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DELHI PUBLIC SCHOOL, SILIGURI Pre-Board examination Session: 2020- 21 CLASS: XII SC Subject: Biology Time: 60 Minutes DATE : 15/01/2021 Descriptive Max. Marks :30 General Instructions: (i) All questions are compulsory. (ii) The question paper has four sections: Section A, Section B, Section C and Section D. There are 33 questions in the question paper. (iii) Section A has 5 questions of 1 mark each. Section B has 4 questions of 2 marks each. Section C has 4 questions of 3 marks each and Section D has 1 question of 5 marks. (iv) There is no overall choice. However, internal choices have been provided in some questions. A student has to attempt only one of the alternatives in such questions. (v) Wherever necessary, neat and properly labeled diagrams should be drawn. SECTION- A (1X5= 5) Q1.Give an example of a chromosomal disjunction of autosomes. disorder caused due to non(1) Q2.The gene I that controls the ABO blood grouping in human beings has three alleles IA, IB and i. (i) How many different genotypes are likely to be present in the human population? (ii) Also, how many phenotypes are possibly present? ( + =1) Q3. Why are some seeds of Citrus referred to as poly embryonic? How are they formed? (1) Q4. What makes the Nucleopolyhedrovirus a desirable biological control agent? OR What are flocs formed during secondary treatment of sewage? (1) Q5. About 200 species of Cichlid fish became extinct when a particular fish was introduced in Lake Victoria of Africa. Name the invasive fish. (1) SECTION- B (2 x 4 = 8) Q6. Hot-spots occupy less than 2% of earth s land surface. Yet, they are given priority for conservation of biodiversity. Give two reasons for this. (2) Q7.Name the organism from where the thermostable DNA polymerase is isolated. State its role in genetic engineering. (2) Q8. Explain four advantages of mycorrhizal association to plants. OR Name the source of streptokinase. How does this bioactive molecule function in our body. (2) Q9. List the adaptive features of water- pollinated flower like Vallisneria.(2) SECTION- C (3 x 4 = 12) Q10. A farmer noticed that nematode infection in tobacco plants has resulted in the reduction in the yield. Suggest a strategy which provides cellular defense for providing resistance to this pest. Explain the technique. Q11. (a) Name the causative agent of typhoid in humans. ( ) (b) Name the test administered to confirm the disease. ( ) (c) How does the pathogen gain entry into the human body? Write the diagnostic symptoms and mention the body organ that gets affected in severe case. (2) Q12. The graph given below represents three categories of organismic responses - A, B and C to cope with stressful conditions. Identify the categories A and B. (1) Given below are examples of some of the activities performed by animals. Categorise these activities into the appropriate kind of the organismic response (A, B or C) as shown in the graph with reasons. i. In summers we sweat profusely. (1) ii. Sometimes desert lizards bask in the sun and sometimes they move into shade. (1) OR Give reasons for the following: (a) Very small animals are rarely found in polar regions. (1) (b) Mammals from colder climate generally have shorter ear and limbs.(1) (c) Cattle and goats do not browse the Calotropis growing fields. (1) Q13. Name two hormones that are constituents of contraceptive pills .(1) Why do they have high and effective contraceptive value? ( 1 ) Name a commonly prescribed non-steroidal oral pill. . ( ) SECTION- D (5 x 1 = 5) Q14. (a) Why is DNA considered a better hereditary material than RNA? (3) (b) How is the translation of mRNA terminated? Explain. (2) OR (a) Give two reasons why both the strands of DNA are not copied during transcription. (2) (b) Draw a labelled diagram of a nucleosome. Where is it formed in the cell. (2) (c) How is the length of DNA usually calculated? *********** (1)
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