AIIMS 2018 Biology Question Paper with Answer and Solution

(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) Leaf tendrils are specialized, thread-like, sensitive structures that are often modified parts of leaves. In the plant $Pisum \text{ } sativum$ (Pea), the terminal leaflets are modified into tendrils to help the plant climb. In contrast, in Grapevine, Watermelon, and Cucumber, the tendrils are stem modifications.

(C) In the given figure:
$A$ represents the caecum,which is a small blind sac that hosts some symbiotic microorganisms.
$B$ is the vermiform appendix,a narrow finger-like tubular projection arising from the caecum.
$C$ is the ascending colon,which is the first part of the colon moving upwards.
$D$ is the transverse colon,which runs across the abdomen.
$E$ is the descending colon,which moves downwards.
$F$ is the sigmoid colon,which is the $S$-shaped part leading to the rectum.
Therefore,the correct identification is $A$ - Caecum,$B$ - Vermiform appendix,$C$ - Ascending colon,$D$ - Transverse colon,$E$ - Descending colon,$F$ - Sigmoid colon.

(B) . Diakinesis: During this stage, the chiasmata undergo terminalisation.
$B$. Pachytene: This stage is characterized by the occurrence of crossing over between non-sister chromatids of homologous chromosomes.
$C$. Zygotene: During this stage, homologous chromosomes pair up, a process known as synapsis.
$D$. Metaphase: In this stage, the bivalent chromosomes align at the equatorial plate.
Therefore, the correct matching is $A-2, B-1, C-4, D-3$.

(A) Apical dominance is a phenomenon where the main central stem of the plant grows more dominantly than the side stems.
This process is primarily caused by the hormone $Auxin$,which is synthesized at the shoot tips.
$Auxin$ inhibits the growth of lateral buds,thereby promoting the vertical growth of the plant.

$(C)$. Thiobacillus performs denitrification, which is the process of converting nitrates back into nitrogen gas $(2)$.
$B$. Nitrosomonas is a nitrifying bacterium that oxidizes $NH_{3}$ to nitrite $(4)$.
$C$. Nostoc is a free-living nitrogen-fixing cyanobacterium $(1)$.
$D$. Azotobacter is a free-living aerobic nitrogen-fixing bacterium $(3)$.
Therefore, the correct matching is $A-2, B-4, C-1, D-3$.

(B) The given plant belongs to the family $Papilionaceae$ (a subfamily of $Fabaceae$).
This family is characterized by a papilionaceous corolla,which consists of $5$ petals.
The arrangement includes one large posterior petal called the standard (or vexillum),two lateral petals called wings (or alae),and two anterior petals fused to form a boat-shaped structure called the keel (or carina).

(D) Based on the structural characteristics shown in the figures:
$1$. Figure $A$ shows cells with thin primary walls and intercellular spaces,which are characteristic of Parenchyma.
$2$. Figure $B$ shows cells with thickened corners (primary wall thickening),which is characteristic of Collenchyma.
$3$. Figure $C$ shows cells with very thick secondary walls and narrow lumens,which is characteristic of Sclerenchyma.
Therefore,the correct identification is $A$ - Parenchyma,$B$ - Collenchyma,$C$ - Sclerenchyma.

(D) The Casparian strip is a band of cell wall material deposited in the radial and transverse walls of the endodermis.
It is composed of suberin and sometimes lignin.
Its primary function is to block the apoplastic pathway,forcing water and dissolved solutes to pass through the plasma membrane of the endodermal cells (symplastic pathway).
This allows the plant to selectively regulate the uptake of water and minerals.
Therefore,if a plant cannot produce the Casparian strip,it loses the ability to control the amount of water and solutes entering the vascular cylinder.

(A) Water potential $\Psi_w$ is the sum of solute potential $\Psi_s$ and pressure potential $\Psi_p$, given by the formula: $\Psi_w = \Psi_s + \Psi_p$.
For cell $A$:
$\Psi_A = -20 + 5 = -15$ bars.
For cell $B$:
$\Psi_B = -18 + 2 = -16$ bars.
Water always moves from a region of higher water potential to a region of lower water potential. Since $-15$ bars is greater than $-16$ bars, the water will flow from cell $A$ to cell $B$.

(A) In the apoplast pathway,water moves from root hairs to the xylem exclusively through the cell wall and intercellular spaces,without crossing any plasma membranes.
The apoplast consists of the non-living parts of the plant,including cell walls and intercellular air spaces,providing a continuous path for water movement.

(A) $C_{4}$ plants are more advantageous than $C_{3}$ plants because they can efficiently fix carbon even in low $CO_{2}$ concentrations.
In hot and dry conditions,plants close their stomata to prevent water loss,which leads to a decrease in internal $CO_{2}$ levels.
$C_{3}$ plants suffer from photorespiration under these conditions because $RuBisCO$ acts as an oxygenase,leading to the loss of $RuBP$ and inhibition of the Calvin cycle.
$C_{4}$ plants possess a specialized anatomy called $Kranz$ anatomy and a $PEP$ carboxylase enzyme that has a high affinity for $CO_{2}$,allowing them to maintain photosynthesis even when $CO_{2}$ levels are low.

(C) Photosynthetic bacteria,such as purple and green sulphur bacteria,perform photosynthesis in anoxic (oxygen-free) environments.
These organisms use bacteriochlorophyll instead of chlorophyll $a$.
Instead of water $(H_{2}O)$,they use other hydrogen donors like hydrogen sulphide $(H_{2}S)$ or organic compounds.
Because water is not split during this process,oxygen is not evolved as a byproduct.

(D) The figures represent different types of aestivation:
$A$ - Valvate: The margins of sepals or petals touch each other without overlapping.
$B$ - Twisted: One margin of the sepal or petal overlaps that of the next one.
$C$ - Imbricate: The margins of sepals or petals overlap one another but not in any particular direction.
$D$ - Quincuncial: Two petals are completely external,two are completely internal,and one is partially external and partially internal.
$E$ - Vexillary: The largest posterior petal (standard) overlaps the two lateral petals (wings),which in turn overlap the two smallest anterior petals (keel).
Therefore,the correct order is $A$ - Valvate,$B$ - Twisted,$C$ - Imbricate,$D$ - Quincuncial,$E$ - Vexillary.

(C) The correct matches are as follows:
$A$. Hinge joint: Found in the knee joint $(4)$.
$B$. Saddle joint: Found between the carpal and metacarpal of the thumb $(3)$.
$C$. Pivot joint: Found between the atlas and axis vertebrae $(1)$.
$D$. Ball and socket joint: Found between the humerus and pectoral girdle $(5)$.
Thus, the correct sequence is $A-4, B-3, C-1, D-5$.

(B) : $XO$ type of sex chromosomes determine male sex in grasshoppers. This type of sex determination comes under $XX-XO$ type. Its common examples are cockroaches,grasshoppers,and bugs. The female has two homomorphic sex chromosomes $(XX)$ and is homogametic. It produces similar eggs,each with $X$-chromosome. The male has one chromosome only and is heterogametic. It produces $2$ types of sperms: gynosperms with $X$ and androsperms without $X$. Fertilization of an egg by $X$-bearing sperm yields female offspring and by no-$X$ sperm yields male offspring. The chromosomal constitution is represented as $AA + XX$ (female) and $AA + XO$ (male).

(B) In grafting,the scion is the upper part of the plant that is selected for its desirable traits,such as fruit quality,yield,or disease resistance. The stock is the lower part that provides the root system. Since the scion contains the genetic information that dictates the development of branches,leaves,flowers,and fruits,the quality of the fruits produced is determined by the genotype of the scion.

(C) Biomagnification is a phenomenon through which certain pollutants get accumulated in tissues in increasing concentrations at successive trophic levels along the food chain.
Many pesticides,such as $DDT$,have a long life.
Thus,they get incorporated into the food chain and get magnified at higher trophic levels.

(D) The frequency of recombination between two genes is inversely proportional to the distance between them on the chromosome.
Statement $I$ is incorrect because tightly linked genes show very few recombinations,not higher.
Statement $II$ is incorrect because genes that are far apart on the same chromosome show a higher frequency of recombinations due to more crossing-over events.
Statement $III$ is incorrect because loosely linked genes show higher recombination frequencies compared to tightly linked genes,and the term 'similar recombinations' is vague and scientifically inaccurate in this context.
Therefore,all three statements are incorrect.

(D) Statement $I$ is incorrect because species diversity decreases,not increases,as we move from the equator towards the poles.
Statement $II$ is correct; Stellar's sea cow and passenger pigeon are classic examples of extinction due to overexploitation by humans.
Statement $III$ is correct; Lantana camara and Eichhornia crassipes (water hyacinth) are well-known invasive alien species in India.
Statement $IV$ is correct; the historic Convention on Biological Diversity (The Earth Summit) was held in Rio de Janeiro in $1992.$

(D) Cancerous cells that break away from the original site and spread to other parts of the body through the bloodstream and lymphatic system constitute malignant tumours.
The spread of cancer to other sites or organs in the body is called metastasis.
Other statements are incorrect because:
$1$. Cancer cells lose the property of contact inhibition,which is why they continue to divide uncontrollably.
$2$. Not all tumours are cancerous; for example,benign tumours remain confined to their original location.
$3$. Not all viruses are oncogenic (cancer-causing).

(A) The Nitrogen cycle is a gaseous nutrient cycle because its reservoir pool is the atmosphere. Sedimentary cycles,such as the Phosphorus or Sulphur cycle,have the lithosphere as their reservoir pool.
The pyramid of biomass in an aquatic ecosystem like the sea is inverted because the biomass of phytoplankton is much lower than that of the zooplankton and fishes that feed on them.
Therefore,statements $I$ and $IV$ are incorrect.

(B) Bacterial blight of chickpea is caused by the bacterium $Xanthomonas$ $campestris$.
The stem and leaves of the infected plant give a blighted or burnt-up appearance.
Control measures for bacterial diseases in crops typically include:
$1.$ Using disease-free seeds to prevent primary infection $(III)$.
$2.$ Using disease-resistant varieties,which is the most effective and sustainable method $(IV)$.
$3.$ Spraying with copper-based fungicides like Bordeaux mixture to manage bacterial populations $(I)$.
While insect vectors are crucial for viral diseases,bacterial blight is primarily seed-borne or soil-borne,making $I, III,$ and $IV$ the most appropriate measures for control.

(C) Statement $I$ is correct: Gene therapy involves the introduction, removal, or alteration of genetic material within a patient's cells to treat a disease at the molecular level.
Statement $II$ is incorrect: Calcitonin is a hormone secreted by the thyroid gland ($C$-cells) that regulates calcium and phosphate levels in the blood. It is not used to treat infertility. Infertility treatments often involve hormones like $FSH$ or $LH$.
Statement $III$ is correct: $Bt$ toxin is produced by the bacterium $Bacillus \text{ } thuringiensis$ and acts as a natural, biodegradable insecticide.
Therefore, statements $I$ and $III$ are correct.

(C) Transformation is the process of introducing foreign $DNA$ into a host cell.
If the introduced $DNA$ carries an ampicillin resistance gene,the cells that successfully take it up (transformed cells) will be able to survive and grow on agar medium containing ampicillin.
Cells that do not take up the $DNA$ (untransformed cells) lack this resistance and will die.
In this context,the ampicillin resistance gene acts as a selectable marker.

(C) $Wuchereria$ $bancrofti$ is a digenetic human parasite that completes its life cycle in two hosts.
The human is the final host that harbours the adult worms in the lymphatic vessels and lymph nodes.
This infection leads to a chronic inflammation of the organs in which they live for many years, usually the lymphatic vessels of the lower limbs, causing elephantiasis.

(C) $Cissus$ $quadrangularis$ is an ancient medicinal plant of India.
It has specific bone-healing properties and helps in the faster increase of bone tensile strength.
It acts as a glucocorticoid antagonist.
Since anabolic compounds are known to act as antagonists to the glucocorticoid receptor as well as promote bone growth and fracture healing,it has been concluded that $Cissus$ possesses significant bone-healing properties.

(D) Genetic code is defined as the sequence of nucleotides in a polynucleotide chain that determines the sequence of amino acids in a polypeptide chain.
Because the genetic code is universal,a specific codon codes for the same amino acid in all living organisms,ranging from bacteria to humans.
Therefore,when a human gene is inserted into a bacterial cell,the bacteria can read the genetic instructions and synthesize the corresponding human protein.

(C) Statement $I$ is incorrect because flippers of whales and wings of bats are homologous organs,as they share a common embryonic origin and basic skeletal structure.
Statement $II$ is incorrect because wings of butterflies and wings of birds are analogous organs; they perform the same function (flight) but have different developmental origins.
Statement $III$ is correct: Analogous organs are defined as structures that have different basic structures and developmental origins but perform similar functions.
Statement $IV$ is correct: Homologous organs are defined as structures that share a similar basic structure and embryonic origin,even if they perform different functions.
Therefore,statements $III$ and $IV$ are correct.

(C) Intergeneric hybridisation involves crossing plants that belong to different genera but the same family.
Triticale is the first man-made cereal crop,which was developed by crossing wheat ($Triticum$ species) and rye ($Secale$ species).
Since wheat and rye belong to two different genera,this process is known as intergeneric hybridisation.
Therefore,the correct option is $C$.

(D) $Azolla$ is a pteridophyte and it is used as a biofertiliser because it has a symbiotic association with nitrogen-fixing cyanobacteria called $Anabaena \ azollae$.

(D) Severe Combined Immunodeficiency $(SCID)$ is a rare genetic disorder that severely compromises the immune system. Individuals with $SCID$ lack functional $T$ and $B$ lymphocytes, making them extremely susceptible to infections. Because they cannot fight off common pathogens, they must live in a sterile, protective environment, often referred to as a 'bubble'. This condition is famously known as '$baby$ $in$ $a$ $bubble$ $syndrome$'. While treatments like bone-marrow transplants or enzyme replacement therapy exist, they are not always curative, leading to the use of gene therapy as a potential permanent solution.

(D) Industrial melanism is an adaptation where the moths living in industrial areas developed melanin pigments to match their body to the black soot (smoke) covered surroundings.
Before industrialisation, the light-coloured moths were well-camouflaged against the whitish lichens on tree barks, making them less visible to predatory birds.
After industrialisation, the tree barks became covered with black soot, making the light-coloured moths highly visible and easily preyed upon by birds.
Conversely, the dark-coloured (melanic) moths were better camouflaged against the soot-covered barks, allowing them to survive and reproduce.
This process, where the environment selects for the better-adapted phenotype, is a classic example of natural selection.

(A) The woman is a carrier for colourblindness because her father was colourblind $(X^{C}X)$. The man is normal $(XY)$.
The cross is as follows:
Parents: $X^{C}X \times XY$
Gametes: $X^{C}, X$ and $X, Y$
Offspring genotypes:
$X^{C}X$ (Carrier daughter,phenotypically normal)
$XX$ (Normal daughter)
$X^{C}Y$ (Colourblind son)
$XY$ (Normal son)
Thus,all daughters are phenotypically normal (though $50 \%$ are carriers),and $50 \%$ of the sons are colourblind.

(D) The provided graph shows a logistic growth curve where the population size stabilizes as it reaches the carrying capacity. At this stage,the intrinsic rate of natural increase $(r)$ becomes zero $(r = 0)$.
When $r = 0$,the population is stable,meaning the number of individuals in the pre-reproductive and reproductive age groups is approximately equal,while the post-reproductive group is the smallest. This age structure is represented by a bell-shaped age pyramid.

(C) Change in population density provides insight into whether a population is increasing or declining.
Population density is influenced by natality,mortality,immigration,and emigration.
In an isolated small tribal population,if individuals marry only within their own tribe,it leads to a restricted gene pool.
Over time,this practice increases the frequency of homozygous recessive traits,including hereditary diseases.
Furthermore,such isolation often leads to a decline in population due to reduced genetic diversity and potential reproductive barriers,ultimately resulting in a decrease in the population size.

(C) The codons for the amino acid glycine are $GGU, GGC, GGA,$ and $GGG$.
In the genetic code,a codon is a sequence of three nucleotides (triplet) that specifies a particular amino acid.
Glycine is a non-essential amino acid,and its codons all start with the nucleotides $GG$ followed by any of the four bases $(U, C, A, G)$.

(B) The correct matching is as follows:

$A$. Fumagillin	$4$. Aspergillus fumigatus
$B$. Bacitracin	$5$. Bacillus licheniformis
$C$. Streptomycin	$2$. Streptomyces griseus
$D$. Viridin	$1$. Gliocladium virens
$E$. Polymixin	$3$. Bacillus polymyxa

Thus, the correct sequence is $A-4, B-5, C-2, D-1, E-3$. Therefore, option $B$ is correct.

(A) The $Bt$ toxin genes are isolated from $Bacillus \text{ } thuringiensis$ and incorporated into several crop plants.
Specifically, the genes $cry \text{ } I \text{ } Ac$ and $cry \text{ } II \text{ } Ab$ are incorporated into cotton to provide resistance against cotton bollworms.
Similarly, the gene $cry \text{ } I \text{ } Ab$ is introduced into corn to protect it from the corn borer.
Therefore, $cry \text{ } II \text{ } Ab$ controls cotton bollworms and $cry \text{ } I \text{ } Ab$ controls corn borer.

(B) Statement $I$ is incorrect because colostrum is rich in antibodies (specifically $IgA$),not antigens.
Statement $II$ is incorrect because Chikungunya is caused by the Chikungunya virus,which is an $RNA$ virus,not a bacterium.
Statement $III$ is correct because meristematic tissues are free from viral infection,allowing the production of virus-free plants through tissue culture.
Statement $IV$ is incorrect because beer is produced by the fermentation of malted cereals (like barley) without distillation. Distillation is used for spirits like whiskey or brandy.
Therefore,only one statement $(III)$ is correct.