Grade 13 — Term 2
12 study notes · 120 practice questions · Aligned with NIE syllabus
Counted in the Idasara Academy app, which you open with a free account. The free notes, revision, practice and library pages for each lesson are linked below.
This term in the Idasara Academy app has 12 lessons (NIE Units 7, 8). For each lesson, this page gives you the points you must know and one exam-style question with its answer. None of it needs an account.
Lessons in This Term
Structure and function of genetic materials
What you must know
- Leading vs Lagging strand synthesis: Leading strand is synthesized continuously towards the replication fork (5' to 3'); Lagging strand is synthesized discontinuously away from the fork as Okazaki fragments.
- Directionality of DNA polymerase: DNA polymerases can ONLY add new nucleotides to the free 3'-OH end of an existing strand; synthesis is strictly in the 5' to 3' direction.
- Hydrogen bond thermal stability: DNA sequences rich in G-C pairs require higher denaturation temperatures than A-T rich sequences due to three hydrogen bonds versus two.
Try one question
In a double-stranded DNA sample, adenine constitutes 28% of the total nitrogenous bases. What percentage of the bases is cytosine?
- A. 22%
- B. 28%
- C. 24%
- D. 44%
Show the answer
Answer: A. 22%. According to Chargaff's rules for double-stranded DNA: %A = %T = 28%, so A + T = 56%. The remaining bases G + C = 100% - 56% = 44%. Since %G = %C, cytosine is 44% / 2 = 22%.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Genes and gene expression
What you must know
- Template vs Coding strand: The template strand (3' to 5') is complementary to mRNA; the coding (non-template) strand (5' to 3') has the identical sequence to mRNA, with T replaced by U.
- tRNA structure: Cloverleaf secondary structure with an anticodon triplet at one loop and an amino acid attachment site (CCA-3') at the 3' terminus.
- Start codon dual role: AUG codes for the amino acid Methionine (Met) as well as serving as the universal translation initiation signal.
Try one question
If the DNA template strand has the base sequence 3'-TAC GGA CTT ACT-5', what is the corresponding mRNA codon sequence synthesized during transcription?
- A. 5'-UAC GGA CUU ACU-3'
- B. 5'-ATG CCT GAA TGA-3'
- C. 3'-AUG CCU GAA UGA-5'
- D. 5'-AUG CCU GAA UGA-3'
Show the answer
Answer: D. 5'-AUG CCU GAA UGA-3'. Transcription produces an antiparallel, complementary RNA strand where A pairs with U, T pairs with A, and G/C pair mutually. 3'-TAC GGA CTT ACT-5' transcribes to 5'-AUG CCU GAA UGA-3'.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Molecular basis of mutations
What you must know
- Sickle-cell disease molecular mechanism: Single base substitution (GAG to GTG in DNA; GAG to GUG in mRNA) replaces hydrophilic Glutamic acid with hydrophobic Valine at position 6 of the hemoglobin β-globin polypeptide chain.
- Somatic vs Germline mutations: Somatic mutations occur in non-reproductive cells and cannot be inherited by offspring; Germline mutations occur in gametes and are transmitted to subsequent generations.
- Aneuploidy vs Polyploidy: Aneuploidy is the gain or loss of individual chromosomes (2n ± 1, e.g., Trisomy 21 Down syndrome); Polyploidy is the possession of entire extra sets of chromosomes (3n, 4n).
Try one question
What type of point mutation occurs when a single nucleotide substitution changes a codon specifying an amino acid into a premature stop codon?
- A. Nonsense mutation
- B. Frameshift mutation
- C. Silent mutation
- D. Missense mutation
Show the answer
Answer: A. Nonsense mutation. A nonsense mutation is a point mutation that converts a sense codon encoding an amino acid into a premature stop codon (UAA, UAG, or UGA), resulting in a truncated, non-functional polypeptide.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Gene technology tools and methods
What you must know
- Gel electrophoresis mobility: DNA is negatively charged (due to phosphate groups) and migrates towards the positive anode (+); smaller fragments travel fastest and farthest through the agarose matrix.
- PCR thermal cycle steps: Denaturation (94-96°C, separates strands), Annealing (50-65°C, primers bind complementary sequences), Extension (72°C, thermostable Taq polymerase synthesizes new strands).
- Reverse transcriptase application: Synthesizes complementary DNA (cDNA) from mature mRNA templates to clone eukaryotic genes without non-functional introns into prokaryotic expression vectors.
Try one question
Which enzyme is used in recombinant DNA technology to catalyze the formation of phosphodiester bonds to join foreign DNA fragments into a plasmid vector?
- A. DNA ligase
- B. Restriction endonuclease
- C. Reverse transcriptase
- D. DNA polymerase
Show the answer
Answer: A. DNA ligase. DNA ligase catalyzes the synthesis of phosphodiester bonds between the 3'-hydroxyl and 5'-phosphate ends of adjacent nucleotides, sealing nicks and joining DNA fragments.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Applications of gene technology
What you must know
- Golden rice nutritional target: Specifically engineered to synthesize β-carotene (pro-vitamin A) in the endosperm to combat childhood blindness in developing nations.
- Bt toxin specificity: The Cry endotoxin crystal proteins are toxic specifically to alkaline-gut insect larvae (lepidopterans), remaining completely harmless to humans and non-target organisms.
- Bioethical concerns: Potential allergenicity of GMOs, transfer of antibiotic-resistant markers, gene flow into wild weed populations, and monoculture ecological risks.
Try one question
In human insulin production via recombinant DNA technology, which bacterium is most widely utilized as the host organism to express insulin polypeptides?
- A. Escherichia coli
- B. Bacillus thuringiensis
- C. Agrobacterium tumefaciens
- D. Streptomyces griseus
Show the answer
Answer: A. Escherichia coli. Escherichia coli is the standard bacterial host organism engineered to express the A and B chains of human insulin, which are subsequently extracted and linked by disulfide bonds.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Ecosystem components
What you must know
- Competitive Exclusion Principle (Gause's Law): Two competing species with identical ecological niches cannot coexist indefinitely on the same limiting resources; the more efficient competitor eventually eliminates the other, leading to niche differentiation (resource partitioning).
- Resource Partitioning mechanism: Sympatric species diverge morphologically or behaviorally to exploit different resources or microhabitats (e.g., Anolis lizards foraging at different tree heights, warblers feeding on different tree canopy zones).
- Keystone Species concept: A species having a disproportionately large impact on its community structure relative to its abundance (e.g., Pisaster sea star controlling mussel monocultures, top carnivores preventing herbivore overgrazing).
Try one question
What ecological concept defines the total functional role, resource utilization, and multidimensional space occupied by a species within its biotic and abiotic community?
- A. Physical habitat
- B. Ecological niche
- C. Trophic level
- D. Ecological biome
Show the answer
Answer: B. Ecological niche. An ecological niche encompasses both the physical habitat and the complete functional role of an organism—what it eats, how it modifies its environment, its tolerance ranges, and its interactions with other species (Hutchinson's multidimensional hypervolume).
A model question written by Idasara on the exam pattern. It is not from a past paper.
Ecosystem processes
What you must know
- Why Energy Pyramids are NEVER inverted: Energy is continually degraded and lost as entropic heat at each trophic transfer in accordance with the Second Law of Thermodynamics; energy cannot be created at higher levels.
- Bioamplification / Biomagnification: Fat-soluble, non-biodegradable toxins (e.g., DDT, methylmercury, microplastics) concentrate progressively at higher trophic levels because consumers eat large quantities of contaminated lower-level biomass without metabolizing or excreting the toxin.
- Nitrogen transformation stoichiometry: Nitrosomonas oxidizes ammonia (NH3 / NH4+) to nitrite (NO2-); Nitrobacter oxidizes nitrite (NO2-) to nitrate (NO3-), the primary form absorbed by plant root transporters.
Try one question
Which of the following ecological pyramids can NEVER be inverted in an undisturbed natural ecosystem, governed by the Second Law of Thermodynamics?
- A. Pyramid of energy
- B. Pyramid of biomass
- C. Pyramid of numbers
- D. Pyramid of standing crop
Show the answer
Answer: A. Pyramid of energy. Energy transfer between successive trophic levels is inefficient due to respiratory heat loss and metabolic work (~10% trophic efficiency). Because energy is constantly dissipated and cannot be recycled, productivity at higher trophic levels is always lower than at preceding levels, making pyramids of energy strictly upright.
A model question written by Idasara on the exam pattern. It is not from a past paper.
World biomes
What you must know
- Climatic drivers of biomes (Whittaker's diagram): Mean annual temperature and mean annual precipitation are the two primary climactic determinants establishing global biome distribution.
- Permafrost ecological constraint: Continuously frozen subsoil in Arctic tundra restricts root growth to shallow active layers and prevents water drainage, creating boggy bogs in brief summers.
- Nutrient paradox of tropical rainforests: Despite colossal aboveground biomass, rainforest soils are notoriously nutrient-poor and acidic because high humidity and heat promote immediate microbial decomposition and instant root reabsorption, leaving zero humus reserves.
Try one question
Which global terrestrial biome is characterized by continuous subsoil permafrost, brief cold summers, absence of arborescent trees, and vegetation dominated by lichens, mosses, and low sedges?
- A. Temperate Deciduous Forest
- B. Chaparral
- C. Arctic Tundra
- D. Boreal Coniferous Forest (Taiga)
Show the answer
Answer: C. Arctic Tundra. The Arctic Tundra encircles the north pole and is defined by permafrost (permanently frozen subsoil) which prevents deep root penetration, high winds, extremely low temperatures, and low precipitation, resulting in treeless landscapes dominated by cryptogams, sedges, and dwarf shrubs.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Ecosystems of Sri Lanka
What you must know
- Sri Lanka Biodiversity Hotspot: Sri Lanka, combined with the Western Ghats of India, is recognized as one of the world's 36 global biodiversity hotspots due to extreme species endemism threatened by habitat fragmentation.
- Mangrove physiological adaptations: Viviparous germination (seeds germinate while still attached to mother plant, e.g., Rhizophora) and negative geotropic breathing roots (pneumatophores with lenticels, e.g., Avicennia, Sonneratia) overcome anaerobic, saline coastal mud.
- Villu ecosystems: Unique shallow floodplain lakes in Mahaweli basin and Wilpattu, inundated during northeast monsoons, providing crucial grazing grounds for Asian elephants and aquatic avifauna.
Try one question
Which protected area in Sri Lanka is recognized as a UNESCO World Heritage site and represents the largest remaining contiguous tract of primary tropical lowland rainforest in the country?
- A. Horton Plains National Park
- B. Sinharaja Forest Reserve
- C. Wilpattu National Park
- D. Knuckles Conservation Forest
Show the answer
Answer: B. Sinharaja Forest Reserve. Sinharaja is Sri Lanka's last viable area of primary tropical lowland wet evergreen forest. It harbours extraordinary endemism across dipterocarp tree canopies, amphibians, reptiles, freshwater fish, and bird feeding flocks, earning UNESCO World Heritage and Biosphere Reserve designations.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Biodiversity and threats
What you must know
- Habitat Fragmentation & Edge Effects: Fragmenting contiguous forests into small isolated patches increases edge-to-interior ratios, altering microclimates (increased wind, dessication, light) and exposing interior species to invasive predators and nest parasites.
- Invasive Alien Species (IAS) in Sri Lanka: Plants like Eichhornia crassipes (water hyacinth), Salvinia molesta, Lantana camara, and animals like Chitala ornata (clown featherback) outcompete native species and destabilize aquatic and terrestrial food webs.
- Endemism vulnerability: Endemic species with narrow geographic ranges, specialized habitats, or low reproductive rates are inherently the most prone to catastrophic extinction following human disturbances.
Try one question
Which aggressive invasive alien woody plant species severely degrades native vegetation and coastal wetlands in the arid zones of Sri Lanka, notably in Bundala National Park?
- A. Salvinia molesta
- B. Prosopis juliflora (Kalapu andara)
- C. Eichhornia crassipes (Japan jabara)
- D. Parthenium hysterophorus
Show the answer
Answer: B. Prosopis juliflora (Kalapu andara). Prosopis juliflora (Mesquite / Kalapu andara) is a thorny drought-resistant shrub/tree native to Central America. Introduced to the Hambantota district, it aggressively colonized lagoons and sand dunes of Bundala National Park, forming impenetrable monocultures that outcompete native scrub.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Global environmental problems
What you must know
- Montreal Protocol vs Kyoto/Paris Agreements: The Montreal Protocol (1987) successfully phased out ozone-depleting substances (CFCs), hailed as the most effective global treaty; Paris Agreement (2015) commits nations to limit global warming well below 2.0°C (ideally 1.5°C) above pre-industrial baselines.
- Ocean Acidification chemistry: Atmospheric CO2 dissolves in ocean water to produce carbonic acid: CO2 + H2O <-> H2CO3 <-> H+ + HCO3-; excess H+ ions react with carbonate (CO3^2-), depleting carbonate needed by calcifying corals to build CaCO3 skeletons.
- Eutrophication sequence: Excessive agricultural runoff of nitrates and phosphates -> algal bloom -> algae die and sink -> aerobic saprotrophic bacteria multiply -> biological oxygen demand (BOD) skyrockets -> dissolved O2 crashes (hypoxia) -> mass fish kills.
Try one question
Which pair of anthropogenic gaseous emissions is primarily responsible for the acidification of precipitation, resulting in wet acid deposition with pH values below 5.6?
- A. Carbon monoxide (CO) and Tropospheric ozone (O₃)
- B. Carbon dioxide (CO₂) and Methane (CH₄)
- C. Chlorofluorocarbons (CFCs) and Halons
- D. Sulfur dioxide (SO₂) and Nitrogen oxides (NOₓ)
Show the answer
Answer: D. Sulfur dioxide (SO₂) and Nitrogen oxides (NOₓ). Sulfur dioxide (SO₂) from coal-fired power plants and nitrogen oxides (NO and NO₂) from vehicle exhaust react with atmospheric water vapor, hydroxyl radicals, and oxygen to form dilute sulfuric acid (H₂SO₄) and nitric acid (HNO₃), lowering rain pH well below the natural unpolluted benchmark of 5.6.
A model question written by Idasara on the exam pattern. It is not from a past paper.
Biodiversity and environmental conservation
What you must know
- In-situ vs Ex-situ tradeoffs: In-situ maintains evolutionary adaptation and ecological interactions at landscape scale; Ex-situ serves as genetic insurance and facilitates captive breeding/propagation when natural populations face imminent wild extinction.
- Ramsar Wetland Sites in Sri Lanka: Bundala National Park (first Ramsar site in SL), Anawilundawa, Maduganga, Vankalai Sanctuary, Kumana wetland cluster, and Wilpattu Ramsar wetland cluster.
- Core vs Buffer vs Transition Zones in Biosphere Reserves: Core zone is strictly protected for zero human interference; Buffer zone permits non-destructive research and educational tourism; Transition zone allows sustainable local agroforestry and economic activity.
Try one question
Which of the following conservation strategies is categorized as ex-situ conservation?
- A. Cryopreservation of gametes and seeds in cryogenic gene banks
- B. Establishment of a National Park under the Fauna and Flora Protection Ordinance
- C. Management of a wildlife sanctuary for migratory bird breeding
- D. Declaration of a Biosphere Reserve protecting virgin forest ecosystems
Show the answer
Answer: A. Cryopreservation of gametes and seeds in cryogenic gene banks. Ex-situ conservation involves the preservation of biological diversity components outside their natural habitats (e.g., cryogenic gene banks, botanical gardens, zoological gardens, seed banks). In-situ conservation protects species within their natural ecosystems (national parks, nature reserves, sanctuaries).
A model question written by Idasara on the exam pattern. It is not from a past paper.
Before you sign up
What does it cost?
I already go to a Biology tuition class
Then you are paying for Biology by the subject. Whether an Idasara plan would cost you less depends on how many classes you go to, so put in your own numbers.
Work it out on the pricing page →