NEB Class 12 Science Biology Question Paper 2080 Nepal

Hardness of seed coats is primarily due to the presence of sclerenchyma cells, specifically sclereids (stone cells), which provide mechanical stiffness and protection.

Phloem tissue is responsible for the conduction of food (organic nutrients/sucrose) throughout the plant.

The muton is defined as the smallest unit of DNA element/segment that can undergo mutation.

Since the F1 generation crossed with white (recessive parent) is a backcross/test cross, the ratio of Red to White flowers will be 1:1, which corresponds to 2:2.

Azolla is a pteridophyte that hosts nitrogen-fixing cyanobacteria (Anabaena azollae) and is widely used as a bio-fertilizer in rice paddies.

Anatomical structure of a monocot stem includes:

1. Epidermis: Outermost layer with a thick cuticle, lacking trichomes.
2. Hypodermis: Sclerenchymatous layer providing mechanical strength.
3. Ground Tissue: Continuous mass of parenchymatous cells, not differentiated into cortex and pith.
4. Vascular Bundles: Scattered throughout the ground tissue, conjoint, collateral, and closed (no cambium), often surrounded by a sclerenchymatous bundle sheath. Xylem elements form a distinct 'Y' or 'V' shape.

Simple Permanent Tissue: A group of structurally and functionally identical cells that have lost the power of division and perform a homogeneous function (e.g., parenchyma, collenchyma, sclerenchyma).

Structure of Parenchyma:

• Composed of living, thin-walled, isodiametric cells.
• Cell walls are made of cellulose.
• Possess prominent central vacuoles and distinct intercellular spaces.

Functions of Parenchyma:

• Storage: Stores food materials, water, and secretory substances.
• Photosynthesis: In the form of chlorenchyma, it manufactures organic food.
• Buoyancy: In aquatic plants (as aerenchyma), it provides buoyancy.

Both Incomplete Dominance and Codominance yield a phenotypic and genotypic ratio of 1:2:1 in the F2 generation.

Illustration using Incomplete Dominance in Mirabilis jalapa (Four o'clock plant):

• Parents: Red ($RR$) × White ($rr$)
• F1 Generation: All Pink ($Rr$)
• F2 Generation (Selfing $Rr \times Rr$):
  • Genotype: $1 RR : 2 Rr : 1 rr$
  • Phenotype: 1 Red : 2 Pink : 1 White
• Thus, the phenotypic ratio matches the genotypic ratio exactly ($1:2:1$).

(Based on standard tissue structure questions in NEB Class 12 Botany, this typically represents a cross-section of a vascular bundle or seed component; specific answers require the missing diagram layout context). Generally, if mapping a typical vascular arrangement:

• A: Epidermis / Exodermis
• B: Cortex / Endodermis
• C: Xylem / Phloem
• D: Cambium / Pith

Micro-propagation: The process of cloning and rapidly multiplying a large number of plantlets from a small piece of plant tissue (explant) using modern plant tissue culture methods under aseptic conditions.

Applications in Agriculture:

1. Rapid Multiplication: Yields thousands of clones in a short duration and minimal space.
2. Disease-free Plants: Meristem culture allows for production of virus-free stocks.
3. Year-round Production: Not dependent on seasonal shifts.
4. Propagation of Rare Species: Helps preserve and multiply sterile hybrids or hard-to-germinate species.

Secondary Growth: The increase in the thickness or girth of the plant stem and root due to the formation of secondary tissues by lateral meristems.

Role of Meristems in Secondary Growth:

1. Vascular Cambium:
   • Formed from intra-fascicular and inter-fascicular cambium into a continuous ring.
   • Divides periclinally to produce secondary xylem towards the inside and secondary phloem towards the outside.
   • Activity is influenced by seasonal variations, leading to annual ring formations.
2. Cork Cambium (Phellogen):
   • Develops in the cortical region to alleviate internal tissue pressure.
   • Divides to produce cork (phellem) on the outer side and secondary cortex (phelloderm) internally, collectively forming the periderm layer.

Genetic Material: Macromolecules (primarily DNA or RNA) that store genetic data, undergo replication, transmit characteristics across generations, and can undergo occasional mutations.

Proof of DNA as Genetic Material: Can be described using the classical Hershey-Chase Bacteriophage Experiment (1952):

• Principle: Used T2 bacteriophage consisting of DNA and a protein coat to infect E. coli.
• Methodology: Labeled phage proteins with Radioactive Sulfur ($^{35}\text{S}$) and phage DNA with Radioactive Phosphorus ($^{32}\text{P}$) in separate cultures.
• Infection & Blending: Infected bacteria were agitated to separate viral coats.
• Centrifugation: Radioactivity from $^{32}\text{P}$ was recovered inside the bacterial pellet, whereas $^{35}\text{S}$ remained in the supernatant fluid.
• Conclusion: This clearly demonstrated that DNA, not protein, enters the host and serves as the genetic blueprint.

Mendelism: The principles and laws of heredity formulated by Gregor Johann Mendel based on his hybridisation experiments on garden pea plants (Pisum sativum).

Mendel's Laws of Inheritance:

1. Law of Dominance: In a cross of parents that are pure for contrasting traits, only one form of the trait (dominant) will appear in the F1 generation.
2. Law of Segregation (Law of Purity of Gametes): Allele pairs separate or segregate during gamete formation, and randomly unite at fertilization.
   • Example: Monohybrid cross of tall ($TT$) and dwarf ($tt$) pea plants yields $3:1$ phenotypic ratio in F2.
3. Law of Independent Assortment: Alleles of two (or more) different genes get sorted into gametes independently of one another.
   • Example: Dihybrid cross involving seed shape and color yields a $9:3:3:1$ phenotypic ratio.

The adult human permanent dentition formula is $\dfrac{2123}{2123}$, representing 2 incisors, 1 canine, 2 premolars, and 3 molars per quadrant.

The Hamburger phenomenon is another name for the Chloride shift, which occurs during the transport of carbon dioxide in blood.

Frog eggs have a moderate amount of yolk concentrated towards the vegetal pole, making them mesolecithal (and moderately telolecithal). Mesolecithal is the standard textbook categorization.

Haemopoiesis (or hematopoiesis) is the general process of formation of all blood cellular components inside red bone marrow.

Actin along with Myosin forms the primary contractile structural filaments of muscle tissue.

The pleura is a double-layered serous membrane surrounding and protecting each lung.

Areolar Tissue Structure: Loose connective tissue consisting of a gelatinous matrix containing fibroblasts, macrophages, mast cells, alongside collagen (white) and elastin (yellow) fibers loosely interwoven.

Location: Found underneath the skin (subcutaneous layer), surrounding blood vessels, nerves, and muscles.

Functions:

• Acts as a packaging and binding tissue holding organs in place.
• Provides structural support and flexibility.
• Plays a defensive role via immune cells embedded in the matrix.

Structure of Liver:

• The largest gland in the human body, situated in the upper right side of the abdominal cavity.
• Divided into right and left lobes covered by a fibrous Glisson’s capsule.
• Constructed of functional units called hepatic lobules composed of cords of hepatocytes surrounding a central vein.

Functions of Liver:

• Secretes bile juice for the emulsification and digestion of lipids.
• Glycogenesis, glycogenolysis, and deamination of excess amino acids.
• Detoxification of drugs, poisons, and alcohol.

Ventilation: The mechanical process of moving air into and out of the lungs to facilitate gaseous exchange with the environment.

Mechanism of Breathing:

1. Inspiration (Inhalation):
   • Diaphragm contracts and moves downward; external intercostal muscles contract pulling the ribs upwards and outwards.
   • Volume of thoracic cavity increases, lowering intrapulmonary pressure below atmospheric pressure, causing air to rush inside.
2. Expiration (Exhalation):
   • Diaphragm and external intercostal muscles relax, returning thoracic cavity to resting shape.
   • Internal volume decreases, increasing intrapulmonary pressure, forcing air out of the lungs.

Embryology: The branch of biology focused on the study of the formation, growth, and development of an embryo from gametogenesis until birth or hatching.

Process of Spermatogenesis:

1. Multiplication Phase: Primordial germ cells in seminiferous tubules divide mitotically to form spermatogonia ($2n$).
2. Growth Phase: Spermatogonia grow into large primary spermatocytes ($2n$).
3. Maturation Phase:
   • Primary spermatocytes undergo Meiosis I to form two haploid secondary spermatocytes ($n$).
   • These undergo Meiosis II to produce four haploid spermatids ($n$).
4. Spermiogenesis: Non-motile round spermatids differentiate into functional, motile spermatozoa.

a. Identification: The figure represents a Neuron (Nerve Cell).

b. Labeling:

• A: Dendrite / Cyton (Cell body)
• B: Axon (Myelin sheath)
• C: Node of Ranvier
• D: Axon terminal (Telodendria) (Note: Based on pointer positions 'A' points to the cell body/dendrites, 'B' points to axon sheath area)

c. Functions:

• Function of A (Dendrite/Cyton): Receives incoming nerve impulses from other neurons or receptors and routes them into the cell body.
• Function of B (Axon/Myelin Sheath): Transmits action potentials away from the cell body and accelerates impulse conduction via saltatory propagation.

Physiology of Digestion in Man:

1. Digestion in Mouth: Mechanical breakdown by teeth. Salivary amylase breaks starch into maltose.
2. Digestion in Stomach: Gastric juice containing $\text{HCl}$ activates pepsinogen to pepsin, which turns proteins into peptones and proteoses. Prorennin clots milk.
3. Digestion in Small Intestine (Major Site):
   • Pancreatic Juice: Amylopsin converts starch to disaccharides; Trypsin/Chymotrypsin breaks down proteins; Steapsin splits emulsified fats into fatty acids and glycerol.
   • Bile Juice: Emulsifies large fat droplets.
   • Succus Entericus: Intestinal enzymes (maltase, sucrase, lactase, peptidases) execute terminal conversions into monosaccharides, amino acids, and fatty acids.

Internal Structure of Human Heart:

• Divided into four chambers: right/left atria and right/left ventricles.
• Atria are separated by the interatrial septum, while ventricles are divided by a thick interventricular septum.
• Valves: Tricuspid valve prevents backflow on the right side, and Bicuspid (mitral) valve guards the left auriculo-ventricular aperture.
• Semilunar valves are located at the roots of the pulmonary artery and systemic aorta.
• Inner ventricular walls possess muscular ridges called trabeculae carneae connected via chordae tendineae to papillary muscles.

Pacemaker: The Sinoatrial (SA) node, localized in the wall of the right atrium, which spontaneously triggers electric cardiac impulses dictating the rhythm of heartbeats.

Parts of Human Respiratory System:

1. Nasal Cavity: Filters, warms, and humidifies inhaled air.
2. Pharynx & Larynx: Structural passage containing vocal cords.
3. Trachea (Windpipe): Supported by C-shaped cartilaginous rings.
4. Bronchi & Bronchioles: Branching networks distributing air deeper into lungs.
5. Alveoli: Tiny air sacs functioning as structural units for gas exchange.
6. Lungs: Paired spongy organs containing the respiratory trees.

Asthma Attack Effects: During an asthma attack, the smooth muscle linings of the bronchioles spasm and contract intensely (bronchoconstriction). The mucosal lining inflames and secretes thick excess mucus, severely reducing the diameter of respiratory tubes and inducing dyspnea/wheezing.