endocrine gland as it lacks the secretory function characteristic of endocrine tissues.

(B) The correct answer is $(b)$.
Antagonistic hormones are those that have opposite effects on a physiological process.
$(a)$ Aldosterone increases blood pressure by retaining sodium,while Atrial Natriuretic Factor $(ANF)$ decreases blood pressure by promoting sodium excretion. They are antagonistic.
$(c)$ Parathormone increases blood calcium levels,while Calcitonin decreases blood calcium levels. They are antagonistic.
$(d)$ Insulin decreases blood glucose levels,while Glucagon increases blood glucose levels. They are antagonistic.
$(b)$ Relaxin is involved in relaxing pelvic ligaments and softening the cervix during childbirth,whereas Inhibin inhibits the secretion of $FSH$ from the anterior pituitary. Since they perform distinct,non-opposing functions,they are not antagonistic.

(C) Tryptophan is an essential amino acid that serves as a precursor for the synthesis of the neurotransmitter serotonin and the hormone melatonin.
Serotonin is synthesized from tryptophan via the intermediate $5-hydroxytryptophan$.
Melatonin is subsequently synthesized from serotonin in the pineal gland.

(D) : Polyploidy is the phenomenon of the occurrence of more than two sets of chromosomes in the nucleus of a cell.
Polyploidy is more common in plants.
It arises as a result of the total non-disjunction of chromosomes during mitosis or meiosis,where the cell fails to divide after the replication of chromosomes,leading to an increase in the ploidy level.

(B) The metabolic rate of an organism is inversely proportional to its body size.
Smaller animals have a higher surface area to volume ratio,which results in a higher metabolic rate to maintain their body temperature and physiological functions.
Because of this higher metabolic rate,small animals can generate energy more rapidly,allowing them to perform strenuous activities like running uphill more easily compared to larger animals,which have a lower metabolic rate relative to their body size.
Therefore,the correct option is $(b)$.

(C) In the anatomy of dicot stems and roots,the cortex is the region located between the epidermis (the outermost layer) and the stele (the central part containing vascular tissues).
Therefore,the cortex is found between the epidermis and the stele.

(D) The correct answer is $D$.
$A$. Protists are eukaryotic organisms,which is correct.
$B$. Methanogens are archaebacteria,which are prokaryotic,which is correct.
$C$. Gas vacuoles are found in blue-green,purple,and green photosynthetic bacteria,which is correct.
$D$. Large central vacuoles are characteristic of plant cells,not animal cells. Animal cells typically have small,temporary vacuoles or lack them entirely. Therefore,this is the incorrect match.

(D) The correct answer is $D$.
$A$: Cyanobacteria (blue-green algae) are photosynthetic prokaryotes that lack flagella throughout their life cycle. This statement is correct.
$B$: Mycoplasma are the smallest known living cells and they completely lack a cell wall,making them resistant to many antibiotics. This statement is correct.
$C$: The cell wall of bacteria is made up of peptidoglycan (murein). This statement is correct.
$D$: Pili and fimbriae are surface structures of bacteria. Pili are involved in conjugation (transfer of $DNA$),and fimbriae are involved in attachment to rocks or host tissues. Neither of these structures is involved in motility. Flagella are the primary structures responsible for bacterial motility. Therefore,this statement is incorrect.

(C) Lysosomes are membrane-bound vesicular structures formed by the process of packaging in the Golgi apparatus. They are rich in hydrolytic enzymes (hydrolases - lipases,proteases,carbohydrases) that are optimally active at the acidic $pH$. These enzymes are capable of digesting carbohydrates,proteins,lipids,and nucleic acids.

(A) Microtubules are hollow, cylindrical structures composed of tubulin proteins. They form the structural components of the cytoskeleton.
Specifically, they are the primary building blocks of:
$1$. $Spindle fibers$: Essential for chromosome movement during cell division.
$2$. $Centrioles$: Cylindrical structures that organize the spindle apparatus.
$3$. $Cilia and flagella$: Hair-like appendages involved in cell movement, which possess a $9+2$ arrangement of microtubules.
Chromatin, nucleosomes, and peroxisomes do not contain microtubules as their structural components.

(B) Mitochondria and chloroplasts are known as semi-autonomous organelles because they possess their own $DNA$ and ribosomes.
Statement $(a)$ is correct as they are semi-autonomous.
Statement $(b)$ is incorrect because,although they contain $DNA$,they also contain ribosomes (specifically $70S$ ribosomes in prokaryotic-like fashion) which are the machinery for protein synthesis.
Therefore,$(a)$ is correct,but $(b)$ is incorrect.

(B) $1$. Chloroplast is a double-membrane-bound organelle.
$2$. Lysosomes are single-membrane-bound organelles containing hydrolytic enzymes.
$3$. Nucleolus is a non-membrane-bound structure found inside the nucleus.
$4$. Mitochondria is a double-membrane-bound organelle.
Therefore, the correct answer is Lysosomes.

(C) In plant cells,the vacuole is a membrane-bound organelle that stores various substances. Water-soluble pigments,such as anthocyanins,are dissolved in the cell sap within the vacuole. These pigments are responsible for the red,purple,or blue colors in many flowers and fruits. Chlorophyll,carotenoids,and xanthophylls are typically found in plastids (like chloroplasts) and are generally fat-soluble,not water-soluble.

(C) Smooth muscle tissue (also known as visceral muscle) is characterized by the following features:
$1$. It is involuntary,meaning it is not under conscious control.
$2$. The cells are fusiform (spindle-shaped) in structure.
$3$. It is non-striated (smooth) because it lacks the regular arrangement of actin and myosin filaments found in skeletal muscle.
$4$. Each cell contains a single nucleus (uninucleate).
Therefore,the correct description is involuntary,fusiform,and non-striated.

(A) : Autoimmunity is a disorder of the body's defence mechanisms in which an immune response is elicited against its own tissues,which are thereby damaged or destroyed.
Autoimmunity may be caused due to genetic or environmental factors.

(C) true breeding plant is one that,when self-pollinated,consistently produces offspring with the same traits as the parent.
This occurs because the plant is near homozygous for the traits in question.
Therefore,it maintains its genetic purity over successive generations.
Option $C$ is the correct definition.

(B) : In interspecific hybridisation,a species is mated with a different related species of the same genus.
Interspecific hybrids are generally difficult to produce,but they are important in plant breeding,particularly in breeding for disease resistance.
This is also called intrageneric hybridisation.

(D) The correct answer is $(d)$.
$Mackerel$ is a marine fish that is a rich source of omega-$3$ fatty acids.
$Mystus$,$Mangur$ (Clarias batrachus),and $Mrigala$ (Cirrhinus mrigala) are freshwater fishes.

(C) : Ley farming is an agricultural system where the field is alternately seeded for grain and left fallow for growing hay or used for pasture.
During the fallow or pasture period,the soil is filled with the roots of grasses and other plants.
Subsequent ploughing mixes these organic materials into the soil,which improves soil structure and increases the nitrogen content,especially when legume forage crops are used.
This practice also protects the soil from erosion by maintaining constant soil coverage.

(B) The correct matches are as follows:
$(A)$ Citric acid is produced by the fungus $Aspergillus$ $niger$ $(iii)$.
$(B)$ Cyclosporin $A$ is an immunosuppressive agent produced by the fungus $Trichoderma$ $polysporum$ $(i)$.
$(C)$ Statins are blood-cholesterol lowering agents produced by the yeast $Monascus$ $purpureus$ $(iv)$.
$(D)$ Butyric acid is produced by the bacterium $Clostridium$ $butylicum$ $(ii)$.
Therefore, the correct matching is $A-(iii), B-(i), C-(iv), D-(ii)$.
The correct option is $(B)$.

(B) The correct answer is $(b)$.
$Clostridium$ $butylicum$ is a bacterium used for the production of butyric acid, not lipase.
Lipases are enzymes produced by microorganisms like $Candida$ $lipolytica$ or $Geotrichum$ $candidum$. These enzymes are used in detergent formulations to help remove oily stains from laundry.
Other options are correctly matched:
- $Streptococcus$ produces streptokinase, which acts as a 'clot buster' for removing clots from the blood vessels of patients who have undergone myocardial infarction.
- $Trichoderma$ $polysporum$ is a fungus that produces Cyclosporin $A$, which is used as an immuno-suppressive agent in organ transplant patients.
- $Monascus$ $purpureus$ is a yeast that produces statins, which act by competitively inhibiting the enzyme responsible for the synthesis of cholesterol.

(A) : Gene therapy is a technique of genetic engineering which involves the replacement of a faulty or disease-causing gene with a normal,healthy,functional gene.
In $1990$,the first clinical gene therapy was administered to a $4$-year-old girl suffering from adenosine deaminase $(ADA)$ deficiency.
This enzyme is crucial for the proper functioning of the immune system.
$A$ deficiency of this enzyme leads to severe combined immunodeficiency $(SCID)$.

(B) Human insulin is composed of $51$ amino acids arranged in two polypeptide chains.
Chain $A$ consists of $21$ amino acids,and chain $B$ consists of $30$ amino acids.
These two polypeptide chains are interconnected by disulphide bridges (or $S-S$ linkages).

(B) $Meloidogyne \ incognita$ is a nematode that infects the roots of tobacco plants,causing a significant reduction in yield. This infection is prevented using $RNA$ interference $(RNAi)$ technology.

(A) $r$-selected species ($r$-strategists) are organisms that can colonize a habitat rapidly by utilizing available resources before competitors arrive.
These organisms typically have short life spans and small body sizes (e.g.,bacteria,many insects).
Their survival strategy relies on producing a large number of offspring to ensure that at least some survive in unstable or temporary environments,rather than investing energy in competitive ability or parental care.

(D) The population interaction represented by $'+, -'$ is known as parasitism.
In this interaction,one species (the parasite) benefits $('+')$ while the other species (the host) is harmed $('-')$.
Parasitism is an association in which one organism (the parasite) lives on (ectoparasitism) or in (endoparasitism) the body of another organism (host),from which it obtains its nutrients.
This association is beneficial for the parasites as they get a continuous supply of nutrients from their host and are able to rapidly multiply their numbers.
However,it is detrimental for the host organism as parasitic infection leads to various complications and diseases in the host body,which may also be fatal to the host under certain circumstances.

(B) : Two or more species with closely similar niche requirements cannot exist indefinitely in the same area as sooner or later they come into competition for possession of it. This is called as Gause's competitive exclusion principle,which states that an ecological niche cannot be simultaneously and completely occupied by established populations of more than one species. Two species can live in the same habitat but not in the same niche. The more similar the two niches are,the more severe the competition is.

(C) The logistic growth model is represented by the equation $dN/dt = rN(1 - N/K)$.
Here,$dN/dt$ represents the population growth rate.
For the growth rate to be zero,$dN/dt = 0$.
Substituting this into the equation: $0 = rN(1 - N/K)$.
Since $r$ (intrinsic rate of natural increase) and $N$ (population size) are generally non-zero in an active population,the term $(1 - N/K)$ must be equal to zero.
This implies $1 - N/K = 0$,which simplifies to $N/K = 1$.
Therefore,the population growth rate becomes zero when the population size $N$ reaches the carrying capacity $K$ (i.e.,$N/K = 1$).

(A) : Two or more species with closely similar niche requirements cannot exist indefinitely in the same area as sooner or later they come into competition for possession of it.
This is called Gause's competitive exclusion principle,which states that an ecological niche cannot be simultaneously and completely occupied by established populations of more than one species.
Two species can live in the same habitat but not in the same niche.
The more similar the two niches are,the more severe the competition is.

(B) The correct answer is $B$.
Hydrothermal vents are fissures in the ocean floor that release jets of hot water rich in minerals and $H_2S$.
In these extreme environments,sunlight is absent,so photosynthesis cannot occur.
Instead,chemosynthetic bacteria act as the primary producers.
These bacteria derive energy by oxidizing hydrogen sulfide $(H_2S)$ present in the vent water.
This energy is utilized to synthesize organic compounds from inorganic carbon sources.
Consequently,these bacteria form the foundation of the hydrothermal vent food web,supporting all other life forms in that ecosystem.

(C) The correct answer is $C$.
Bare rocky habitats are extremely hostile environments for living organisms.
These areas lack water because the rocky substratum cannot absorb or retain rainwater.
Additionally, there is no mechanism for nutrient retention, making it difficult for most plants to grow.
$Lichens$ are the pioneer species in such habitats because they are highly resistant to desiccation (drying out) and can withstand extreme temperature fluctuations.
They secrete acids that help in the weathering of rocks, which eventually leads to soil formation and allows other successional species to colonize the area.

(D) The correct answer is $(d)$.
Cropland ecosystems are artificial or man-made terrestrial ecosystems that are created and maintained by humans for agricultural productivity.
Because these systems are designed to favor a specific crop species (monoculture),they exhibit the least genetic diversity compared to natural ecosystems.
Weeds are often present,soil organisms are essential for soil health,and ecological succession is typically suppressed by human intervention.

(D) The term ecosystem was coined by the British ecologist $A. G. Tansley$ in $1935$. An ecosystem is a functional unit of nature where living organisms interact among themselves and also with the surrounding physical environment.

(C) The concept of biodiversity hotspots was first proposed by Norman Myers in $1988$ to identify regions that are rich in biodiversity but are under serious threat of habitat loss.
Initially,he identified $10$ hotspots,which he later expanded to $25$ and eventually,with further research and global assessment,the number of identified biodiversity hotspots reached $34$.
These regions are characterized by high levels of plant endemism and significant loss of habitat.
Therefore,the correct answer is $34$.

(C) : $Parthenium$ $hysterophorus$ is commonly known as congress grass or carrot weed. It is an herbaceous annual plant of the family $Asteraceae$.
It is a deadly invasive,noxious weed infesting both cropped and non-cropped areas.
It rapidly colonizes areas,replacing native vegetation,and causes a number of human health-related problems such as skin allergy,rhinitis,and eye irritation.
Also,being toxic and unpalatable,it causes fodder scarcity.
Hence,it is considered a significant threat to biodiversity.

(C) The correct answer is $C$. The Red Data Book or Red List is a comprehensive catalogue of taxa (species,subspecies,etc.) that are facing a risk of extinction. It provides information on the conservation status of these threatened species to help in their protection and management. The Red Data Book was first initiated in $1963$ by the International Union for Conservation of Nature $(IUCN)$.

(D) The correct answer is $D$. Dachigam National Park,located in Jammu and Kashmir,is famous for being the natural habitat of the Hangul,also known as the Kashmir stag. The Hangul is a subspecies of the Central Asian red deer and is critically endangered. While musk deer are also found in high-altitude regions of the Himalayas,the Hangul is specifically associated with the Dachigam National Park.

(D) : The River dolphin (Platanista gangetica) found in the holy rivers Ganga,Brahmaputra,Indus,and their tributaries is the National aquatic animal of India.
The presence of the River dolphin in the Ganga indicates the purity and health of the freshwater ecosystem.

(A) The correct answer is $(A)$.
Habitat loss and fragmentation is the most important cause driving animals and plants to extinction.
Destruction of natural habitats poses the most serious threat to biodiversity.
Overpopulation,urbanization,and industrialization lead to the destruction or fragmentation of natural habitats to fulfill the requirement for additional land.
Loss of habitat results in the annihilation of plants and microorganisms,and forces animals out of their natural homes,where they often die in alien environments.
Fragmentation of habitats disrupts complex interactions among species,destroys species in the cleared regions,eliminates species restricted to deeper undisturbed parts of forests,and decreases overall biodiversity in the habitat fragments.

(C) $(BOD)$ is a measure of the amount of dissolved oxygen required by aerobic microorganisms to break down the organic matter present in a given volume of water.
It is an effective indicator for biodegradable organic pollutants.
Domestic sewage,dairy industry effluents,and sugar industry effluents contain high amounts of biodegradable organic matter.
However,effluents from the petroleum industry contain non-biodegradable hydrocarbons and toxic chemicals that cannot be easily broken down by aerobic decomposers.
Therefore,$(BOD)$ is not a reliable index for measuring pollution in water bodies receiving effluents from the petroleum industry.

(D) : High amount of organic waste in a lake will trigger and activate decomposer microbes,which will decompose the organic waste.
Biochemical Oxygen Demand $(BOD)$ of this lake will shoot up,and decomposers will utilize most of the dissolved oxygen present in the lake.
Consequently,the level of dissolved oxygen will decrease to an alarming extent.
Aquatic animals like fish,which totally depend on the oxygen dissolved in water,will ultimately die due to suffocation.

(B) $DDT$ is non-biodegradable and is not metabolized within the body of an organism. Instead,it accumulates in the fat tissues.
Therefore,its concentration increases from one trophic level to another in a food chain,with the highest amount being present in the top consumer,a process known as biomagnification.
In an aquatic food chain,the seagull is the top carnivore and therefore will possess the highest concentration of $DDT$.

(D) The correct answer is $D$.
Ozone $(O_3)$ layer is present in the stratosphere and acts as a protective shield against harmful $UV$ radiations coming from the sun.
$UV$ radiations are highly energetic and can cause mutations in the $DNA$ of living organisms.
Depletion or thinning of the ozone layer allows a higher intensity of $UV$ radiations to reach the Earth's surface.
Increased exposure to these radiations leads to various health issues,including an increased incidence of skin cancers,cataracts,impaired vision,sunburns,and a weakened immune system.

(A) The Joint Forest Management $(JFM)$ concept was introduced in India in the $1980s$.
Under this initiative,the government works in collaboration with local village and tribal communities to protect and manage degraded forest lands.
This approach ensures that local communities receive benefits from the forest resources in exchange for their active participation in conservation and sustainable management efforts.

(B) : High amount of organic waste in a river will trigger and activate decomposer microbes which will decompose the organic waste.
Biochemical Oxygen Demand $(BOD)$ of this river will shoot up,and decomposers will utilize most of the dissolved oxygen present in the water.
Consequently,the level of dissolved oxygen will decrease to an alarming extent.
Aquatic animals like fish,which depend on the oxygen dissolved in water,will ultimately die due to suffocation.

(D) Lichens are considered pioneer organisms in primary succession on bare rocks. They secrete acids that help in the weathering of rocks,which leads to the formation of soil. This process creates a suitable environment for the growth of other organisms like mosses and eventually higher plants.

(A) Translocation is a type of chromosomal mutation in which a segment of a chromosome breaks off and attaches to another non-homologous chromosome. This results in the movement of a genetic segment from one linkage group to another. Therefore,the correct answer is translocation.

(A) $DNA$ fingerprinting involves identifying differences in specific regions of $DNA$ sequence called repetitive $DNA$. The steps involved are:
$1$. Isolation of $DNA$.
$2$. Digestion of $DNA$ by restriction endonucleases.
$3$. Separation of $DNA$ fragments by electrophoresis.
$4$. Transferring (blotting) of separated $DNA$ fragments to synthetic membranes,such as nitrocellulose or nylon.
$5$. Hybridization using labelled $VNTR$ probe.
$6$. Detection of hybridized $DNA$ fragments by autoradiography.
Polymerase chain reaction $(PCR)$ is often used to amplify the $DNA$ sample if the amount of $DNA$ is very small.
Zinc finger analysis is a technique used for studying $DNA$-binding proteins and is not a part of the standard $DNA$ fingerprinting procedure.

(A) $BOD$ measures the amount of dissolved oxygen required by aerobic microorganisms to decompose the organic matter present in a water sample. It is an effective indicator for organic pollutants,such as those found in domestic sewage,sugar industry waste,and dairy industry waste,which are rich in biodegradable organic matter. However,petroleum industries release non-biodegradable or toxic chemical pollutants that cannot be easily broken down by microorganisms. Therefore,$BOD$ is not a suitable index for measuring pollution caused by petroleum industry effluents.

(C) The correct answer is $C$.
$1$. $Monascus$ $purpureus$ produces statins,which act as blood-cholesterol-lowering agents.
$2$. $Streptococcus$ produces streptokinase,which is used as a 'clot buster' for removing clots from the blood vessels of patients who have undergone myocardial infarction.
$3$. $Clostridium$ $butylicum$ is a bacterium that produces butyric acid. Lipase is produced by the fungus $Candida$ $lipolytica$ or $Rhizopus$ $arrhizus$,not $Clostridium$ $butylicum$. Lipases are used in detergent formulations and are helpful in removing oily stains from the laundry.
$4$. $Trichoderma$ $polysporum$ is a fungus that produces cyclosporin-$A$,which is used as an immunosuppressive agent in organ transplant patients.

(A) Stirred-tank bioreactors are cylindrical vessels designed to facilitate the mixing of the reactor contents.
They are equipped with an agitator system,an oxygen delivery system,a foam control system,a temperature control system,a pH control system,and sampling ports.
The primary purpose of the stirring mechanism is to ensure even mixing and,crucially,to provide optimal oxygen availability throughout the entire culture medium for the aerobic growth of microorganisms.

(B) $1$. When a foreign $DNA$ fragment and a plasmid vector are cut with the same restriction endonuclease,they produce complementary sticky ends.
$2$. These complementary sticky ends allow the foreign $DNA$ to base-pair with the plasmid vector.
$3$. However,the sugar-phosphate backbone of the $DNA$ remains broken at the nicks.
$4$. The enzyme $DNA$ ligase is responsible for catalyzing the formation of phosphodiester bonds between the adjacent nucleotides,thereby sealing the nicks and creating a stable recombinant $DNA$ molecule.

(B) Downstream processing refers to the stages of processing that occur after the biosynthesis of the product in a bioreactor.
It includes the following steps:
$1$. Separation of the product from the culture medium.
$2$. Purification of the product.
$3$. Formulation with suitable preservatives.
$4$. Quality control and clinical trials.
'Expression' is a part of the upstream processing,where the gene of interest is expressed in the host organism to produce the desired product. Therefore,it is not a part of downstream processing.

(D) Restriction enzymes are classified based on the type of ends they produce after $DNA$ cleavage.
$1$. Sticky ends (cohesive ends) are produced by enzymes like $Eco-RI$,$Hind-III$,$Xho-I$,and $Sal-I$,which create staggered cuts.
$2$. Blunt ends (flush ends) are produced by enzymes that cut both strands of $DNA$ at the same position,typically in the center of the recognition site.
$3$. $Eco-RV$ is a well-known restriction enzyme that recognizes the sequence $5'-GATATC-3'$ and cuts between the $T$ and $A$ residues,resulting in blunt ends.
Therefore,the correct option is $D$.

(D) Restriction endonucleases are enzymes that cut $DNA$ at specific recognition sequences.
$Hind-II$ was the first restriction endonuclease to be isolated and characterized.
Proteases are enzymes that break down proteins.
DNase-$I$ and RNase are enzymes that degrade $DNA$ and $RNA$,respectively,but they are not restriction endonucleases.

(C) Plasmids are small,circular,double-stranded $DNA$ molecules that are distinct from a cell's chromosomal $DNA$.
They are found primarily in bacteria and can replicate independently of the chromosomal $DNA$.
They are often used as vectors in genetic engineering because they can be transferred between cells.
Since plasmids are double-stranded,the statement that they are 'single-stranded' is incorrect.

(D) The $Taq$ polymerase enzyme is isolated from a thermophilic bacterium called $Thermus$ $aquaticus$.
This bacterium thrives in high-temperature environments such as hot springs.
Because of its origin,the $Taq$ polymerase enzyme is thermostable,meaning it can withstand the high temperatures required during the denaturation step of the Polymerase Chain Reaction $(PCR)$ process.
Therefore,it is widely used in $PCR$ to amplify $DNA$ segments.

(C) When organic-rich domestic sewage is discharged into a river,it acts as a nutrient source for microorganisms like bacteria.
These microorganisms decompose the organic matter,a process that consumes a large amount of dissolved oxygen $(DO)$ from the water.
This leads to a sharp decline in the $BOD$ (Biochemical Oxygen Demand) levels and a significant reduction in dissolved oxygen.
As a result,aquatic organisms,especially fish,die due to the lack of oxygen (hypoxia).

(B) The Government of India introduced the concept of Joint Forest Management $(JFM)$ in the year $1980$.
This initiative was launched to involve local communities in the management and restoration of degraded forests.
Under this scheme,local communities are entitled to intermediary benefits like non-timber forest products and a share in the timber harvested by successful protection and management.