NEB Class 12 · Past paper
The complete NEB Class 12 2082 exam paper for Biology, all 12 questions with solved model answers.
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Differentiate between simple permanent tissue and complex permanent tissue on the basis of structure, types, cell composition and functions. [4]
The diagram below illustrates a type of artificial vegetative propagation in an angiosperm. Answer the following: i) Identify the type of vegetative propagation and name the parts labelled P and Q. [1.5] ii) Briefly explain this type of propagation with an example. [1.5] iii) Mention any two applications of this method. [1]
i) The illustrated method is grafting. P = stock (rooted lower part) and Q = scion (the detached shoot/bud of the desired plant).
ii) In grafting, the scion of a desired variety is joined to the rooted stock so that their cambium layers unite and grow as one plant; the stock supplies water and minerals while the scion produces the shoot. Example: grafting of mango, apple, citrus and rose.
iii) Applications: (a) to multiply superior/desired varieties that do not come true from seed and combine good root (stock) with good shoot (scion) qualities; (b) to propagate seedless plants and improve disease resistance/early fruiting.
In which process are the linked genes inherited together from one generation to another with parental and non-parental combinations? Explain the process with a suitable example. [4] (Or) How does diffusion differ from osmosis based on the types of molecules involved across a cell membrane? Explain how environmental factors influence the rate of diffusion. [2+2]
The process is linkage together with crossing over. Linkage is the tendency of genes located on the same chromosome to be inherited together; crossing over between them occasionally separates them, giving recombinant (non-parental) combi...
How can GMOs be used to address global food security challenges and what are the potential environmental concerns and health risks associated with their use? [2+2]
Use of GMOs for food security: genetically modified crops can give higher yields, resistance to pests, diseases, herbicides and abiotic stress (drought, salinity), longer shelf life, and enriched nutrition (e.g. Golden Rice with pro-vitamin A, Bt cotton/maize), helping feed a growing population on limited land.
Environmental concerns and health risks: possible escape of transgenes to wild/weedy relatives, harm to non-target organisms and loss of biodiversity, development of resistant pests/weeds, and reduced use of traditional varieties. Health risks include possible allergenicity, antibiotic-resistance marker genes and unknown long-term effects, raising ethical and biosafety concerns.
How do different photosynthetic pigments contribute to capturing light energy? Explain how cyclic and non-cyclic photo-phosphorylation generate ATP and NADPH, and illustrate these with appropriate charts. [2+3+3]
Photosynthetic pigments: Chlorophyll a is the primary pigment (reaction centre) that converts light into chemical energy; chlorophyll b, carotenoids and xanthophylls are accessory pigments that absorb light of different wavelengths and transfer the captured energy to chlorophyll a, widening the range of light used and protecting against photo-oxidation.
Non-cyclic photophosphorylation: involves both PS II and PS I. Light splits water (photolysis) at PS II, releasing $O_2$, $H^+$ and electrons. Electrons flow PS II → plastoquinone → cytochrome → plastocyanin → PS I → ferredoxin → NADP$^+$, forming NADPH, and ATP is generated along the way. It is a non-cyclic (open, Z-scheme) path producing ATP + NADPH + $O_2$.
$$H_2O \to PS,II \to PS,I \to NADP^+ \Rightarrow NADPH,\ ATP,\ O_2$$
Cyclic photophosphorylation: involves only PS I. The excited electron from PS I passes through the carriers and returns to PS I itself, generating only ATP (no NADPH, no $O_2$, no water splitting). It operates when NADPH is sufficient but more ATP is needed.
$$PS,I \to ferredoxin \to cytochrome \to PS,I \Rightarrow ATP\ \text{only}$$
Explain how colour blindness is inherited as a sex-linked trait in humans. Illustrate the inheritance of colour blindness using suitable genetic crosses and analyze the resulting phenotypic outcomes. Based on your analysis, justify why this pattern is referred to as criss-cross inheritance. [2+4+2] (Or) Scientists proposed different models of DNA replication. Identify the biologically feasible model that occurs during the S-phase of interphase. Explain the mechanism of this replication process highlighting the role of enzymes, and illustrate with necessary diagrams. [3+5]
Colour blindness is an X-linked recessive disorder; the defective allele ($X^c$) lies on the X chromosome. A male ($X^c Y$) is colourblind with a single allele, while a female is colourblind ($X^c X^c$) only when she has the allele on both X chromosomes; $X^C X^c$ females are normal carriers.
Cross - carrier mother × normal father ($X^C X^c \times X^C Y$):
$$\text{Offspring: } X^C X^C,\ X^C X^c,\ X^C Y,\ X^c Y$$
Daughters: 50% normal, 50% carrier (none colourblind); Sons: 50% normal, 50% colourblind.
Cross - colourblind mother × normal father ($X^c X^c \times X^C Y$): all daughters are carriers ($X^C X^c$, normal) and all sons are colourblind ($X^c Y$).
Justification (criss-cross inheritance): the trait passes from a colourblind father to his carrier daughters and then to his grandsons through them, and a colourblind mother passes it to all her sons - the character crosses from father to daughter to grandson (and mother to son), i.e. it skips the same sex in alternate generations, hence 'criss-cross' inheritance.
(Or) The feasible model is semi-conservative replication (Meselson-Stahl), occurring in the S-phase. Helicase unwinds the helix; primase lays RNA primers; DNA polymerase adds nucleotides $5'\to3'$ making a continuous leading strand and discontinuous lagging strand (Okazaki fragments joined by DNA ligase), so each daughter DNA has one old + one new strand.
"Squamous epithelium is called pavement epithelium". Give reason. Write any three differences between squamous epithelium and cuboidal epithelium. [1+3]
Reason: Squamous epithelium is called pavement epithelium because its cells are flat, thin and plate-like, and fit together edge to edge like the tiles/slabs of a pavement.
Differences:
Write about neurulation process in development of frog. [4] (Or) Name any two indigenous fish species. Discuss how integrated fish farming is advantageous over traditional fish farming. [1+3]
Neurulation in frog is the formation of the neural tube after gastrulation, forming the neurula stage. The notochord induces the overlying dorsal ectoderm to thicken into a neural plate. The margins of the plate rise up as neural folds enclosing a neural groove; the folds then move towards each other and fuse middorsally to form the neural tube, which sinks below the ectoderm. The anterior part of the neural tube forms the brain and the posterior part forms the spinal cord; neural crest cells give rise to nerves and other structures.
(Or) Indigenous fishes: Sahar (Tor putitora) and Asala (Schizothorax) (also Naini/Rohu). Integrated fish farming (combining fish with poultry, pig, duck or paddy) is advantageous because animal wastes act as manure/feed for fish, reducing feed cost; it gives two or more products from the same land and water, uses resources efficiently, increases income and reduces pollution, unlike traditional single-species fish farming.
Study the given figure of human internal ear and answer the questions that follow: i) Label part A, B, C, D. ii) Write function of A and D. [2+2]
i) Labels (as read from the figure, the internal ear consists of): semicircular canals, utricle and saccule (vestibule), cochlea, and the auditory (vestibulocochlear) nerve.
ii) Functions:
(The utricle and saccule maintain static balance, and the auditory nerve carries impulses to the brain.)
Renal disorder is the malfunctioning of kidneys. Identify any two risk factors responsible for kidney disorders and suggest any two healthy habits to protect our kidneys. [2+2]
Two risk factors for kidney disorders: - Diabetes mellitus and high blood pressure (hypertension), which damage the delicate filtering blood vessels of the kidney. - Dehydration and habitual low water intake, and misuse of pain-killers/d...
Mention the causative agent, mode of transmission, symptoms of pulmonary tuberculosis. Highlight various strategies for its prevention. Why is it called "disease of poverty"? [1+2+2+2+1]
Causative agent: the bacterium Mycobacterium tuberculosis (Koch's bacillus).
Mode of transmission: mainly by droplet (air-borne) infection - when an infected person coughs, sneezes or spits, bacteria-laden droplets are inhaled by others; also through contaminated milk/utensils and prolonged close contact in crowded places.
Symptoms: persistent cough for more than three weeks (often with blood-stained sputum), chest pain, evening rise of temperature (mild fever), night sweats, loss of appetite, weight loss and weakness.
Prevention strategies: BCG vaccination, early diagnosis and complete treatment with anti-TB drugs (DOTS), isolation and covering the mouth while coughing, good ventilation, avoiding overcrowding, nutritious diet, pasteurisation of milk and health awareness.
Why 'disease of poverty': it spreads most in poor communities because of overcrowding, malnutrition, poor sanitation, poor ventilation and lack of access to health care, all linked to poverty.
Reproduction is a process of continuation of the human race. Describe the female reproductive system with a well labelled diagram of human being. How would you explain ovulation in the female human? [4+3+1] (Or) Nephrons are the structural and functional unit of the kidney. Illustrate the structure of nephron with a labelled diagram. What would happen if ADH is secreted insufficiently? [4+3+1]
Female reproductive system consists of:
Ovulation: it is the release of a mature ovum (secondary oocyte) from the Graafian follicle of the ovary, occurring about the 14th day of the menstrual cycle. A surge of luteinizing hormone (LH) from the pituitary triggers the mature follicle to rupture and release the egg into the oviduct; the empty follicle becomes the corpus luteum.
(Or) The nephron has a Malpighian body (Bowman's capsule + glomerulus) and a tubule (PCT, loop of Henle, DCT, collecting duct). If ADH is secreted insufficiently, the tubules reabsorb little water, so large volumes of dilute urine are passed (a condition called diabetes insipidus), leading to excessive thirst and dehydration.
Photosynthetic pigments: Chlorophyll a is the primary pigment (reaction centre) that converts light into chemical energy; chlorophyll b, carotenoids and xanthophylls are accessory pigments that absorb light of different wavelengths and transfer the captured energy to chlorophyll a, widening the range of light used and protecting against photo-oxidation.
Non-cyclic photophosphorylation: involves both PS II and PS I. Light splits water (photolysis) at PS II, releasing , and electrons. Electrons flow PS II → plastoquinone → cytochrome → plastocyanin → PS I → ferredoxin → NADP, forming NADPH, and ATP is generated along the way. It is a non-cyclic (open, Z-scheme) path producing ATP + NADPH + .
Cyclic photophosphorylation: involves only PS I. The excited electron from PS I passes through the carriers and returns to PS I itself, generating only ATP (no NADPH, no , no water splitting). It operates when NADPH is sufficient but more ATP is needed.
Colour blindness is an X-linked recessive disorder; the defective allele () lies on the X chromosome. A male () is colourblind with a single allele, while a female is colourblind () only when she has the allele on both X chromosomes; females are normal carriers.
Cross - carrier mother × normal father ():
Daughters: 50% normal, 50% carrier (none colourblind); Sons: 50% normal, 50% colourblind.
Cross - colourblind mother × normal father (): all daughters are carriers (, normal) and all sons are colourblind ().
Justification (criss-cross inheritance): the trait passes from a colourblind father to his carrier daughters and then to his grandsons through them, and a colourblind mother passes it to all her sons - the character crosses from father to daughter to grandson (and mother to son), i.e. it skips the same sex in alternate generations, hence 'criss-cross' inheritance.
(Or) The feasible model is semi-conservative replication (Meselson-Stahl), occurring in the S-phase. Helicase unwinds the helix; primase lays RNA primers; DNA polymerase adds nucleotides making a continuous leading strand and discontinuous lagging strand (Okazaki fragments joined by DNA ligase), so each daughter DNA has one old + one new strand.