Biology
जीवविज्ञान · Class 12 · Code 2021 (CDC Bio.202)
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chapters done
- Botany
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The syllabus asks for
- Concept of tissues
- Meristems and permanent tissues
- Anatomy of dicot and monocot root
- Anatomy of dicot and monocot stem
- Anatomy of dicot and monocot leaf
- Secondary growth of dicot stem
Key points
- A tissue is a group of cells of common origin doing one job. Tissues are meristematic (still dividing) or permanent (differentiated), and simple (one cell type) or complex (several). The three tissue systems are epidermal, ground and vascular.
- Meristems are classified by origin (promeristem, primary, secondary), by position (apical → length, intercalary → grasses regrow, lateral → girth) and by function (protoderm → epidermis, procambium → vascular tissue, ground meristem → cortex and pith).
- Simple permanent tissues: parenchyma (living, thin-walled, storage; variants chlorenchyma, aerenchyma), collenchyma (living, pectin-rich, unevenly thickened, flexible support in growing dicot organs — angular, lamellar, lacunar), sclerenchyma (dead, lignified, rigid — fibres and sclereids).
- Complex tissues: xylem = tracheids + vessels + fibres + parenchyma (upward, one way); phloem = sieve tubes + companion cells + fibres + parenchyma (both ways). Protoxylem is exarch in roots, endarch in stems.
- Root: epiblema without cuticle, wide cortex, endodermis with Casparian strips and passage cells, pericycle giving endogenous lateral roots, radial bundles, exarch xylem. Dicot 2–6 xylem groups with little or no pith and later a cambium; monocot polyarch with a large pith and no cambium.
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The syllabus asks for
- Water relation
- Photosynthesis
- Respiration
- Plant hormones
- Plant growth and movement
Key points
- Water potential Psiw = Psis + Psip for an ordinary cell; pure free water is 0 MPa and every solution is negative. Water moves from higher to lower water potential, and Psip is negative in the xylem. DPD is the obsolete equivalent, with Psiw = -DPD. Water crosses the root cortex by apoplast, symplast and transmembrane routes, and the Caspa
- Ascent of sap is explained by the cohesion–tension theory of Dixon and Joly (1895): evaporation from mesophyll wall menisci creates tension, cohesion holds the column together and adhesion keeps it attached. Strasburger's poisoned-trunk experiment killed the vital-force theories; the capillarity arithmetic kills that one.
- Transpiration is 80–90 % stomatal. Stomata open when K^+ floods into the guard cells, lowering their solute potential; radial micellation turns the resulting turgor into opening. ABA closes them. The starch–sugar theory is obsolete — onion guard cells have no starch and work perfectly. Guttation is different: liquid xylem sap forced out t
- Light reaction, in the thylakoid: PS II (P680) and PS I (P700) work in series along the Z-scheme; photolysis at the Mn4CaO5 complex releases O2 and dumps 4 H^+ into the lumen. Non-cyclic flow gives ATP + NADPH + O2; cyclic flow (PS I alone) gives ATP only, and exists because the Calvin cycle needs a 3 : 2 ratio that linear flow cannot sup
- Calvin cycle, in the stroma: carboxylation, reduction, regeneration. Per CO2, 3 ATP + 2 NADPH; per glucose, 6 CO2 + 18 ATP + 12 NADPH. First stable product 3-PGA.
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The syllabus asks for
- Genetic Materials
- Mendelian genetics
- Linkage and crossing over
- Mutation and polyploidy
Key points
- DNA is the genetic material — Griffith's transformation, Avery's enzyme test, and the Hershey–Chase ³²P/³⁵S experiment; in some viruses (TMV) it is RNA. Chargaff's rules: A = T, G = C, so A + G = T + C.
- B-DNA is a right-handed antiparallel double helix, diameter 2 nm, pitch 3.4 nm, 10 bp per turn, rise 0.34 nm per base pair; A=T has two hydrogen bonds, G≡C has three.
- Replication is semi-conservative (Meselson–Stahl): helicase opens the fork, primase lays an RNA primer, DNA polymerase III builds only 5′ → 3′, so one strand is leading (continuous) and one lagging (Okazaki fragments joined by ligase).
- Central dogma: DNA → (transcription) → mRNA → (translation) → protein. The code is a triplet, degenerate, non-overlapping, comma-less and nearly universal code of 64 codons; AUG starts, UAA, UAG and UGA stop.
- Mendel's laws: dominance, segregation (monohybrid 3 : 1 phenotypic, 1 : 2 : 1 genotypic) and independent assortment (dihybrid 9 : 3 : 3 : 1; test cross 1 : 1 : 1 : 1). A heterozygote of n genes makes 2ⁿ kinds of gamete.
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The syllabus asks for
- Asexual and sexual reproduction in angiosperms
- Pollination
- Fertilization
- Development of male gametophyte
- Development of female gametophyte
- Dicot and monocot embryo
- Endosperm
Key points
- Asexual reproduction (rhizome, tuber, bulb, runner, cutting, grafting, apomixis) gives clones — quick and uniform but with no variation; sexual reproduction gives variation.
- Pollination is the transfer of pollen from anther to stigma: autogamy, geitonogamy and xenogamy; agents are wind (anemophily), water (hydrophily), insect (entomophily), bird (ornithophily) and bat (chiropterophily).
- Cross-pollination is enforced by dicliny, dichogamy (protandry and protogyny), herkogamy, heterostyly and self-incompatibility.
- The anther is dithecous and tetrasporangiate; its wall is epidermis, endothecium (dehiscence), middle layers and tapetum (nutrition and sporopollenin).
- Microsporogenesis: microspore mother cell (2n) → meiosis → tetrad of microspores (n). Microgametogenesis: microspore → 2-celled pollen grain (vegetative + generative cell) → two male gametes in the pollen tube.
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The syllabus asks for
- Tissue culture
- Plant breeding
- Disease-resistant plants
- Green manure, bio-fertilizer, bio-pesticide
- Genetic engineering and GMOs
- Bio-engineering
- Food safety and food security
Key points
- Tissue culture works because plant cells are totipotent. Explant → surface sterilisation → MS medium → callus → shoots (high cytokinin) → roots (high auxin) → hardening. It gives disease-free, genetically identical clones all year round.
- Plant breeding improves crops by selection, introduction, hybridisation and mutation breeding; the Green Revolution used dwarf, fertiliser-responsive varieties such as wheat Bhrikuti and rice Sabitri in Nepal.
- Disease resistance is vertical (one R gene, complete but easily broken, gene-for-gene) or horizontal (many genes, partial but durable). Bacillus thuringiensis cry genes make a protoxin that becomes active only in the alkaline insect gut, so Bt crops kill the pest but not the eater.
- Green manure adds organic matter and nitrogen by ploughing in a legume; bio-fertilizers are living organisms (Rhizobium, Azotobacter, Azolla–Anabaena, mycorrhiza, blue-green algae) that supply nutrients; bio-pesticides are living or natural agents (Bt, Trichoderma, neem) that control pests without residues.
- Genetic engineering needs four tools: restriction enzymes to cut (EcoRI cuts G↓AATTC and leaves sticky ends), DNA ligase to join, a vector with ori + marker + unique sites, and a host cell. Steps: isolate → cut → ligate → transform → select → multiply.
- Zoology
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The syllabus asks for
- Epithelial tissue
- Connective tissue
- Muscular tissue
- Nervous tissue
Key points
- A tissue is a group of similar cells with a common origin doing one job; the four animal tissues are epithelial, connective, muscular and nervous.
- Epithelium: cells packed, matrix negligible, avascular, polar, on a basement membrane, high regeneration. Simple (squamous, cuboidal, columnar, ciliated, pseudostratified), compound (stratified squamous, transitional) or glandular.
- Connective tissue: few cells, abundant matrix of collagen, elastic and reticular fibres. Loose (areolar, adipose), dense (tendon = collagen, muscle→bone; ligament = elastin, bone→bone), skeletal (cartilage, bone) and fluid (blood, lymph).
- Cartilage: chondrocytes in lacunae in chondrin, avascular, covered by perichondrium. Bone: osteocytes in lacunae joined by canaliculi, matrix of ossein + calcium salts, organised into Haversian systems in mammals.
- Blood = 55% plasma + 45% formed elements. RBC 4.5–5.5 million per mm³, WBC 4000–11000 per mm³, platelets 150,000–400,000 per mm³.
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The syllabus asks for
- Gametogenesis
- Development of frog
Key points
- Gametogenesis has three phases — multiplication (mitosis), growth, and maturation (two meiotic divisions); spermatogenesis adds a fourth, spermiogenesis.
- One primary spermatocyte gives 4 sperms; one primary oocyte gives 1 ovum + 3 polar bodies. Both divide meiotically twice; only the cytoplasm is shared unequally.
- In the frog 2n = 26, so every gamete carries n = 13 and the zygote is restored to 2n = 26.
- The frog's egg is mesolecithal and telolecithal: dark animal pole with the nucleus, pale vegetal pole with the yolk.
- Fertilisation is external and monospermic; the fertilisation membrane blocks extra sperms, and the grey crescent appears opposite the point of entry and fixes the dorsal side.
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The syllabus asks for
- Digestive system
- Respiratory System
- Circulatory System
- Excretory System
- Nervous system
- Sense organs
- Endocrinology
- Reproductive System
Key points
- Digestion is chemical hydrolysis by enzymes, each with its own optimum pH: salivary amylase 6.8, pepsin 1.5–2.0, pancreatic and intestinal enzymes 7–8. Absorption happens in the ileum across villi, which raise the surface area to about 250 m².
- Breathing moves air; respiration releases energy. Tidal volume 500 ml, vital capacity 4500 ml, total lung capacity 5800 ml, residual volume 1200 ml. Pulmonary ventilation = tidal volume × breathing rate ≈ 500 × 12 = 6000 ml min⁻¹.
- Gas exchange is pure diffusion across the 0.5 µm alveolar wall over about 70 m² of surface. Oxygen travels as oxyhaemoglobin (1 g Hb carries 1.34 ml O₂); most CO₂ travels as bicarbonate.
- Cardiac output = stroke volume × heart rate = 70 × 72 ≈ 5000 ml min⁻¹. The cardiac cycle lasts 0.8 s (atrial systole 0.1 s, ventricular systole 0.3 s, joint diastole 0.4 s); normal BP is 120/80 mmHg. The heart beats myogenically, from the SA node.
- Urine formation = ultrafiltration + selective reabsorption + tubular secretion. Net filtration pressure = 55 - (15 + 30) = 10 mmHg; GFR = 125 ml min⁻¹ = 180 L day⁻¹, of which more than 99 % is reabsorbed to leave 1–1.5 L of urine.
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The syllabus asks for
- Human Population
- Health disorders
Key points
- CBR = (B/P)× 1000; CDR = (D/P)× 1000; RNI = CBR-CDR; IMR uses live births as denominator.
- Exponential growth: Pt = P0e^rt, so r = (1)/(t)ln(Pt/P0) and doubling time Td = 0.693/r ≈ 70/R%.
- Dependency ratio = dfracP0-14+P65+P15-64× 100; for Nepal in 2021 it is 53.3 (child 42.7, old-age 10.6).
- Census 2021: 29,164,578 people, growth 0.92 % per year, sex ratio 95.59, density 198 km⁻², 66.17 % urban, 2.19 million absent abroad, life expectancy 71.3 years. NDHS 2022: TFR 2.1, IMR 28, U5MR 33. NMMS 2021: MMR 151.
- Population pyramids are expanding (broad base), stable or declining (narrow base); Nepal's base is now narrower than the band above it.
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The syllabus asks for
- Application of Zoology
- Microbial diseases and application of microbiology
Key points
- Apiculture is the keeping of Apis species in hives. Nepal's hive bee is Apis cerana; A. mellifera was introduced in 1992; A. dorsata, A. laboriosa and A. florea build single open combs and are hunted, not kept. The movable-frame hive works because bees leave a bee space of 6.4–9.5 mm, discovered by Langstroth in 1851 and patented in 1852.
- A colony has one queen, sterile female workers and haploid drones from unfertilised eggs. All larvae get royal jelly for about three days; only the queen larva stays on it.
- Sericulture rears Bombyx mori on mulberry (Morus alba). The larva passes five instars and four moults in 29–30 days, then spins a cocoon of one filament 600–900 m long. The pupa is killed by stifling before it can emerge and cut the filament. 4,000–7,000 cocoons make one kilogram of raw silk, which is 75–80 % fibroin and 20–25 % sericin.
- Composite fish culture stocks six carps with different feeding niches in one pond: silver carp (phytoplankton), catla (zooplankton), rohu (column), grass carp (weeds), mrigal and common carp (bottom). Carps mature in ponds but will not spawn there, so breeding is induced by hormone injection — Hiralal Chaudhuri, 1956.
- Poultry: Gallus gallus domesticus from the red junglefowl; broilers reach slaughter weight in 4–6 weeks, layers approach 500 eggs by 100 weeks. Nepal has exactly three indigenous breeds — Sakini, Ghanti Khuile, Pwankh Ulte. Ranikhet is Newcastle disease, caused by avian paramyxovirus type 1.
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The paper
3hr, FM 75 written (+25 internal)
| Section | Questions | Marks each | Total |
|---|---|---|---|
| Section I Botany 37 + Section II Zoology 38 | — | — | — |
Where the hours go
Teaching hours are the board's guide to how much of the paper each part carries.
- Botany64 h
- Zoology64 h