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Board Paper of Class 12-Science 2020 Biology Delhi(Set 3) - Solutions

General Instructions:
Read the following instructions very carefully and strictly follow them:
(i) Question paper comprises five sections – A, B, C, D and E.
(ii) There are 27 questions in the question paper. All questions are compulsory.
(iii) Section A question number 1 to 5 are multiple choice questions, carrying one mark each.
(iv) Section B question number 6 to 12 are short answer questions type-I, carrying two marks each.
(v) Section C question number 13 to 21 are short answer questions type-II, carrying three marks each.
(vi) Section D question number 22 to 24 are short answer questions type-III, carrying three marks each.
(vii) Section E question number 25 to 27 are long answer questions, carrying five marks each.
(viii) Answer should be brief and to the point also the above word limit be adhered to as far as possible.
(ix) There is no overall choice in the question paper. However, an internal choice has been provided in two questions of 1 mark, one question of 2 marks, two questions of 3 marks and three questions of 5 marks questions. Only one of the choices in such questions have to be attempted.
(x) The diagram drawn should be neat proportionate and properly labelled, wherever necessary.
(xi) In addition to this, separate instructions are given with each section and question, wherever necessary.

  • Question 1
    Louis Pasteur demonstrated that
    (a) early life came from outer space
    (b) non-living chemicals produced living molecules
    (c) life comes from pre-existing life
    (d) life originated spontaneously VIEW SOLUTION

  • Question 2
    Mating of a superior male of a breed of a cattle to a superior female of another breed is called
    (a) in breeding
    (b) out crossing
    (c) out breeding
    (d) cross breeding


    Large-holes in 'Swiss-Cheese' are due to
    (a) Propionibacterium sharmanii
    (b) Saccharomyces cerevisae
    (c) Penicillium chrysogenum
    (d) Acetobacter aceti VIEW SOLUTION

  • Question 3
    Increased concentration of DDT in fish-eating birds is due to
    (a) eutrophication
    (b) bio-magnification
    (c) cultural eutrophication
    (d) accelerated eutrophication


    Species-Area relationship is represented on a log scale as
    (a) hyperbola
    (b) rectangular hyperbola
    (c) linear
    (d) inverted VIEW SOLUTION

  • Question 4
    Which one of the following part of the plant when put into the soil is likely to produce new offspring?
    (a) Part of an internode
    (b) A stem cutting with a node
    (c) Part of a primary root
    (d) A flower VIEW SOLUTION

  • Question 5
    In a bacterium when RNA-polymerase binds to the promoter on a transcription unit during transcription, it
    (a) terminates the process
    (b) helps remove introns
    (c) initiates the process
    (d) inactivates the exons VIEW SOLUTION

  • Question 6
    Name one air-borne and a water borne disease in humans. List one specific symptom of each one of them. VIEW SOLUTION

  • Question 7
    (a) Name the two techniques employed to meet the increasing demand of fish in the world.
    (b) Name any two fresh water fishes.


    Describe the contributions of Alexander Fleming, Ernest Chain and Howard Florey in the field of microbiology. VIEW SOLUTION

  • Question 8
    All cloning vectors do have a 'selectable marker'. Describe its role in recombinant DNA-technology. VIEW SOLUTION

  • Question 9
    Mention how have plants developed mechanical and chemical defence against herbivores to protect themselves with the help of one example of each. VIEW SOLUTION

  • Question 10
    How is humus formed ? Mention any three characteristics of humus. VIEW SOLUTION

  • Question 11
    State what are Mendelian disorders. Both thalassemia and colour blindness categorised as Mendelian disorders. Justify. VIEW SOLUTION

  • Question 12
    State two advantages of an apomictic seed to a farmer. VIEW SOLUTION

  • Question 13
    (a) Write the palindromic nucleotide sequence EcoRI recognises.

    (b) Draw the vector DNA and a foreign DNA showing the sites where EcoRI has acted to form the sticky ends.

    (c) Name the enzyme that helps in forming recombinant DNA. VIEW SOLUTION

  • Question 14
    Differentiate between “Pioneer-species”; “Climax-community” and “Seres”.


    Explain any three ways other than zoological parks, botanical gardens and wildlife safaries, by which threatened species of plants and animals are being conserved 'ex situ'. VIEW SOLUTION

  • Question 15
    Explain 'Integrated organic' farming as successfully practiced by Ramesh C. Dagar, a farmer in Sonepat (Haryana). VIEW SOLUTION

  • Question 16
    (a) Explain the mode of action of Cu++ releasing IUDs as a good contraceptive. How is hormone releasing IUD different from it?
    (b) Why is “Saheli' a preferred contraceptive by women (any two reasons)? VIEW SOLUTION

  • Question 17
    What are 'SNPs'? Where are they located in a human cell ? State' any two ways the discovery of SNPs can be of importance to humans. VIEW SOLUTION

  • Question 18
    (a) Rearrange the following in the correct order of their appearance on Earth between two million years and 40,000 years back.
    Neanderthals, Australopithecus, Homo erectus and Homo habilis.

    (b) Which one of the above
    (i) had the largest brain size
    (ii) ate fruits

  • Question 19
    Explain Mendel’s “Law of segragation” in a typical monohybrid cross with the help of a suitable example. VIEW SOLUTION

  • Question 20
    (a) Explain why bee-hives are setup on the farms for some of our crop-species. Name any two such crop species.
    (b) List any three important steps to be kept in mind for successful bee keeping. VIEW SOLUTION

  • Question 21
    Explain three different modes of pollination that can occur in chasmogamous flower.


    Explain the formation of placenta after implantation in a human female. VIEW SOLUTION

  • Question 22
    "The population of a metro city experiences fluctuations in its population density over a period of time.”
    (a) When does the population in a metro city tend to increase?
    (b) When does the population in metro city tend to decline?
    (c) If 'N' is the population density at the time 't', write the population density at the time 't + 1'. VIEW SOLUTION

  • Question 23
    Study the diagram showing the entry of HIV into the human body and be processes that are followed :

    (a) Name the human cell 'A' HIV enters into.
    (b) Mention the genetic material 'B' HIV releases into the cell.
    (c) Identify enzyme 'C'. VIEW SOLUTION

  • Question 24
    Following a road accident four injured persons were brought to a nearby clinic. The doctor immediately injected them with tetanus antitoxin.
    (a) What is tetanus antitoxin ?
    (b) Why were the injured immediately injected with this antitoxin?
    (c) Name the kind of immunity this injection provided. VIEW SOLUTION

  • Question 25
    (a) Name the insect that attacks cotton crops and causes lot of damage to the crop. How has Bt cotton plants overcome this problem and saved the crop ? Explain.
    (b) Write the role of gene Cry IAb.


    (a) Explain the different steps carried out in Polymerase Chain Reaction, and the specific roles of the enzymes used.

    (b) Mention application of PCR in the field of

    (i) Biotechnology

    (ii) Diagnostics

  • Question 26
    (a) Describe the process of double fertilisation in angiosperms.

    (b) Trace the development of polyploidal cell that is formed after double fertilisation in a non-albuminous seed and albuminous seed.


    (a) List any two reasons other than physical and congenital disorders for causing infertility in couples.

    (b) Explain how IVF as a technique helped childless couples in having children.

    (c) Compare GIFT with ICSI. VIEW SOLUTION

  • Question 27
    (a) Why did T.H. Morgon select Drosophila melanogaster for his experiments?
    (b) How did he disprove Mendelian dihybrid F2 phenotypic ratio of 9 : 3 : 3 : 1? Explain giving reasons.


    (a) List any four major goals of Human Genome project.
    (b) Write any four ways the knowledge from HGP is of significance for humans.
    (c) Expand BAC and mention its importance. VIEW SOLUTION
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