ISC Class 12 Biology Board Exam Question Paper 2019 with Solutions

ISC Class 12 Biology Board Exam Question Paper with Solutions

PART - I (20 Marks)

Question I

a) Answer the following questions briefly to the point. [8 Marks]

i) Name the antibody which is more effective in allergies. [1 Mark]

Answer: The antibody which is most effective in allergies is IgE type.

Teacher's Note:
a) IgE antibodies bind to mast cells and basophils, triggering the release of histamine during allergic reactions.
b) Students must remember the primary immunoglobulin associated with hypersensitivity reactions.

ii) What is the function of GEAC? [1 Mark]

Answer: GEAC (Genetic Engineering Appraisal Committee) is setup for making decisions regarding the validity of Genetically Modified (GM) research and the safety of introducing GM organisms for public services.

Teacher's Note:
a) GEAC functions under the Ministry of Environment, Forest and Climate Change (MoEFCC) in India.
b) Mentioning its regulatory role regarding biosafety is essential for full credit.

iii) What is a clone? [1 Mark]

Answer: Clones are organisms that are exact morphological and genetic copies of each other, having identical DNA. They can occur naturally or be produced in the lab by the technique of genetic engineering.

Teacher's Note:
a) Key terms like "genetic copies" and "identical DNA" must be included in the definition.
b) Note that natural clones include identical twins and vegetatively propagated plants.

iv) What does Detritus food chain begin with? [1 Mark]

Answer: Detritus food chain starts from dead organic matter of decaying animals and plant bodies, which is consumed by microorganisms and detritivores, followed by their predators.

Teacher's Note:
a) Differentiate clearly between grazing food chain (starting with green plants) and detritus food chain (starting with dead organic matter).
b) Mentioning decomposers and detritivores completes the pathway.

v) Give the full form of EFB. [1 Mark]

Answer: EFB stands for European Federation of Biotechnology.

Teacher's Note:
a) Memorize standard biological acronyms carefully as spelling errors in full forms lead to loss of marks.
b) EFB defines biotechnology in a way that integrates classical and modern molecular views.

vi) How many chromosomes are present in the meiocytes of a fruit fly? [1 Mark]

Answer: In the meiocyte of a fruit fly, 8 chromosomes are present.

Teacher's Note:
a) Meiocytes are diploid cells (2n = 8 for Drosophila melanogaster) that undergo meiosis to form gametes (n = 4).
b) Do not confuse meiocyte chromosome number with haploid gametic chromosome number.

vii) Name the common ancestor of Apes and Man. [1 Mark]

Answer: Dryopithecus was the ancestor of Apes and Ramapithecus was the ancestor of Man (from common dryopithecine ancestors).

Teacher's Note:
a) Dryopithecus was more ape-like, whereas Ramapithecus was more man-like.
b) Both evolved from common primate ancestors during the Miocene epoch.

viii) Give the scientific term used for the preservation of germplasm at very low temperature. [1 Mark]

Answer: Cryopreservation is the term used for preservation of germplasm at very low temperature.

Teacher's Note:
a) Tissues, gametes, and embryos are typically stored in liquid nitrogen at \(-196^{\circ}\text{C}\).
b) This technique halts metabolic activities, ensuring long-term viability.

B) Each of the following sub parts i to iv have four choices. Choose the best option in each case. [4 Marks]

i) Eyelid in human fetus separates in [1 Mark]
(1) 14 week
(2) 16 week
(3) 24 week
(4) 40 week

Answer: (2) 16 week

Eyelids separate and eyelashes are formed during the second trimester, around 16 weeks.

Teacher's Note:
a) Human gestation milestones are important factual questions in human reproduction.
b) Ensure students distinguish between weeks of gestation and months.

ii) Study the given monohybrid cross
P₁ = TT X tt
F₁ = Tt
A test cross for this F1 will be [1 Mark]
(1) Tt x TT
(2) Tt x tt
(3) Tt x Tt
(4) TT x tt

Answer: (2) Tt x tt

A test cross involves crossing an organism with an unknown dominant genotype with a homozygous recessive parent.

Teacher's Note:
a) A test cross is specifically used to determine the zygosity of a dominant phenotype.
b) Crossing with a homozygous recessive (\(tt\)) yields a 1:1 phenotypic ratio if heterozygous.

iii) Montreal protocol aims at: [1 Mark]
(1) Reduction of ozone depleting substances
(2) Biodiversity Conservation
(3) Control of water pollution
(4) Control of CO₂ emission

Answer: (1) Reduction of ozone depleting substances

The Montreal Protocol is an international treaty designed to protect the ozone layer by phasing out the production of numerous substances that are responsible for ozone depletion.

Teacher's Note:
a) Note that Montreal Protocol relates to ozone, whereas Kyoto Protocol relates to greenhouse gases.
b) This distinction is frequently tested in environmental biology.

iv) In the given Pedigree chart the trait shown is [1 Mark]
[Figure: Pedigree chart showing affected and unaffected parents with affected offspring in multiple generations, with no skipped generations and transmission through both sexes.]
(1) Autosomal Dominant
(2) Autosomal Recessive
(3) X-linked
(4) Y-linked

Answer: (1) Autosomal Dominant

The trait appears in every generation without skipping, affects both males and females equally, and is transmitted by affected individuals directly.

Teacher's Note:
a) Pedigree analysis requires checking for skipping of generations (recessive) and male-to-male transmission (rules out X-linked recessive).
b) Vertical transmission across generations is a classic hallmark of dominant traits.

c) Give one significant contribution of each of the following scientist: [4 Marks]

i) Alfred Russel [1 Mark]

Answer: Alfred Russel Wallace independently formulated the theory of natural selection and sent his ideas to Charles Darwin, prompting the joint presentation of evolution by natural selection.

Teacher's Note:
a) Wallace worked extensively on the Malay Archipelago, establishing biogeographical boundaries (Wallace's Line).
b) He shares equal credit with Darwin for discovering natural selection.

ii) R. Mishra [1 Mark]

Answer: Professor Ramdeo Misra is known as the father of Ecology in India. His research laid the foundation for understanding tropical communities, ecological succession, environmental responses of plant populations, and productivity and nutrient cycling in tropical forests and grassland ecosystems.

Teacher's Note:
a) R. Misra formulated the first postgraduate ecology course in India at Banaras Hindu University.
b) His ecological contributions established institutional ecological research in the country.

iii) G. Gamow [1 Mark]

Answer: George Gamow, a physicist, suggested that in order to code for all 20 amino acids, the genetic code should be made of a combination of three nucleotides (triplet code), giving \(4^3 = 64\) codons.

Teacher's Note:
a) Gamow's combinatorial calculation solved the mathematical requirement for coding 20 amino acids with 4 nucleotide bases.
b) A doublet code (\(4^2 = 16\)) would be insufficient.

iv) Sanger [1 Mark]

Answer: Frederick Sanger determined the amino acid sequence of the insulin molecule and developed the dideoxy chain termination method for DNA sequencing.

Teacher's Note:
a) Sanger is one of the few scientists who won two Nobel Prizes in Chemistry.
b) His sequencing techniques enabled the mapping of genomes.

d) Define the following: [2 Marks]

i) Carrying Capacity [1 Mark]

Answer: The maximum number of individuals of a population that a given environment can sustainably support with its available resources is called its carrying capacity, denoted by K.

Teacher's Note:
a) In logistic population growth, population growth rate zeroes as population size approaches K.
b) Environmental resistance increases as the population nears carrying capacity.

ii) Homologous Chromosome [1 Mark]

Answer: Homologous chromosomes are chromosome pairs (one from each parent) that are similar in length, gene position, and centromere location, though they may contain different alleles for the same genes.

Teacher's Note:
a) Homologous chromosomes pair up during meiosis I (synapsis).
b) They govern the same set of inherited traits.

e) Give reason for each of the following: [2 Marks]

i) Bagging is necessary in artificial hybridization. [1 Mark]

Answer: Bagging is necessary to prevent contamination of the stigma of an emasculated flower from unwanted pollen grains, ensuring that only desired pollen is used for pollination.

Teacher's Note:
a) Emasculation involves the removal of anthers from bisexual flowers before pollen dehiscence.
b) Bagging uses butter paper or similar material bags to protect the emasculated flower.

ii) Climax stage is achieved quickly in secondary succession as compared to primary succession. [1 Mark]

Answer: Secondary succession begins in an area that has previously supported life and already possesses fertile soil and organic seeds/roots, whereas primary succession starts from a completely bare, lifeless area where soil must be formed over a long period.

Teacher's Note:
a) Presence of pre-existing soil and seed banks accelerates secondary succession.
b) Primary succession requires pioneer species like lichens to weather rocks into soil first.

PART - II

SECTION - A (14 Marks)

Question 2: Enumerate any two four good and effective Poultry farm management practices [2 Marks]

Answer:
1. Selection of disease-free and suitable breeds of poultry birds.
2. Maintenance of proper hygiene, sanitation, and daily cleaning of the poultry houses with adequate ventilation and light.
3. Provision of a balanced and nutritious feed containing carbohydrates, proteins, fats, minerals, and vitamins.
4. Regular vaccination and disease control measures to prevent infectious outbreaks.

Teacher's Note:
a) Although the question mentions "any two four", students should list appropriate management practices clearly.
b) Disease control and proper nutrition are vital pillars of poultry management.

Question 3: What is a single cell protein? How is it significant for Human Welfare? [2 Marks]

Answer: Single Cell Protein (SCP) refers to dried microbial biomass (from unicellular and multicellular bacteria, yeasts, filamentous fungi, or algae) cultivated as a protein-rich food supplement. It is significant because conventional agricultural production cannot meet the rapidly growing demand for food by human and animal populations, making SCP an alternate, high-protein nutrition source.

Teacher's Note:
a) Examples include Spirulina and Methylophilus methylotrophus.
b) SCP production utilizes waste materials like molasses, sewage, and animal manure, helping reduce environmental pollution.

Question 4 (a) List any four reasons for drug addictions. [2 Marks]

Answer:
1. Curiosity and experimentation among young people.
2. Peer pressure and the desire to fit into a social group.
3. Escapism from stress, depression, anxiety, or academic pressure.
4. Feelings of loneliness, isolation, or lack of support from family and friends.

Teacher's Note:
a) Psychological and social factors are primary contributors to initial drug abuse.
b) Addictive behavior often starts as a perceived harmless experiment.

OR

(b) List any four effects of alcoholism on human health. [2 Marks]

Answer:
1. Damage to the central nervous system, reducing brain activity and causing coordination loss.
2. Liver damage, leading to conditions like fatty liver syndrome and cirrhosis.
3. Development of psychological and neurological disorders such as Korsakoff's syndrome (memory loss).
4. Disruption of physical, mental, professional, domestic, and social areas of life.

Teacher's Note:
a) Chronic alcohol consumption severely impairs metabolic and neurological functions.
b) Liver cirrhosis is one of the classic long-term consequences of alcoholism.

Question 5: List any four features of flower pollinated by insects. [2 Marks]

Answer:
1. Flowers are generally large, brightly colored, and possess pleasant fragrance and nectar to attract insects.
2. Small flowers are grouped into conspicuous inflorescences to make them easily visible.
3. Pollen grains are sticky or spiny so that they easily adhere to the body of visiting insects.
4. Stigma is sticky and positioned suitably within the flower to receive pollen from visiting insects.

Teacher's Note:
a) Entomophilous flowers possess distinct adaptations to encourage animal pollination.
b) Nectar and floral rewards act as attractants for pollinators.

Question 6: What is reproductive fitness? Explain it with the help of an example. [2 Marks]

Answer: Reproductive fitness (Darwinian fitness) refers to the relative ability of an organism to survive and produce fertile offspring in a given environment. For example, according to Darwin, giraffes with longer necks and legs were better able to reach food on tall trees during food scarcity, leaving more surviving offspring than those with shorter necks, eventually making long-necked giraffes dominant in the population.

Teacher's Note:
a) Fitness is ultimately measured by reproductive success, not just individual survival.
b) Natural selection favors traits that enhance reproductive output.

Question 7: Give one significant difference between primary lymphoid organs and secondary lymphoid organs. Give one example each. [2 Marks]

Answer: Primary lymphoid organs are sites where lymphocytes originate and mature into antigen-sensitive cells (e.g., Thymus or Bone marrow), whereas secondary lymphoid organs are sites where lymphocytes interact with antigens and proliferate (e.g., Spleen, Lymph nodes, or Tonsils).

Teacher's Note:
a) B-lymphocytes mature in bone marrow and T-lymphocytes mature in the thymus.
b) Secondary organs like lymph nodes filter antigens from interstitial fluid.

Question 8 (a) Explain the term Bio-fortification. How is this technique useful in production of Golden rice? [2 Marks]

Answer: Bio-fortification is the breeding of crops with higher levels of vitamins, minerals, higher protein, and healthier fats to improve public health. This technique is used to produce Golden rice, a transgenic rice variety enriched with provitamin A (beta-carotene) to combat vitamin A deficiency.

Teacher's Note:
a) Bio-fortification can be achieved through selective breeding or genetic engineering.
b) Golden rice helps prevent blindness and other health issues in populations dependent on rice as a staple food.

OR

b) Write short notes on Electrophoresis. [2 Marks]

Answer: Electrophoresis is a laboratory technique used to separate charged macromolecules (such as DNA, RNA, or proteins) based on their size, shape, and net electrical charge by applying an electric field through a gel matrix (such as agarose or polyacrylamide).

Teacher's Note:
a) Smaller DNA fragments move faster and travel further through the gel pores than larger fragments.
b) Ethidium bromide staining followed by UV visualization is commonly used to view separated DNA bands.

SECTION - B (21 MARKS)

Question 9 Explain the Evolution of long neck of Giraffe according to Charles Darwin. [3 Marks]

Answer: According to Charles Darwin, ancestral giraffes had variable neck lengths in the population. When food sources on the ground became scarce, individuals with longer necks could reach foliage on taller trees and obtain enough food to survive and reproduce. Those with shorter necks could not reach leaves, suffered starvation, and left fewer offspring. Over generations, the advantageous long-neck trait was naturally selected and became predominant, while short-necked variants gradually declined.

Teacher's Note:
a) Darwin's explanation is based on natural selection acting on pre-existing hereditary variations.
b) Contrast this with Lamarck's theory of inheritance of acquired characters (stretching necks).

Question 10 a) Draw a labeled diagram of the T.S. of a mature anther. [3 Marks]

Answer:
[Figure: Diagram of T.S. of a mature anther showing Epidermis, Endothecium, Middle layers, Tapetum, and Pollen grains inside the microsporangia/pollen sacs.]
1. The transverse section of a mature anther typically reveals a bilobed structure with four microsporangia.
2. Wall layers include outer Epidermis, Endothecium, Middle layers, and the innermost nutritive Tapetum.
3. The central sporogenous tissue develops into pollen grains.

Teacher's Note:
a) Neat labeling of wall layers and pollen grains is essential for full marks.
b) Tapetum is multinucleated and provides nourishment to developing microspores.

OR

b) Draw the labeled diagram of the internal structure of Human Ovary [3 Marks]

Answer:
[Figure: Diagram of internal structure of human ovary showing Germinal epithelium, Primary follicles, Secondary follicles, Graafian follicle, Antrum, Released ovum, Corpus luteum, and Ruptured follicles.]
1. The human ovary is covered by germinal epithelium and encloses an ovarian stroma divided into cortex and medulla.
2. The cortex contains ovarian follicles in various stages of development (Primary, Secondary, Graafian follicles).
3. It also shows Corpus luteum formed after ovulation and degenerating follicles (Corpus albicans).

Teacher's Note:
a) Focus on clearly depicting the Graafian follicle with its antrum and secondary oocyte.
b) Correct labeling of Corpus luteum adds high scoring value.

Question 11: Describe the structure of a nucleosome with the help of a well labelled diagram. [3 Marks]

Answer:
[Figure: Diagram of a nucleosome showing DNA wrapped around a histone octamer with H1 histone locking the DNA entry and exit points.]
1. Structure of Nucleosome: Each nucleosome consists of a negatively charged DNA double helix wrapped around a positively charged core of histone octamer.
2. Histone Octamer: The core protein consists of two molecules each of four histone proteins: \(H_2A\), \(H_2B\), \(H_3\), and \(H_4\) (forming an octamer core particle).
3. Linker DNA: The core particles are connected by linker DNA, which is associated with another histone molecule, \(H_1\).

Teacher's Note:
a) Histones are rich in basic amino acids lysine and arginine.
b) Nucleosomes represent the repeating unit of chromatin structure, often described as 'beads-on-a-string'.

Question 12: (a) Explain the Rivet Popper Hypothesis. [3 Marks]

Answer: The Rivet Popper Hypothesis, proposed by Stanford ecologist Paul Ehrlich, compares an ecosystem to an airplane. In an airplane (ecosystem), all parts are joined together by thousands of rivets (species). If passengers remove a rivet (extinction of a species) here and there, it may not affect flight safety initially, but as more and more rivets are removed, the plane becomes dangerously weak and eventually crashes. This illustrates how the loss of even a few key species can destabilize and collapse an entire ecosystem.

Teacher's Note:
a) This hypothesis emphasizes the importance of biodiversity and keystone species in ecological stability.
b) Mentioning the author (Paul Ehrlich) and the airplane analogy is crucial.

OR

(b) Define: [3 Marks]

(i) Standing Crops [1 Mark]

Answer: The amount of living biomass at each trophic level in a given ecosystem at any given time is known as standing crop, usually expressed in terms of mass or number per unit area.

Teacher's Note:
a) Differentiate between standing crop (amount of living matter) and standing state (amount of inorganic nutrients like carbon, nitrogen in the ecosystem).
b) Biomass is measured as fresh or dry weight.

(ii) Stenothermal Organism [1 Mark]

Answer: Organisms that can tolerate only a narrow range of temperatures are called stenothermal organisms (e.g., penguins, crocodiles).

Teacher's Note:
a) Contrast with eurythermal organisms which can tolerate wide temperature ranges.
b) Temperature is a major abiotic factor limiting the geographic distribution of stenothermal species.

(iii) Niche [1 Mark]

Answer: An ecological niche is a complete description of how an organism relates to its physical and biological environment, including the physical space it occupies and its functional role (trophic position) in the community.

Teacher's Note:
a) No two species can occupy the exact same niche in the same habitat indefinitely (Competitive Exclusion Principle).
b) Niche includes habitat plus functional adaptations.

Question 13 Give the biological names for the following: [3 Marks]
i) The Mould from which Penicillin is obtained.
ii) Baker's Yeast
iii) Microbes used to control the insect larvae growing on cotton.
iv) Microbes used to produce Swiss Cheese.
v) The fungus is being grown as biocontrol agent.
vi) A symbiotic Nitrogen fixing bacterium found in the root nodules

Answer:
i) Penicillium notatum
ii) Saccharomyces cerevisiae
iii) Bacillus thuringiensis
iv) Propionibacterium shermanii
v) Trichoderma species
vi) Rhizobium

Teacher's Note:
a) Binomial nomenclature rules require genus name to start with a capital letter and species name with a small letter, and both must be italicized or underlined when handwritten.
b) Factual microbial identification questions carry direct marks.

Question 14: Explain the different types of endosperm in angiosperm. [3 Marks]

Answer:
1. Nuclear Endosperm: The primary endosperm nucleus undergoes repeated free-nuclear divisions without cell wall formation, followed by wall formation later (e.g., Coconut water).
2. Cellular Endosperm: Every division of the primary endosperm nucleus is immediately accompanied by cell wall formation, so the endosperm is cellular from the beginning (e.g., Balsam, Petunia).
3. Helobial Endosperm: Intermediate between nuclear and cellular types, where the first division is accompanied by a transverse wall dividing the embryo sac unequally, followed by free-nuclear divisions in the larger chamber (e.g., Asphodelus).

Teacher's Note:
a) Nuclear endosperm is the most common type found in about 50% of angiosperms.
b) Tender coconut water is free-nuclear endosperm, and the white kernel is cellular endosperm.

Question 15: A homozygous pea plant with round seed coat and yellow cotyledon is crossed with another pea plant having wrinkled seed coat and green cotyledon [3 Marks]
i) Give the types of gametes produced by plants of F1 generation
ii) Give the dihybrid phenotypic ratio with the corresponding phenotypes
iii) State the Mendel's Principle involved in this cross.

Answer: A homozygous round yellow pea plant (\(RRYY\)) is crossed with a wrinkled green pea plant (\(rryy\)).
i) Gametes produced by plants of F₁ generation (\(RrYy\)) are RY, Ry, rY, and ry.
ii) The dihybrid phenotypic ratio is 9 : 3 : 3 : 1 (9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green).
iii) The principle demonstrated is Mendel's Law of Independent Assortment.

Teacher's Note:
a) Law of Independent Assortment states that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.
b) Students should write all four gamete types correctly for F₁ heterozygotes.

Section - C (15 Marks)

Question 16 (a) Describe the physiochemical events that takes place during fertilization in Humans. [5 Marks]

Answer:
Fertilization involves a series of physiochemical events that lead to syngamy and initiation of development:
1. Acrosome Reaction: Sperm releases lytic enzymes (hyaluronidase and acrosin) to digest the corona radiata and zona pellucida layers of the ovum.
2. Cortical Reaction: Entry of the first sperm induces cortical granules to release substances that alter the zona pellucida, preventing polyspermy.
3. Completion of Meiosis II: Sperm entry triggers the secondary oocyte to complete its second meiotic division, forming a mature ovum and a second polar body.
4. Syngamy: Fusion of male and female pronucleus forms a diploid zygote (\(2n\)).
5. Cleavage and Morula formation: The zygote undergoes mitotic divisions (cleavage) moving down the oviduct to form a solid 8-16 celled mass called a morula.

Teacher's Note:
a) Fertilization takes place in the ampullary-isthmic junction of the fallopian tube.
b) Mentioning prevention of polyspermy (cortical reaction) is essential for full credit.

OR

(b) [5 Marks]

i) Define and give the role of Amniocentesis. [1 Mark]

Answer: Amniocentesis is a prenatal diagnostic technique in which a small sample of amniotic fluid is extracted from the amniotic cavity using a hypodermic needle to detect chromosomal abnormalities, genetic disorders, and fetal infections.

Teacher's Note:
a) It analyzes fetal cells sloughed into the amniotic fluid.
b) It is legally checked and regulated to prevent female foeticide.

ii) Name the causative agent and give any one symptom of Gonorrhoea. [1 Mark]

Answer: Gonorrhoea is a sexually transmitted disease caused by the bacterium Neisseria gonorrhoeae. Symptoms in males include a burning sensation during urination or penile discharge.

Teacher's Note:
a) Bacterial STIs can be completely cured if detected early and treated with antibiotics.
b) Ensure correct spelling of the causative organism.

iii) What is the significance of dispersal of seeds. Give any two points. [1 Mark]

Answer:
1. It helps plants colonize new areas and expand their distribution range.
2. It reduces intense competition for light, water, and nutrients between parent plants and offspring.

Teacher's Note:
a) Seed dispersal mechanisms include wind, water, animals, and explosive mechanisms.
b) It ensures survival and continuation of plant species.

iv) What are seasonal breeders? Give one example. [1 Mark]

Answer: Seasonal breeders are animals that reproduce only during specific, favorable seasons of the year. Example: Sheep or dogs (canines).

Teacher's Note:
a) Contrast with continuous breeders who can reproduce throughout their reproductive maturity.
b) Photoperiod regulates reproductive cycles in seasonal breeders.

v) How is chromosome number maintained in sexually reproducing organism. [1 Mark]

Answer: The chromosome number is maintained by meiosis, which reduces the diploid number (\(2n\)) to haploid (\(n\)) in gametes, followed by fertilization where male and female haploid gametes fuse to restore the diploid zygote (\(2n\)).

Teacher's Note:
a) Meiosis and fertilization form the two complementary pillars of sexual reproduction.
b) This ensures genetic constancy across generations while introducing variations.

Question 17 [5 Marks]

(a)(i) What is restriction endonuclease? Give the rules of their nomenclature? [2 Marks]

Answer: Restriction endonucleases are enzymes produced by bacteria that cut DNA molecules at specific internal recognition sequences. Nomenclature rules: The first letter of the name comes from the genus, and the next two letters come from the species of the prokaryotic cell from which they were isolated (e.g., EcoRI from Escherichia coli RY13).

Teacher's Note:
a) They are called "molecular scissors" in recombinant DNA technology.
b) Recognition sites are typically palindromic nucleotide sequences.

(ii) Explain the mechanism of action of restriction endonuclease that makes them suitable for genetic engineering. [3 Marks]

Answer: Restriction endonucleases inspect the DNA sequence, recognize specific palindromic recognition sites, and bind to DNA. They cut both strands of the double helix at specific points, often leaving single-stranded overhangs called sticky ends. These sticky ends can easily form hydrogen bonds with complementary cut ends of foreign DNA, allowing recombinant DNA molecules to be constructed using DNA ligase.

Teacher's Note:
a) Sticky ends facilitate ligation by DNA ligase much more efficiently than blunt ends.
b) Specificity of cleavage is what makes these enzymes indispensable for genetic engineering.

OR

b (i) Explain what are the desirable characteristics of an ideal cloning vector used in rDNA technology. [3 Marks]

Answer:
1. Origin of replication (Ori): Must possess an ori site to initiate independent replication inside the host.
2. Selectable marker: Must carry genes (like antibiotic resistance) to help identify and select transformants.
3. Unique restriction sites: Must have single recognition sites for various restriction enzymes where foreign DNA can be inserted.

Teacher's Note:
a) Plasmids and bacteriophages are commonly used vectors.
b) Small size of the vector makes it easy to isolate and introduce into host cells.

ii) Describe two vector less method used in gene transfer in rDNA technology. [2 Marks]

Answer:
1. Direct gene transfer (Microinjection/PEG method): Foreign DNA is directly introduced into host plant protoplasts through the naked plasma membrane, aided by chemical agents like polyethylene glycol.
2. Electroporation: High-voltage electrical pulses are applied to transiently create microscopic pores in cell membranes, facilitating the uptake of foreign DNA into cells.

Teacher's Note:
a) Vectorless methods are particularly useful when biological vectors cannot infect specific host cells.
b) Other methods include biolistics (gene gun).

Q 18 (a) Give a graphic representation of Carbon Cycle in Nature. [5 Marks]

Answer:
[Figure: Graphic representation of Carbon Cycle showing atmospheric CO2 pool, photosynthesis by plants, respiration by plants and animals, decomposition of dead organic matter, fossil fuels, sedimentary rocks (limestone/chalk), volcanic activity, and industrial combustion/deforestation.]
1. Atmospheric Pool: Carbon exists in the atmosphere primarily as CO₂.
2. Assimilation: Producers (green plants) fix atmospheric carbon into organic compounds via photosynthesis.
3. Transfer and Respiration: Carbon is transferred to consumers through food chains and returned to the atmosphere as CO₂ through respiration by plants, animals, and decomposers.
4. Long-term storage: Carbon is stored in fossil fuels, peat, and sedimentary rocks (limestone), returning via combustion and volcanic activity.

Teacher's Note:
a) Carbon recycling connects living organisms with geological reservoirs.
b) Human activities like fossil fuel burning and deforestation significantly disrupt the global carbon balance.

OR

(b) Give a graphic representation of Phosphorus Cycle in Nature [5 Marks]

Answer:
[Figure: Graphic representation of Phosphorus Cycle showing phosphate rocks, erosion/weathering, dissolved phosphates in soil/water, uptake by plants (roots), assimilation into food chains (animals, birds, fish), death and decay by phosphatising bacteria, and loss to deep marine sediments.]
1. Reservoir Pool: Phosphate rocks and geological deposits serve as the primary natural reservoir of phosphorus.
2. Weathering: Weathering and erosion of phosphate rocks release soluble phosphates into the soil and water bodies.
3. Uptake and Cycling: Plant roots absorb dissolved phosphates from soil, incorporating them into organic molecules (protoplasm, bones, teeth).
4. Return: Decomposers mineralize organic phosphorus from dead organisms and animal excreta, returning phosphates to soil and aquatic sediments.

Teacher's Note:
a) Unlike the carbon cycle, there is no appreciable atmospheric input of phosphorus.
b) The phosphorus cycle is a sedimentary biogeochemical cycle.