AIIMS 2008 Biology Question Paper with Answer and Solution

(A) The leech (Annelida),cockroach (Arthropoda),and scorpion (Arthropoda) all belong to the phyla of invertebrates that possess a ventral nerve cord.
$1$. Leech belongs to Phylum Annelida,which is characterized by a ventral nerve cord.
$2$. Cockroach and scorpion belong to Phylum Arthropoda,which also possess a ventral nerve cord.
$3$. Nephridia are specific to Annelids.
$4$. Cephalization is not a defining common feature for all three.
$5$. Antennae are present in insects (cockroach) but absent in leeches and scorpions.

(A) Regeneration is broadly classified into two types: $Morphallaxis$ and $Epimorphosis$.
$Morphallaxis$ is a type of regeneration where the lost body parts are replaced by the remodeling of existing tissues with little or no new cellular proliferation. A classic example is the $Hydra$, where a piece cut transversely can reorganize its existing cells to form a complete, smaller $Hydra$.
$Epimorphosis$ involves the replacement of lost parts through significant cellular proliferation and differentiation, such as limb regeneration in salamanders or wound healing in higher vertebrates.
Option $A$ correctly identifies $Morphallaxis$ with the $Hydra$ example, where the organism reorganizes its existing body structure to regenerate.

(A) In eukaryotic cells,extranuclear $DNA$ is primarily found in mitochondria and chloroplasts (plastids). These organelles contain their own genome. Mitochondrial $DNA$ is inherited through the egg cytoplasm because the egg provides the bulk of the cytoplasm to the zygote,whereas the sperm contributes primarily nuclear $DNA$.

(D) Plastids are organelles found in plant cells and algae. They are classified into three types based on the type of pigments they contain: chloroplasts,chromoplasts,and leucoplasts.
All these plastids originate from proplastids and possess the ability to multiply by a fission-like process.
Chloroplasts contain chlorophyll and carotenoid pigments which are responsible for trapping light energy essential for photosynthesis.
Chromoplasts contain fat-soluble carotenoid pigments like carotene,xanthophylls,and others,which give plants their yellow,orange,or red color.
Leucoplasts are the colourless plastids of varied shapes and sizes with stored nutrients: amyloplasts store carbohydrates (starch),elaioplasts store oils and fats,and aleuroplasts store proteins.
Therefore,the common feature among all these plastids is their origin from proplastids and their ability to divide by fission.

(A) The fluid mosaic model,proposed by $Singer$ and $Nicolson$ in $1972$,describes the cell membrane as a fluid structure.
The fluidity of the membrane is due to the lateral movement of lipids and proteins within the bilayer.
The membrane is described as a mosaic because it is composed of a diverse arrangement of proteins embedded in a phospholipid bilayer.
Since the fluid nature of the membrane allows for the movement of these embedded proteins and lipids,the mosaic composition is directly related to its fluid behavior.
Therefore,both the Assertion and Reason are correct,and the Reason is the correct explanation of the Assertion.

(A) During the $M-$phase of the cell cycle,specifically during telophase,the nuclear envelope begins to reform around the daughter chromosomes.
Before this reformation,the chromosomes,which were highly condensed during metaphase and anaphase,must undergo decondensation to return to their chromatin state.
Simultaneously,the nuclear lamina,which is a dense fibrillar network inside the nucleus,must reassemble to provide structural support to the newly forming nuclear envelope.
Therefore,the decondensation of chromosomes and the reassembly of the nuclear lamina are the critical events that precede the reformation of the nuclear envelope.

(A) Oxyntic cells,also known as parietal cells,are located in the gastric glands of the stomach.
These cells are responsible for the secretion of hydrochloric acid $(HCl)$ and intrinsic factor.
The secretion of $HCl$ results in a highly acidic environment in the stomach,typically maintaining a $pH$ between $2.0$ and $3.0$.
Therefore,the pair of oxyntic cells and their secretion is correctly matched.

(A) The first heart sound, $Lubb$, is produced primarily due to the closure of the atrioventricular $(AV)$ valves (bicuspid and tricuspid valves) at the beginning of ventricular systole.
$Dup$ is the second heart sound, caused by the closure of the semilunar valves at the beginning of ventricular diastole.
$Purkinje$ fibres are responsible for the conduction of the cardiac impulse throughout the ventricles, not for the initiation of the heartbeat (which is the function of the $SA$ node).

(B) The normal fasting blood glucose level in a healthy adult is approximately $70-110\, mg/100\, ml$ (or $mg/dl$).
Option $B$ provides the range $70-100\, mg/100\, ml$, which is clinically accepted as the normal fasting blood sugar range.
Total blood volume in a normal adult human is typically $5-6\, litres$.
Blood is a fluid connective tissue consisting of formed elements (corpuscles) suspended in plasma.
It is mesodermal in origin and slightly alkaline with a $pH$ of $7.3-7.4$.

(B) The thymus is an organ located in the upper anterior portion of the chest cavity just behind the sternum.
The thymus continues to grow between birth and puberty and then begins to atrophy.
Proportional to thymic size,thymic activity is most active before puberty.
Upon atrophy,the size and activity are dramatically reduced,and the organ is primarily replaced with fat.
The atrophy is due to the increased circulating level of sex hormones,and chemical or physical castration of an adult results in the thymus increasing in size and activity.
In contrast,other options are incorrect: Thyroid hyperactivity causes hyperthyroidism,not cretinism (which is caused by hypothyroidism); Parathormone increases blood calcium levels by mobilizing it from bones; and Delta cells of the pancreas secrete somatostatin,not a hormone that stimulates glycolysis.

(C) Both $corpus \ luteum$ and $macula \ lutea$ are characterized by a yellow colour.
$1$. The $corpus \ luteum$ is a temporary endocrine structure in the ovary of mammals,formed after ovulation,which secretes progesterone to maintain pregnancy.
$2$. The $macula \ lutea$ is an oval-shaped yellowish pigmented area near the center of the retina in the human eye,responsible for sharp,detailed central vision.

(B) The genetic code is degenerate and specific.
- $UUA$ codes for Leucine.
- $AAA$ codes for Lysine.
- $AUG$ codes for Methionine (it also acts as a start codon).
- $CCC$ codes for Proline.
Therefore,the pair $AAA$-Lysine is correctly matched.

(D) The human genome contains approximately $3.1647$ billion nucleotide base pairs. According to the Human Genome Project,the total number of nitrogenous bases is estimated to be about $3.1$ billion.

(B) $Spirulina$ is a type of blue-green alga (cyanobacteria) that is widely used as a dietary supplement.
It is considered a superfood because it is exceptionally rich in high-quality proteins,containing all essential amino acids.
In addition to protein,$Spirulina$ is packed with vitamins,minerals,carotenoids,and essential fatty acids.
Due to its high nutritional density,it is often recommended for individuals recovering from long illnesses to help restore strength and provide essential nutrients efficiently.

(B) The Assertion is correct because tapeworms $(Taenia)$,roundworms $(Ascaris)$,and pinworms $(Enterobius)$ are well-known endoparasites that inhabit the human intestine.
The Reason is also correct as the consumption of improperly cooked or contaminated food and water is a primary source of transmission for these intestinal parasites.
However,the Reason is not the direct explanation for the Assertion. The Assertion describes the biological classification and habitat of these organisms,while the Reason describes the mode of transmission. Therefore,both statements are true,but the Reason does not explain why they are endoparasites.

(D) The plant part which consists of two generations one within the other is the $seed$.
In a $seed$, the embryo represents the next generation (sporophyte), which is enclosed within the maternal tissues (seed coat and remains of the ovule) of the parent plant.
Specifically, the $seed$ contains the embryo (new generation) protected by the seed coat, which is derived from the integuments of the ovule (parental generation).

(B) Colour blindness is an $X$-linked recessive trait.
$1$. The woman is normal but her father was colour blind,so she must be a carrier for the trait. Her genotype is $X^hX$.
$2$. The man is normal,so his genotype is $XY$.
$3$. The cross between the carrier woman $(X^hX)$ and the normal man $(XY)$ results in the following offspring:
- $X^hX$ (Carrier daughter)
- $XX$ (Normal daughter)
- $X^hY$ (Colour blind son)
- $XY$ (Normal son)
$4$. Among the sons,one is colour blind $(X^hY)$ and one is normal $(XY)$. Therefore,$50\%$ of the sons would be colour blind.

(B) In genetics,a test cross,first introduced by Mendel,is used to determine if an individual exhibiting a dominant trait is homozygous or heterozygous for that trait.
Test crosses involve breeding the individual in question with another individual that expresses a recessive version of the same trait.
If all offspring display the dominant phenotype,the individual in question is homozygous dominant.
If the offspring display both dominant and recessive phenotypes,then the individual is heterozygous.
$A$ test cross is a specific type of back cross where an individual is crossed with a recessive homozygote.

(C) Haemophilia is a group of hereditary genetic disorders that impair the body's ability to control blood clotting or coagulation.
In its most common form,Haemophilia $A$,the blood clotting factor $VIII$ is absent or deficient.
In Haemophilia $B$,factor $IX$ is deficient.
Factor $VIII$ acts as a cofactor for factor $IXa$,which in the presence of $Ca^{+2}$ and phospholipids,forms a complex that converts factor $X$ to its activated form,$Xa$.
Defects in the gene encoding factor $VIII$ result in Haemophilia $A$,which is a recessive $X$-linked coagulation disorder.
The Reason statement is incorrect because the concentration of platelets in haemophilia patients is typically normal; the disorder is caused by a deficiency in specific clotting factors,not a deficiency in platelet count or prothrombin production.

(A) $DNA$ replication and transcription occur inside the nucleus in eukaryotic cells. After transcription,the $mRNA$ is processed and exported from the nucleus into the cytoplasm. In the cytoplasm,the $mRNA$ binds to ribosomes,where translation (protein synthesis) takes place using available amino acids and $tRNA$. This flow of genetic information from $DNA$ to $RNA$ to protein is known as the Central Dogma. Since the Reason correctly explains why translation must occur in the cytoplasm (due to the presence of necessary machinery),the Assertion and Reason are both correct,and the Reason is the correct explanation.

(D) Amphetamines are stimulants that increase the activity of the $CNS$.
Lysergic acid diethylamide $(LSD)$ is a well-known hallucinogen.
Heroin is a depressant (opioid) that slows down body functions.
Benzodiazepines are sedatives or tranquilizers used for anxiety and sleep disorders,not primarily as pain killers (analgesics). Therefore,the pair 'Benzodiazepine $-$ pain killer' is incorrect as they are not classified as analgesics.

(B) An insect bite causes tissue damage,which triggers an inflammatory response at the site of the injury.
This response is mediated by the release of chemical mediators known as alarm chemicals.
$Histamine$ is released by mast cells and basophils,which causes vasodilation and increased capillary permeability,leading to redness and swelling.
$Kinins$ (such as bradykinin) are polypeptides that also promote vasodilation,increase vascular permeability,and stimulate pain receptors.
Therefore,the correct combination of alarm chemicals involved in this process is $histamine$ and $kinins$.

(C) An antibody molecule is $Y-$shaped and consists of four polypeptide chains: two identical heavy $(H)$ chains and two identical light $(L)$ chains. The antigen-binding site,also known as the paratope,is located at the variable region of the antibody. This site is formed by the interaction between the variable domain of one heavy chain $(V_H)$ and the variable domain of one light chain $(V_L)$. Therefore,the antigen-binding site is found between one heavy and one light chain.

(B) Diarrhoeal diseases involve the frequent and excessive discharge of watery stools from the bowel.
These diseases are primarily caused by the ingestion of harmful pathogens through contaminated food and water.
$Escherichia$ $coli$,$Shigella$ $sp.$,and $Salmonella$ $sp.$ are well-known bacterial pathogens that cause diarrhoea.
Diarrhoea caused by these organisms involves two main mechanisms: first,the pathogens colonize the intestine,causing inflammation (enteritis); second,they disrupt the balance of intestinal fluid absorption and secretion,significantly increasing fluid loss,which manifests as frequent,watery stools.
Dehydration is a common and dangerous consequence of all types of diarrhoeal diseases.
Therefore,maintaining an adequate supply of fluids and electrolytes is essential for the management and treatment of these conditions.
Since both statements are factually correct but the reason describes the management of the disease rather than explaining why these specific bacteria cause the disease,the correct option is $B$.

(D) The Assertion is incorrect because a 'Dope test' is specifically used to detect the presence of performance-enhancing drugs in athletes,not to measure blood alcohol levels. Breathalyzers are used for alcohol testing.
The Reason is also incorrect because a person under the influence of alcohol typically exhibits symptoms like loss of inhibition,increased talkativeness,and reduced coordination,rather than feeling tense or less talkative.
Therefore,both the Assertion and the Reason are incorrect.

(D) Interferons are not antibodies; they are a class of glycoproteins known as cytokines produced by cells in response to viral infections,not bacterial infections. Therefore,the Assertion is incorrect.
Interferons do not primarily stimulate inflammation at the site of injury; their main function is to inhibit viral replication and induce an antiviral state in neighboring cells. Therefore,the Reason is also incorrect.
Since both the Assertion and the Reason are false,the correct option is $D$.

(A) Mast cells contain granules rich in histamine,serotonin,and heparin.
When allergens enter the body,they bind to $IgE$ antibodies on the surface of mast cells in sensitive individuals.
This binding triggers the degranulation of mast cells,causing them to release large amounts of inflammatory chemicals like histamine into the bloodstream.
Histamine acts as an initiator of the inflammatory response,leading to symptoms such as capillary dilation,increased permeability,mucus secretion,and swelling,which are characteristic of allergic reactions.
Therefore,the Reason correctly explains why mast cells release these chemicals.

(A) $1$. The graph shows the relationship between the weight of the Khapra beetle larvae and the concentration of cholesterol in their diet.
$2$. At $0 \, \mu g/g$ cholesterol,the growth is minimal or zero,indicating that cholesterol is necessary for the development of these larvae.
$3$. As the concentration of cholesterol increases,the weight of the larvae increases,showing that cholesterol is an essential dietary requirement for the Khapra beetle.
$4$. Option $A$ is the most accurate interpretation because the graph demonstrates that without cholesterol,the larvae do not grow,confirming it is an essential nutrient.

(A) The concept of totipotency states that any living plant cell has the inherent capacity to generate a whole new plant.
In plant tissue culture,somatic embryos can be induced from various plant cells because of this property of totipotency.
Since any viable plant cell possesses the potential to differentiate into somatic embryos under appropriate culture conditions,the Reason correctly explains the Assertion.
Therefore,both the Assertion and Reason are correct,and the Reason is the correct explanation of the Assertion.

(C) The Southern blot technique is a method used to detect specific $DNA$ sequences in a sample.
The process begins with the isolation of $DNA$ from the sample,followed by the digestion of the $DNA$ using restriction enzymes.
Restriction enzymes act as molecular scissors that cut the $DNA$ at specific recognition sites,resulting in $DNA$ fragments of varying lengths.
After digestion,these fragments are separated based on their size using gel electrophoresis.

(A) In recombinant $DNA$ technology,human genes are inserted into host organisms like bacteria or yeast to produce large quantities of desired proteins.
Bacteria and yeast are preferred as host cells because they have a very short generation time and multiply rapidly,allowing for the creation of a large population of cells in a short period.
This large population of cells expresses the inserted gene,leading to the efficient production of the recombinant protein.
Therefore,the Reason correctly explains why human genes are transferred into these organisms.

(D) $Bt$ cotton refers to the cultivation of genetically modified cotton plants.
The prefix $Bt$ stands for the bacterium $Bacillus$ $thuringiensis$.
This bacterium naturally produces a specific protein that acts as an endotoxin,which is toxic to certain insect pests such as lepidopterans,coleopterans,and dipterans.
By inserting the gene responsible for this endotoxin into the cotton plant,the plant becomes resistant to these pests,reducing the need for chemical insecticides.

(C) food web is a complex network of interconnected food chains within an ecosystem,representing the various pathways through which energy and nutrients flow.
In a food web,organisms occupy different trophic levels,such as producers,primary consumers,secondary consumers,and tertiary consumers.
Since a kite is a bird of prey (a secondary or tertiary consumer),it feeds on various organisms like small mammals,reptiles,or insects,making it an integral part of multiple food chains within a food web.
Therefore,the Assertion is correct,but the Reason is incorrect.

(C) The Eastern Himalayas and the Western Ghats are recognized as two of the most significant biodiversity hotspots in India.
Biodiversity hotspots are regions that harbor a high level of species richness and a high degree of endemism,meaning many species found there are not found anywhere else on Earth.
The Eastern Himalayas are home to a vast array of flora and fauna,including $163$ globally threatened species such as the Asian elephant $(Elephas \text{ } maximus)$,the greater one-horned rhinoceros $(Rhinoceros \text{ } unicornis)$,and the tiger $(Panthera \text{ } tigris)$.
The Western Ghats are also globally renowned for their exceptional biodiversity,containing a large number of endemic plant and animal species.
Therefore,these two regions are considered to have the maximum biodiversity in our country.