NEB Class 12 Science Biology Question Paper 2082 Nepal

Parenchyma is the most common ground tissue; its thin-walled living cells store food and water and carry out functions like photosynthesis and storage.

Monosomy is the loss of one chromosome from a diploid set, giving $2n-1$.

Squamous (pavement) epithelium consists of flat, hexagonal, plate-like cells.

Hydathodes are pores at leaf margins/tips through which excess water is exuded in liquid form (guttation).

tRNA (transfer RNA) carries specific amino acids to the ribosome and pairs its anticodon with the mRNA codon during translation.

Adrenaline (epinephrine), secreted by the adrenal medulla, prepares the body for 'fight or flight' and is called the emergency hormone.

The cerebellum is part of the hind brain (along with the pons and medulla oblongata).

The vascular cambium is a lateral meristem that produces secondary xylem (inward) and secondary phloem (outward), causing increase in girth (secondary growth).

Hemoglobin binds and transports oxygen (forming oxyhaemoglobin) from the lungs to body tissues; this is its principal essential function in RBCs.

A graft between genetically identical individuals (identical twins) is an isograft (syngraft).

Colour blindness is X-linked recessive. The woman's father is colourblind ($X^c Y$), so she is a carrier ($X^C X^c$). Her husband is normal ($X^C Y$). Sons receive $Y$ from father and one X from mother: half get $X^C$ (normal) and half get $X^c$ (colourblind). So among all sons, $50\%$ are colourblind. Option (D) 50%.

Diffusion: Diffusion is the net movement of molecules (or ions) of a substance from a region of higher concentration to a region of lower concentration along a concentration gradient, until uniform distribution is reached. It requires no metabolic energy.

Significance:

• Exchange of respiratory gases ($O_2$ in, $CO_2$ out) in lungs, tissues and leaves.
• Absorption of mineral salts and gases by plant cells.
• Movement of substances within cells and across membranes.
• Helps in transpiration and spreading of ions.

Factors affecting diffusion:

1. Concentration gradient – steeper gradient gives faster diffusion.
2. Temperature – higher temperature increases kinetic energy and rate.
3. Density / molecular size – lighter, smaller molecules diffuse faster (Graham's law).
4. Medium – diffusion is faster in gases than in liquids.
5. Surface area and distance – larger area and shorter distance increase rate.
6. Pressure – higher pressure increases rate of diffusion of gases.

OR — Osmosis: Osmosis is the movement of solvent (water) molecules from a region of higher water potential (dilute/less concentrated solution) to a region of lower water potential (more concentrated solution) through a semipermeable membrane.

Significance: Absorption of water by roots, maintenance of turgidity and cell shape, opening and closing of stomata, movement of water from cell to cell, and turgor movements.

Factors affecting osmosis: concentration gradient of solutions, temperature, pressure, nature/permeability of the membrane, and surface area available.

Mendel's Law of Dominance: When two homozygous individuals having contrasting (alternative) forms of a character are crossed, only one of the two characters (the dominant) is expressed in the $F_1$ generation, while the other (recessive) remains hidden/masked. A factor (allele) that expresses itself in the presence of the other is dominant; the suppressed one is recessive.

Example: In garden pea (Pisum sativum), when a pure tall plant (TT) is crossed with a pure dwarf plant (tt):

Parents: $TT$ (tall) $\times$ $tt$ (dwarf)

$F_1$: all $Tt$ — all tall.

Though the dwarf (recessive) allele $t$ is present in $F_1$, only the dominant tall character (T) is expressed. This shows the law of dominance. (On selfing $F_1$, the $F_2$ shows tall : dwarf = $3:1$, the recessive reappearing.)

Similarities: Both DNA and RNA are nucleic acids made of nucleotide units (each = pentose sugar + phosphate + nitrogen base); both contain the bases adenine, guanine and cytosine; both are involved in storing/transferring genetic information and in protein synthesis; both have a sugar–phosphate backbone with phosphodiester bonds.

Differences:

T.S. of monocot stem (labelled features to draw): epidermis with cuticle; hypodermis of sclerenchyma; ground tissue (parenchyma, not differentiated into cortex/pith); numerous scattered vascular bundles each surrounded by a sclerenchymatous bundle sheath; vascular bundles are conjoint, collateral and closed (no cambium); each bundle shows protoxylem with a water-containing lysigenous cavity giving a Y/V-shaped arrangement of xylem.

Comparison of monocot vs dicot stem:

Menstrual cycle (definition): The menstrual cycle is the cyclic series of changes that occur in the ovary and uterus of a sexually mature human female, recurring on average every 28 days and controlled by hormones, preparing the body for possible pregnancy.

Phases:

1. Menstrual phase (days 1–5): The thickened uterine endometrium breaks down and is shed along with blood (menstruation/bleeding). Levels of estrogen and progesterone are low.
2. Follicular / proliferative phase (days 6–13): FSH stimulates growth of an ovarian (Graafian) follicle, which secretes estrogen; estrogen causes the endometrium to thicken and repair.
3. Ovulatory phase (around day 14): A surge of LH (with FSH) causes rupture of the mature Graafian follicle and release of the secondary oocyte (ovulation).
4. Luteal / secretory phase (days 15–28): The ruptured follicle becomes the corpus luteum, which secretes progesterone (and estrogen); these maintain the endometrium for implantation. If fertilization does not occur, the corpus luteum degenerates, hormone levels fall, and a new cycle begins.

OR — Parathyroid gland: Structure: Four small (pea-sized) oval glands embedded on the posterior surface of the thyroid gland (two pairs), made of chief (principal) cells and oxyphil cells. Function: Secretes parathormone (PTH), which raises blood calcium level by promoting release of $Ca^{2+}$ from bone, increasing $Ca^{2+}$ reabsorption in kidneys, and activating vitamin D for intestinal calcium absorption; it also regulates phosphate. Disorder due to hypersecretion: Hyperparathyroidism, leading to osteoporosis / osteitis fibrosa cystica (weakening of bones due to excess calcium withdrawal).

Both areolar tissue and adipose tissue are loose connective tissues of animal origin with cells embedded in a matrix and derived from mesenchyme.

Transpiration is the loss of water in vapour form from aerial parts of a plant. It occurs as stomatal (through stomata, ~80–90%), cuticular (through the cuticle of leaves/stems) and lenticular (through lenticels of woody stems) transpiration. Environmental factors influence these as follows:

1. Light: Light opens stomata, so it increases stomatal (and total) transpiration; it has little effect on cuticular/lenticular transpiration except by raising temperature.
2. Temperature: Higher temperature increases the rate of evaporation and the vapour-holding capacity of air, increasing all types of transpiration, especially stomatal and cuticular.
3. Relative humidity: High humidity lowers the vapour-pressure gradient and decreases transpiration; dry air increases it.
4. Wind / air movement: Moving air removes the saturated layer around leaves, increasing transpiration (mainly stomatal and cuticular); very strong wind may close stomata and reduce it.
5. Atmospheric pressure: Low atmospheric pressure increases the rate of transpiration.
6. Soil water availability: Adequate soil water maintains transpiration; water deficit causes stomatal closure, reducing stomatal transpiration so cuticular transpiration becomes relatively more important.

Thus stomatal transpiration is most strongly affected by light and humidity, cuticular transpiration by temperature and humidity, and lenticular transpiration (smallest in amount) is least affected by light.

Genetic engineering is the deliberate manipulation/alteration of an organism's genetic material (DNA) using recombinant DNA technology to obtain desired products or traits.

Benefits in medical applications:

• Production of medicines: large-scale production of insulin, human growth hormone, interferons, and clotting factors using recombinant bacteria.
• Vaccines: safer recombinant/subunit vaccines (e.g., hepatitis B vaccine).
• Gene therapy: correcting or replacing defective genes to treat hereditary disorders (e.g., SCID, cystic fibrosis).
• Diagnosis: DNA probes, PCR and genetic screening for early and accurate diagnosis of diseases and prenatal disorders.
• Monoclonal antibodies and improved disease research.

Potential risks:

• Possible unintended/harmful mutations or unknown long-term side effects.
• Ethical concerns (designer babies, germline modification, consent).
• Risk of allergic or immune reactions to engineered products.
• Possibility of escape of modified organisms or misuse (bioweapons) and high cost limiting access.

Conclusion: Genetic engineering offers great medical benefits but must be applied with careful regulation and ethical oversight to minimize its risks.

Influenza: Influenza (flu) is an acute, highly contagious viral respiratory disease affecting the nose, throat and lungs.

Causative agent: The influenza virus (an orthomyxovirus), an RNA virus of types A, B and C.

Mode of transmission: Spreads from person to person mainly through droplets released during coughing, sneezing or talking; also by contact with contaminated surfaces and then touching the nose/mouth (droplet and contact transmission).

Symptoms: Sudden fever, chills, headache, body/muscle aches, sore throat, dry cough, runny or blocked nose, fatigue and weakness.

Treatment: Rest, plenty of fluids, and symptomatic relief (antipyretics/analgesics); antiviral drugs such as oseltamivir (Tamiflu) in severe cases; antibiotics only for secondary bacterial infections; prevention by annual influenza vaccination and hygiene.

What happens when a vaccine is introduced: A vaccine contains weakened/killed pathogens or their antigens. When introduced into the body, the antigens stimulate the immune system to produce specific antibodies and memory cells (active immunity) without causing the disease. On later exposure to the real pathogen, the memory cells respond rapidly and the body is already protected (immunized) against the disease.

Primary excretory organ — the kidney (with nephron): Draw and label a longitudinal section of the kidney and a nephron.

Kidney structure: Humans have a pair of bean-shaped kidneys on the dorsal abdominal wall. Each kidney is covered by a renal capsule and shows an outer cortex and inner medulla; the medulla has cone-shaped renal pyramids opening into the renal pelvis, which continues as the ureter. The concave notch is the hilum, where the renal artery, renal vein and ureter are attached.

Nephron (structural and functional unit): Each kidney has about a million nephrons. A nephron consists of a Malpighian/renal corpuscle (Bowman's capsule enclosing a tuft of capillaries, the glomerulus) and a renal tubule — proximal convoluted tubule (PCT), loop of Henle (descending and ascending limbs), distal convoluted tubule (DCT) and collecting duct. Blood is supplied by the afferent arteriole and drained by the efferent arteriole.

(Diagram to be drawn and labelled: cortex, medulla, renal pyramid, pelvis, ureter, hilum, and a nephron showing Bowman's capsule, glomerulus, PCT, loop of Henle, DCT, collecting duct.)

How hydrostatic pressure is created in the glomerulus: The afferent arteriole bringing blood into the glomerulus is wider than the efferent arteriole that carries blood away. Because more blood enters than can leave, and the efferent (outgoing) vessel is narrower, blood accumulates and builds up a high glomerular hydrostatic (blood) pressure. This high pressure forces water and small solutes out of the glomerular capillaries into Bowman's capsule, bringing about ultrafiltration.

How mutations occur & mutagens: A mutation is a sudden, heritable change in the genetic material (DNA sequence or chromosome) of an organism. Mutations occur due to errors in DNA replication, faulty repair, or the action of mutagens. Mutagens are physical or chemical agents that increase the rate of mutation — e.g., physical mutagens (UV rays, X-rays, gamma rays, high temperature) and chemical mutagens (nitrous acid, colchicine, mustard gas, base analogues). They damage or alter DNA bases, cause breaks, or insert/delete bases, thereby producing mutations.

Types of gene (point) mutations:

• Substitution – one base pair is replaced by another. It may be a transition (purine↔purine or pyrimidine↔pyrimidine) or transversion (purine↔pyrimidine). This can be silent, missense or nonsense.
• Insertion – addition of one or more nucleotides.
• Deletion – loss of one or more nucleotides.
• Insertions/deletions not in multiples of three cause frameshift mutations, altering the whole reading frame downstream.

Significance in evolution and genetic disorders: Mutations are the ultimate source of genetic variation and new alleles, providing the raw material on which natural selection acts, thus driving evolution and adaptation. However, many mutations are harmful and cause genetic disorders such as sickle-cell anaemia (a substitution mutation), thalassaemia, cystic fibrosis and certain cancers. Hence mutations have both beneficial (evolutionary) and harmful (disease) significance.

OR — Dihybrid cross (Law of Independent Assortment): This cross illustrates Mendel's Law of Independent Assortment (alleles of two different genes assort independently of each other).

Let T = tall (dominant), t = dwarf; R = round (dominant), r = wrinkled.

Parents: $TTRR$ (tall, round) $\times$ $ttrr$ (dwarf, wrinkled).

$F_1$: all $TtRr$ — tall with round seeds.

On selfing $F_1$ ($TtRr \times TtRr$), each parent forms four gamete types: $TR$, $Tr$, $tR$, $tr$. The $4\times4$ Punnett square gives 16 combinations:

Phenotypic ratio: Tall round : Tall wrinkled : Dwarf round : Dwarf wrinkled = 9 : 3 : 3 : 1.

Genotypic ratio: $1\,TTRR : 2\,TTRr : 2\,TtRR : 4\,TtRr : 1\,TTrr : 2\,Ttrr : 1\,ttRR : 2\,ttRr : 1\,ttrr$ (i.e. 1:2:2:4:1:2:1:2:1).