Answers to Scanner Mock Paper (Biology) Class XII Section-A 1. 33% for plains, and 67% for hills. 2. In tubectomy, the surgical intervention blocks the transport of ova and hence, the conception; so it is considered a contraceptive method. 3. Parthenogenesis; female gamete/ovum. 4. Monocytes are phagocytic and destroy the pathogenic microbes. 5. Xenogamy Section-B 6. Histones are rich in amino acids, lysine and arginine, which carry positive charge in their side chains. 7. (a) Chlorofluorocarbon. (b) Reserpine. 8. (i) Monocistronic RNA – It is the length of RNA which has information to code for one polypeptide; it is found mostly in eukaryotes. (ii) Polycistronic RNA – It is a long mRNA, which has information to code for more than one polypeptide; it is found in prokaryotes. or Isolation of DNA from bacterial cells – The bacterial cells are treated with enzyme, lysozyme, to break the cells open to release DNA along with RNAs and proteins. – RNAs are removed by treatment with ribonucleases. – Proteins are removed by treatment with proteases; thus, the DNA is purified. – The purified DNA is precipitated out by using chilled ethanol. 9. (a) It works on antigen-antibody interaction. (b) The protein produced in a transgenic cell/organism, by the recombinant gene/ DNA, is called recombinant protein. 10. (i) The first form of life could have come from pre-existing non-living organic molecules. (ii) The formation of life was preceded by chemical evolution, i.e. formation of diverse organic molecules from inorganic constituents. Analysis of meteorites revealed similar compounds (amino acids, sugars, etc) indicating that similar processes are occurring elsewhere in space. Answers to Scanner Mock Paper (Biology–XII) 1 Section-C 11. Outbreeding refers to the breeding of unrelated animals, either of the same breed or of different breeds or even of different species. Outbreeding is of the following types: (i) Outcrossing – Outcrossing is the practice of mating of animals of the same breed, but that have no common ancestor on either side of their pedigree for 4-6 generations. – It is the best breeding method for animals that are below average in productivity and growth rate. (ii) Cross-breeding – It is a method of outbreeding in which superior males of one breed are mated with the superior females of another breed of the same species. – This helps in combining the desirable qualities of the two different breeds into the progeny. (iii) Interspecific hybridisation – It is a method of outbreeding in which male and female animals of two different species are crossed to combine the desirable features of both the parents into one, e.g. Mule is produced by a cross between male donkey and female horse. or (a) (i) Sporozoite (ii) Gametocytes. (b) Sexual reproduction of parasite occurs in mosquito body, while its asexual reproduction occurs in human body. (c) Haemozoin. This toxin is released when the red blood cells are ruptured after the asexual reproduction of the pathogen. 12. (a) Baculoviruses belong to the genus Nucleopolyhedrovirus. – They are the pathogens which attack insects and other arthropods. – They are species-specific, narrow spectrum insecticides, which have no negative impacts on plants, birds, mammals, fish, and even non-target insects. – This is desirable because beneficial insects are conserved to aid in integrated pest management (IPM) programmes. (any two) 2 Answers to Scanner Mock Paper (Biology–XII) (b) – Organic farming is a holistic approach that seeks to develop an understanding of the webs of interaction among the myriads of organisms that form the flora and fauna of the field. – The organic farmer works to create a system where the insects (called pests) are not eradicated, but kept at manageable levels by a complex system of checks and balance within a living and vibrant ecosystem. – According to the organic farmer, the eradication of the creatures, called pests, is not only possible but also undesirable, because many beneficial predatory and parasitic insects cannot survive without them. – Such a use of biocontrol measures reduces the use of chemical pesticides and thereby the pollution. 13. (a) Thermoregulation is an energetically expensive phenomenon. – Heat loss or heat gain is a function of surface area. – Since, larger animals have a smaller surface area relative to their body volume, they do not lose as much of body heat as the small animals, which have a larger surface area to the body volume ratio. So, it is efficient in large animals. (b) Predation: (i)It acts as a conduit for energy transfer across trophic levels. (ii) They keep the prey population under control. (iii) They help in maintaining species diversity in a community, by reducing the intensity of competition. (iv) Biological control of pests is based on the principle of predation. (any two) 14. In eukaryotes, there are three RNA-polymerases. (i) RNA-polymerase I catalyses transcription of rRNAs (28 S, 18 S and 5.8 S). (ii) RNA-polymerase II catalyses transcription of precursor of mRNA; it is called hnRNA. (iii) RNA-polymerase III catalyses tRNA, 5srRNA and snRNAs. 15. Accelerated eutrophication is the phenomenon that accelerates the ageing process of a natural water body, i.e. nutrient enrichment leading to depletion of dissolved oxygen. Consequences: – The nitrates and phosphates that enter the water body stimulate the excess growth of algae and some plants, (called algal bloom) and causes an unsighty scum and unpleasant odour. – The dissolved oxygen content of water decreases leading to death of aquatic organisms. Answers to Scanner Mock Paper (Biology–XII) 3 16. (a) (i) Methionine. (ii) Phenylalanine. (iii) It is a termination codon that does not code for any amino acid. (b) AUG - TAC, UUU - AAA and UAG - ATC. 17. 18. – The secretion of gonadotropin–releasing hormone (GnRH) by the hypothalamus increases significantly during puberty. – GnRH acts on the anterior pituitary and stimulates the secretion of two gonadotropins namely follicle-stimulating hormone (FSH) and luteinising hormone (LH), (also called interstitial cell stimulating hormone (ICSH)). – LH acts on the Leydig cells of testes and stimulates them to secrete testosterone, which stimulates the process of spermatogenesis. – FSH acts on the Sertoli cells and stimulates them to secrete certain factors which are necessary for the process of spermiogenesis. – Seeds have better adaptive strategies for dispersal to new habitats to colonise those areas. – The hard seed coat provides protection to the young embryo. – Seeds have sufficient food reserves (in the cotyledons/endosperm) to support the embryo and seedlings, till they become photosynthetic/independent. – Since they are formed as a result of sexual reproduction, they generate new genetic combinations resulting in variation. (any three) 19. (a) – The first letter of the name comes from the genus of the bacterium, i.e. Escherichia. – The second and the third letters come from the name of the species of the bacterium, i.e. from coli. – The next letter comes from the strain of the bacterium, RY 13. – The Roman number at the end indicates the order in which the enzymes were isolated from this strain of the bacterium. (b) – It recognises and cuts the strands as follows: 5′ – G↓AATTC – 3′ 3′ – CTTAA↑G – 5′ 20. Since, dwarf plants have appeared in the progeny, the plant must be heterozygous, i.e. Tt. Selfing : Tall × Tall (Tt) (Tt) Gametes : T , t T, t Progeny : T t T t TT Tall Tt Tall Tt Tall tt Dwarf 25% of the plants in the progeny are dwarf. 4 Answers to Scanner Mock Paper (Biology–XII) 21. Diseases can be broadly classified as (i) Infectious diseases, and (ii) Non-infectious diseases. Infectious Diseases Non-infectious Diseases – These are the diseases, which are – These are the diseases which are not transmitted from one person to easily transmitted from one person the other. to the other. – They are caused by biological agents called pathogens, e.g. measles, pneumonia. – They occur due to deficiencies, habits, hereditary factors, etc. e.g. cancer, arthritis. The fatal infectious disease is pneumonia and the fatal non-infectious disease is cancer. 22. (a) Divergent evolution is the evolutionary process, where the same structure develops along different directions in different groups of organisms as adaptations to different needs. – All the different structures evolved are homologous structures, as they all have a similar anatomical structure, though they perform different functions, e.g the thorn of Bougainvillea and tendril of Cucurbita are homologous organs, as both of them are modified stems, which perform different functions. (b) It represents genetic equilibrium. Section-D 23. (a) It is called monoculture. Since, the same nutrients are used by the (same) crop, the land becomes deficient in a particular nutrient and hence, the yield is decreased. (b) The roots of pulse crop make symbiotic association with Rhizobium, which fixes atmospheric nitrogen as nitrogen compounds in the field and thereby enriches soil fertility. It is called crop rotation. (c) Nostoc is a cyanobacterium that lives freely and helps to increase the yield in the following ways: (i) It fixes the atmospheric nitrogen in the soil, in a form usable by the plants. (ii) Through its photosynthetic activity, it adds organic matter to the soil and improves soil quality. (d) His concern for his villager, helping nature, awareness and knowledge about these practices, etc. Answers to Scanner Mock Paper (Biology–XII) 5 Section-E 24. Alfred Hershey and Martha Chase proved that DNA is the genetic material. Experiment of Hershey and Chase Infection Blending Centrifugation Hershey-Chase Experiment – They made two different preparations of the phage; in one, the DNA was made radioactive with 32P and in the other, the protein coat was made radioactive with 35S. – These two phage preparations were allowed to infect the bacterial cells separately. – Soon after infection, the cultures were gently agitated in a blender to separate the adhering protein coats of the virus from the bacterial cells. – The culture was also centrifuged to separate the viral coat and the bacterial cells. – It was found that when the phage containing radioactive DNA was used to infect the bacteria, its radioactivity was found in the bacterial cells (in the sediment) indicating that the DNA has been injected into the bacterial cell. – So, DNA is the genetic material and not proteins. 6 Answers to Scanner Mock Paper (Biology–XII) or (a) Trisomy is the condition where a particular chromosome is present in three copies, in a diploid cell/organisms, e.g. Down’s syndrome is due to trisomy of 21st chromosome. (b) Parents : Gametes: Axial, violet × flowers AaVv AV , Av , aV , av Terminal, white flowers aavv av Progeny : av AV Av aV av AaVv Axial, violet flowers Aavv Axial, white flowers aaVv Terminal, violet flowers aavv Terminal, white flowers The phenotypic and genotypic ratios are 1 : 1 : 1 : 1 as seen from the Punnett square. (c) It is a dihybrid test cross. The genotype of the organism with a dominant phenotype can be determined by this cross. 25. (a) A develops into the embryo. B develops into the endosperm. (b) – The endosperm development starts from the primary endosperm cell containing the primary endosperm nucleus. – In the most common type of endosperm development, the primary endosperm nucleus undergoes successive free nuclear divisions to give rise to a number of free nuclei; at this stage, it is called free nuclear endosperm. – Subsequently wall formation takes place from the periphery and proceeds towards the centre and the endosperm becomes cellular. – This type of endosperm development is seen in coconut; the water of the tender coconut represents the free nuclear endosperm with thousands of nuclei in it and the white kernel around it represents the cellular endosperm. – In albuminous seeds, some amount of endosperm persists in the mature seed, as the developing embryo does not consume it fully, e.g. wheat/maize. – In exalbuminous seeds, the endosperm is completely consumed by the developing embryo before seed maturation, e.g. groundnut and pea. Answers to Scanner Mock Paper (Biology–XII) 7 or Development of Female Gametophyte – The megaspore mother cell undergoes meiosis to form four megaspores. – Three of them degenerate and the functional megaspore enlarges to form the embryo sac. – Its haploid nucleus undergoes a mitotic division and the two daughter nuclei move towards the poles. Megasporogenesis – Each of them undergoes two successive mitotic divisions. – Cell wall formation starts in the 8-nucleate stage. – Three of them at the micropylar end form the egg apparatus, consisting of a female gamete and two synergids. – Three of them at the chalazal end form the antipodal cells. – One nucleus from each of the poles moves to the centre; they are called polar nuclei. – The two polar nuclei fuse in the centre (now the central cell) to form a diploid secondary nucleus. – Thus, the mature embryo sac is 7-celled and 8-nucleate. 8 Answers to Scanner Mock Paper (Biology–XII) 26. (a) – Organic farming is a cyclical, zero-waste procedure, where waste products from one process are the raw materials for the other processes. – There is maximum utilisation of the resources and increased effeciency of product. (b) – Since, sewage has a lot of biodegradable organic matter, there is an increase in the oxygen consumption by the decomposers for decomposition; this causes death of many aquatic organisms – Since, more dead/decaying organic matter is added, there is more need of oxygen for the decomposition and hence, the dissolved oxygen content decreases. or Process of decomposition (i) Fragmentation: – It is the process of breaking of the detritus into smaller particles by detritivores like earthworm. (ii) Leaching: – It is the process in which water-soluble inorganic substances run down into soil horizon and get precipitated as unavailable salts. (iii) Catabolism: – The enzymatic conversion of the detritus into simple organic compounds and then into inorganic compounds, is called catabolism. – The enzymes are secreted by the decomposers like bacteria and fungi. (iv) Humification: – Humification during decomposition leads to the accumulation of a dark coloured, amorphous substance, called humus. (v) Mineralisation: – It is the process in which the humus is degraded by certain microbes and the inorganic nutrients are released. Answers to Scanner Mock Paper (Biology–XII) 9