CBSE previous Year Solved  Papers  Class 12 Biology Delhi 2016

Time allowed : 3 hours                                                                                           Maximum Marks: 70

General Instructions :

  1.  There are a total of 26 questions and five sections in the question paper, All questions are compulsory.
  2. Section A contains question number 1 to 5, Very Short Answer type questions of one mark each.
  3.  Section B contains question number 6 to 10, Short Answer type I questions of two marks each.
  4.  Section C contains question number 11 to 22, Short Answer type II questions of three marks each.
  5.  Section D contains question number 23, Value Based Question of four marks.
  6. Section E contains question number 24 to 26, Long Answer type questions of five marks each.
  7. There is no overall choice in the question paper, however, an internal choice is provided in one question of two marks, one question of three marks and all three questions of five marks. An examined is to attempt any one of the questions out of two given in the question paper with the same question number.

SECTION-A

Question.1. According to de-vries what is saltation ?
Answer : Saltation is single step, large mutation.

Question.2. Excessive nutrients in a fresh water body cause fish mortality. Give two reasons.
Answer: Excessive nutrients result in excessive algal growth which produce toxins in water. Water quality becomes poor as Dissolved Oxygen decreases leading to increase in BOD.

Question.3. Suggest the breeding method most suitable for animals that are below average in milk productivity.
Answer : To improve productivity any one of the following methods can be followed :
Outbreeding/Outcrossing/Cross-breeding/artificial insemination/hybridisation etc.

Question.4. State a difference between a gene and an allele.
Answer : Gene : It contains information that is required to express a particular trait.
Allele : Genes which code for a pair of contrasting traits.

Question.5. Suggest a technique to a researcher who needs to separate fragments of DNA.
Answer : Gel electrophoresis.

SECTION—B

Question.6. Explain the significance of meiocytes in a diploid organism.
Answer : (i) Meiocytes undergo meiosis or gametogenesis to produce haploid gametes.
(ii) They help to restore diploidy through zygote formation or syngamy.

Question.7. Mention the kind of biodiversity of more than a thousand varieties of mangoes in India represent. How is it possible ?
Answer : Varieties of mango show genetic diversity. Single species of mango show high diversity at genetic level over its distributional range. .

Question.8. List the events that reduce the Biological Oxygen Demand (BOD) of a primary effluent during sewage treatment.
Answer : (i) Effluent from primary settling tank is passed into aeration tank agitated mechanically and air is pumped into it.
(ii) This allows vigorous growth of aerobic microbes into floes which consume major part of organic matter in the effluent.

Question.9. Discuss the role the enzyme DNA ligase plays during DNA replication.
Answer : (i) Discontinuous DNA fragments are joined or sealed by DNA ligase.
(ii) DNA ligase adds on nucleotide in the usual 5’ to 3’ direction along the DNA strand.

Question.10. Name the causative organism of the disease amoebiasis. List three symptoms of the disease.
OR
Identify ‘A’, ‘B’, ‘C’and ‘D’ in the given table.
cbse-previous-year-solved-papers-class-12-biology-delhi-2016-1
Answer: Amoebiasis is caused by Entamoeba histolytica.
The symptoms of this disease are : constipation, abdominal pain, cramps, stools with excess mucous and blood clots.
OR
A—Wheat
B—Black rot/Curl blight black rot
C—White rust
D—Pusa Komal

SECTION—C

Question.11. Why is breast-feeding recommended during the initial period of an infant’s growth ? Give reasons.
Answer: During initial period of infant’s growth, colostrum is produced. It is rich in nutrients. It is also rich in antibodies (IgA) which provide passive immunity to thfc new born.

Question.12. Give an example of an autosomal recessive trait in humans. Explain its pattern of inheritance with the help of a cross.
Answer : Sickle cell anaemia is an autosomal recessive trait disease than can be transmitted from parents to the off spring when both the partners are carrier for the gene. The disease is controlled by a single pair of allele, HbA and HBS. Out of three possible genotypes only homo2ygous individuals for Hbs (Hbs Hbs) show the diseased phenotype white heterozygous (HbAHbs) individuals are carrier of the disease.
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Question.13. Describe the experiment that helped Louis Pasteur to dismiss the theory of spontaneous generation of life.
Answer : Leuis Pasteur took two pre-sterilised flasks with killed yeast. One flask was sealed while the other was kept open to air. Differential growth of life was observed in the flasks-life was found only in the open flask’. It proved that life comes from pre-existing life (theory of biogenesis).

Question.14. Plant breeding technique has helped sugar industry in North India. Explain how.
Answer: Saccharum barberi was originally grown in North India, but had poor sugar content and yield. Sugar cane grown in South India, Saccharum officinarum had thicker stems and higher sugar content but did not grow well in North India. The two species were crossed to get desirable qualities of high yield, thick stems, high sugar and ability to grow in N orth India.

Question.15. Suggest and describe a technique to obtain multiple copies of a gene of interest in vitro.
Answer : PCR : Polymerase Chain Reaction.
Multiple copies of the gene of interest is synthesised in vitro using two sets of primers and enzyme DNA polymerase. The enzyme extends the primers using nucleotides provided and genomic DNA as template. The process of DNA replication is repeated several times for amplification of DNA with the help of thermostable DNA polymerase which remains active during high temperature induced denaturation of double stranded DNA.
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Question.16. What is a GMO ? List any five possible advantages of GMO to a farmer.
Answer : Those plants, bacteria, fungi or animals whose genes have been altered by manipulation are called Genetically Modified Organisms (GMOs).
Advantages :

  1. Tolerance to abiotic stresses like cold, drought, salt, heat etc.
  2.  Reduce reliance on chemical pesticides.
  3.  Reduced post harvest losses.
  4. Increased efficiency of mineral usage by plants.
  5. Enhanced nutritional value.
  6. To create tailor made plants.

Question.17. During a school trip to ‘Rohtang Pass’, one of your classmate suddenly developed ‘altitude sickness’. But, she recovered after sometime.
(a) Mention one symptom to diagnose the sickness.
(b) What caused the sickness ?
(c) How could she recover by herself after some time ?
Answer : (a) The symptoms may be nausea, fatigue or heart palpitation.
(b) The sickness was caused due to low atmospheric pressure which prevails at high altitude. The body does not get enough oxygen.
(c) The body compensates low oxygen availability by
increasing RBC production, decreasing the binding affinity of haemoglobin and by increasing breathing rate.

Question.18. How has RNA technique helped to prevent the infestation of roots in tobacco plants by a nematode ?
Answer: Using Agrobacterium vectors, nematode specific genes were introduced into the host plant. This DNA produced both sense and anti sense RNA in the host cells. These two RNAs being complementary to each other formed a double strand (dsRNA) that initiated RNAi and thus silenced the specific mRNA of the nematode. Hence the parasite could not survive in the transgenic host.

Question.19. “In a food-chain, a trophic level represents a functional level, not a species.” Explain.
OR
(a) Name any two places where it is essential to install electrostatic precipitators. Why it is required to do so ?
(b) Mention one limitation of the electrostatic precipitator.
Answer : Position of a species in any trophic level is determined by the function performed by that mode of nutrition of species in a particular food chain. A given species may occupy more than one trophic level in the same ecosystem at a given time. If the function of the mode of nutrition of species changes, its position shall change in the trophic levels. The same species can be at the primary level of consumer in one food chain and at the secondary consumer level in another food chain in the same ecosystem at a given time.
OR
(a) Electrostatic precipitators can be installed in thermal power plants, smelters or other particulate matter releasing industries. They are important for removing particulate matter.
(b) Limitations :

  1. Very, very small particulate matter which are less than 2.5 micrometres are not removed.
  2.  The velocity of air between the plates must be low enough to allow the dust to fall.
  3.  It cannot work without electricity.
    (Any one can be mentioned)

Question.20. Prior to a sports event blood & urine samples of sports persons are collected for drug tests.
(a) Why is there a need to conduct such tests ?
(b) Name the drugs the authorities usually look for.
(c) Write the generic names of two plants from which these drugs are obtained.
Answer : (a) To detect drug abuse or use of banned drugs cannabinoids, narcotic analgesic, diuretics, hormones or drugs used to accelerate performance, increase muscle strength etc.
(b) Cannabinoids/cocaine/coka alkaloid/coke/crack/
hashish/charas/ganja etc. ’
(c) Cannabis/Atropa/Erythoxylem/Datura etc.

Question.21. Describe the experiment that helped demonstrate the semi-conservative mode of DNA replication.
Answer.
cbse-previous-year-solved-papers-class-12-biology-delhi-2016-4
Heavy Hybrid Light Hybrid
Meselson and Stahl grew E. coli in a medium containing\( ^{ 15 }{ N{ H }_{ 4 }Cl }\) (\(N^{ 15 }\) is the heavy isotopes of nitrogen) for many generations to get\( N^{ 15 }\) incorporated into DNA. Then the cells were transferred into \( ^{ 15 }{ N{ H }_{ 4 }Cl }\). The extracted DNA was centrifuged in a CsCl density gradients to measure the densities of DNA. The DNA extracted from the culture after one generation (20 minutes) showed intermediate or hybrid density. The DNA extracted after two generations (40 minutes) showed equal amounts of hybrid and of ‘light’ DNA.

Question.22. Given below is a list of six micro-organisms. State their usefulness to humans.
(a) Nucleopolyhedrovirus
(b) Saccharomyces cerevisiae
(c) Monascus purpureus
(d) Trichoderma polysporum
(e) Penicillium notation
(f) Propionibacterium sharmanii
Answer : (a) As bio control agents for Integrated Pest Management.
(b) It is used in bread making/brewing industry or for production of ethanol.
(c) It is a cholesterol lowering agent.
(d) It produces Cyclosporin A which is an immuno-suppressive agent.
(e) It produces antibiotic penicillin.
(f) It produces large holes in swiss cheese by releasing large amount of CO2.

SECTION—D
Question.23. Reproductive and Child Healthcare (RCH)
programmes are currently in operation. One of the major tasks of these programmes is to create awareness amongst people about the wide range of reproduction related aspects. As this is important and essential for building a reproductively healthy society.
(a) “Providing sex education in schools is one of the ways to meet this goal.” Give four points in support of your opinion regarding this statement.
(b) List any two ‘indicators’ that indicate a reproductively healthy society.
Answer : (a) It is a means of providing right information to the young so as to discourage children from believing in myths and misconceptions about sex related aspects.
Knowledge is also imparted about reproductive organs, adolescence and related changes, safe hygienic practices, STD/AIDS, available birth control options, care of pregnant mothers, post-natal care, importance of breast feeding, sex abuse and sex related crimes. .
(b)

  1.  Decreasae in IMR (Infant Mortality Rate), MMR (Maternal Mortality Rate).
  2.  Increase in number of couples with small families, better detection and cure of STDs.
  3.  Total well being in all aspects of reproduction, normal emotional and behavioural interaction among all sex related aspects.

SECTION—E
Question.24. (a) Explain the post-pollination events leading to seed production in angiospersms.
(b) List the different types of pollination depending upon the source of pollen grain.
OR
(a) Briefly explain the events of fertilization and implantation in an adult human female.
(b) Comment on the role of placenta as an endocrine gland.
Answer : (a) As a result of pollen-pistil interaction, germination of pollen tube takes’ place carrying two male gametes. One male gamete fuses with the egg cell (syngamy), while the other fuses with two polar nuclei to form primary endosperm nucleus (PEN). The zygote develops into an embryo while the PEN develops to form endosperm. After double fertilisation, the ovule matures into a seed while the ovary matures into a fruit.
(b) Different types of pollination depending upon the source of pollen grain are :

  1.  Autogamy: Transfer of pollen grains from the anther to the stigma of the same flower.
  2.  Geitonogamy : Transfer of pollen grain from the anther to the stigma of another flower of the same plant.
  3.  Xenogamy : Only types of pollination which brings genetically different types of pollen grains to the stigma.

OR
(a) Fertilization : A sperm comes in contact with the zona pellucida layer ovum and induces changes to block entry of additional sperms. The entry of sperm induces completion of meiosis II leading to the formation of anootid and second polar body. The haploid nucleus of the sperm and that of the ovum fuse to form a dipolid zygote.
Implantation : The trophoblast layer of the blastocyst attaches to the endometrium of the uterus. The uterine
cells divide rapidly and cover the blastocyst which becomes embedded in the endometrium and implantation is completed.
(b) Placenta acts as an endocrine tissue and produces several hormones like :

  1.  human chorionic gonadotropin (hCG)
  2. human placental lactogen (hPL)
  3.  Estrogens, progestogens, etc.

Question.25 . (a) How are the following formed and involved in DNA packaging in a nucleus of a cell ?
(i) Histone octomer
(ii) Nucleosomes
(iii) Chromatin
(b) Differentiate between Euchromatin and Hetero-chromatin.
OR
Explain the role of lactose as an inducer in a facoperon.
Answer : (a) (i) Eight molecules of positively charged basic proteins called histones are organised to form histone octomer.
(ii) Negatively charged DNA is wrapped around positively charged histone octomer to give rise to a nucleosome.
(iii) Nucleosomes constitute repeating unit of a structure:
cbse-previous-year-solved-papers-class-12-biology-delhi-2016-5
Lactose is the substrate for the enzyme beta galactosidase and it regulates switching ON and OFF of the operon. In the presence of an inducer such as lactose, the repressor is inactivated by interaction with the inducer. This allows RNA polymerase access to the promoter and transcription proceeds.

Question.26. (a) Why should we conserve biodiversity ? How can we do it ?
(b) Explain the importance of biodiversity hot-spots and sacred groves.
OR
(a) Represent diagrammatically three kinds of age pyramids for human populations.
(b) How does an age pyramid for human population
at given point of time helps the policy-makers in planning for future.
Answer: (a) (i) Narrowly utilitarian : We derive economic benefits from nature food (cereals/pulses/fruits). We also get firewood, fibre, construction material, industrial products (tannins, lubricants, dyes, resins, perfumes), products of medicinal importance etc.
Broadly utilitarian : We get 20% of the total O2 from the Amazon rain forests. Pollination is also achieved. We derive several aesthetic pleasures from them.
Ethical Argument : Millions of species of plants, animals and microbes share this planet with us. We need to realise that every species has an intrinsic value. We have a moral duty to care for their well being and pass on our biological legacy to future generations.
(ii) In situ conservation in biosphere reserves, national parks, sanctuaries, sacred groves etc.
Ex situe conservation in zoological parks, botanical gardens, safari parks, cryoprese-rvation, seed banks, tissue culture etc.
(b) Three of these hotspots are Western Ghats and Sri lanka, Indo-Burma and Himalaya-cover our country’s exceptionally high biodiversity regions.
Sacred Groves : They are tracts of forests containing wild life which are venerated and given total protection. Such sacred groves are found in Khasi and Jaintia Hills in Meghalaya, Aravalli hills of Rajasthan, Chand and Baster areas of Madhya Pradesh. In Meghalaya, the sacred groves are the last refuges for Meghalaya, for a large number of rare and threatened plants.
OR
(a)
Post-reproductive Reproductive Pre-reproductive Expanding
(b) Age pyramid analysis of a population helps in planning and health, education, transport, infrastructure, finance, food or employment.
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SET-II

SECTION—A

Question.3. Give an example of a human disorder that is caused due to a single gene mutation.
Answer : Sickle cell anaemia/Thalassemia/Phenylketonuria

SECTION—B

Question.8. Explain the importance of syngamy and meiosis in a sexual life cycle of an organism.
Answer : Syngamy : It ensures restoration of diploid chromosome number through zygote formation. Variations are an important characteristic of this process.
Meiosis : Gamete formation takes place as a result of meiosis which involves reduction in chromosome number or haploidy. It also leads to variations (due to crossing over).

Question.9. List the events that lead to biogas production from waste water whose BOD has been reduced significantly.
Answer : After significant reduction of BOD, the effluent is passed into a settling tank where the floes are allowed to sediment to form activated sludge. This sludge is pumped into anaerobic sludge digesters where anaerobic bacteria digest the microbes of the sludge to release a mixture of gases such as methane, H2S and CO2. These gases form biogas which can be used as a source of energy.
10. Why the plants that inhabit a desert are not found in a mangrove ? Give reasons.
Answer : Desert plants are not adapted to survive in saline or aquatic conditions prevailing in a mangrove. Plants are conformers. They are also stenothermal. They cannot maintain constant internal environment. The osmotic concentration of their body fluids affect the kinetics of enzymes through basal metabolism.

SECTION—C

Question.12. Differentiate between somaclones and somatic hybrids. Give one example of each.
Answer : Somaclones are produced through micro¬’ propagation or tissue culture. They are genetically identical, e.g., apple, tomato or banana.
Somatic hybrids are produced by fusion of protoplast of two different plants. They are genetically dissimilar e.g., Pomato (hybrid of potato and tomato).

Question.17. A couple with normal vision bear a colour blind child. Work out a cross to show how it is possible and mention the sex of the affected child.
Answer :
cbse-previous-year-solved-papers-class-12-biology-delhi-2016-7
The affected child is male.

Question.19. In certain seasons we sweat profusely while in some other season we shiver. Explain.
Answer : Mammals are able to maintain homeostasis means which ensure constant body temperature.

  1.  In summer, the outside temperature is higher than the body temperature. Hence sweating causes cooling by evaporation of sweat.
  2. In winter the outside temperature is lower than the body temperature. Hence shivering is an involuntary exercise which produces heat.
  3.  Both the above exercises help to regulate our body temperature.

SECTION—E

Question.26. List the criteria a molecule that cart act as genetic material must fulfill. Which one of the criteria are best fulfilled by DNA or by RNA thus making one of them a better genetic material than the other ? Explain.
OR
(a) Differentiate between analogy and homology giving one example each of plant and animal respectively.
(b) How are they considered as an evidence in support of evolution ?
Answer : (i) The genetic material should be able to carry out replication or generate a replica.
(ii) It should be chemically or structurally stable.
(iii) It should provide scope for slow mutation.
(iv) It should be able to express itself as characters.
Out of the two,clearly, DNA is more stable because of the following factors :
• Presence of H and not OH at 2’ position.
• Presence of thiamine instead of uracil.
• It is less reactive.
• It is structurally more stable because of its double stranded structure with hydrogen bonding.
• DNA is slower to mutate than RNA.
• Complementary strands of DNA further resist changes by evolving a process of repair.
OR
(a) Homology : Those structures which have similar origin but perform different functions show homology.
e.g., Forelimbs of mammals, heart of vestebrates, brain of vertebrates etc.
Thorns of bougainvilleas and tendrils of cucurbits. Analogy : Those structure which have a different origin, but perform similar functions show analogy, e.g., Wings of bat and birds, flippers of penguin and dolphin, eye of octopus and mammals etc.
Sweet potato and potato tuber.
(b) Homology shows common ancestry and divergent evolution.
Analogy does not show common ancestry. It shows convergent evolution.

SET-III

SECTION — A

Question.5. Give an example of a codon having dual functional]
Answer: AUG codes for methionine and also act as initiator codon.

SECTION — B

Question.7. Distinguish between the roles of flocks and anaerobic sludge digesters jn sewage treatments.
Answer.
cbse-previous-year-solved-papers-class-12-biology-delhi-2016-8

Question.9. Plants that inhabit a rain-forest ‘are not found in a wetland. Explain.
Answer : Plant inhabiting a rain forest are not adapted to survive in aquatic conditions or wetlands. Plants are conformers. They are stenothermal. They cannot maintain a constant internal environment or temperature. The osmotic concentration of their body fluids affects the kinetics of enzymes through basal metabolic activity.

Question.10. Angiosperms bearing unisexual flowers are said to be either monoecious or dioecious. Explain with the help of one example each.
Answer : Monoecious : Plants bear both male and female unisexual flowers on the same plant. e.g., cucurbits, coconut, maize etc.
Dioecious : Plants bear Cither male or female unisexual flowers on different plants, e.g. papaya, date palms etc.

SECTION—C

Question.13. (a) Name any two fowls other than chicken reared in a poultry farm.
(b) Enlist four important components of poultry farm management.
Answer : (a) Ducks, turkey, geese etc.
(b)

  1. Selection of disease free and suitable breeds.
  2.  Proper and safe farm conditions.
  3.  Proper food and water.
  4. Maintenance of hygiene and health care.

Question.18. Explain with the help of suitable examples the three different ways by which organisms overcome their stressful conditions lasting for short duration.
Answer. Three different ways are :

  1.  Migration: Organisms can move away temporarily from stressful habitat to a more hospitable area and return when stressful period is over. e.g. humans moving from Delhi to Shimla during summer.
  2.  Spore Formation : Various kinds of thick walled spores are formed which germinate on availability of suitable environment, e.g., bacteria, fungi etc.
  3.  Dormancy: Seeds or Vegetative reproductive structures help to tide over stress by reducing their metabolic activity e.g., seeds or vegetative reproductive structures of higher plants.
  4.  Hibernation : It takes place during winter e.g., bear.
  5. Aestivation : It takes place during summer to avoid heat and dessication e.g., Snails/fish etc.
  6.  Diapause : Under unfavourable conditions, zooplanktons enter a stage of suspended metablic activity e.g., zooplanktons.
    (Note : Mention any three from the above)

Question.22. How would you find genotype of a tall pea plant bearing white flowers ? Explain with the help of a cross. Name the type of cross you would use.
Answer : Test cross should be used.
cbse-previous-year-solved-papers-class-12-biology-delhi-2016-9

SECTION—E

Question.24. Answer the following questions based on Hershey and Chases’s experiments 
(a) Name the kind of virus they worked with and why ?
(b) Why did they use two types of culture media to grow viruses in ? Explain.
(c) What was the need for using a blender and later a centrifuge during their experiments ?
(d) State the conclusion drawn by them after the experiments.
OR
(a) How did Darwin explain adaptive radiation ? Give another example exhibiting adaptive radiation.
(b) Name the scientist who influenced Darwin and how ?
Answer : (a) They worked with bacteriophage which infect bacteria because they want to discover whether it was protein or DNA from the viruses that entered the bacteria.
(b) They used two types of culture media in order to make protein of viruses radioactive with the help of 35S in one case, and DNA molecule in virus radioactive by using 32P in the other case. This was done to identify which one of the two had entered into the bacteria during viral infection.
(c) Blender was used to separate viral protein coats that – were still attached to the surface of bacteria.
Centrifuge was used to separate lighter supernatent containing viral protein coats from denser residue containing bacteria.
(d) They concluded that DNA is the genetic material that is passed from virus to bacteria.
OR
(a) Darwin observed that from original seed eating features in finches, altered breaks arose enabling them to become insectivorous and vegetarian finches.
Adaptive Radiation : Is the process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography.
e.g., Australian marsupials, placental mammals in Australia.
(b) Thomas Malthus. Population size grows exponentially. However population size remains limited due to limited natural resources leading to competition.