CBSE Previous Year Question Papers Class 12 Biology With Solutions

In the upright pyramid of number the producers / grass are more than the number of primary consumer / secondary consumer

Eg. grassland ecosystem

In inverted pyramid of number the producer is less in number than the number of primary consumer

Eg. Tree and insects feeding on it

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In the upright pyramid of biomass the producers / tree are more in biomass than the primary consumer / secondary consumer

Eg. forest ecosystem

In inverted pyramid of biomass the biomass of the producer/phytoplanktons are less than the biomass of the zooplanktons / small standing crop of phytoplanktons supports large standing crop of zooplanktons

Eg. Pyramid of biomass in sea

SECTION C

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Question 13: (Marks 3)

‘‘Cotton bollworms enjoy feeding on cotton plants, but get killed when feed on Bt cotton plant.’’ Justify the statement.

OR

(a) Mention the cause of ADA deficiency in humans.

(b) How is gene therapy carried out to treat the patients suffering from this disease ?

(c) State the possibility of a permanent cure of this disease.

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Answer :

Once a bollworm feeds on Bt cotton plant the inactive protoxin produced by Bacillus thuringiensis, is converted into an active form of toxin, due to the alkaline pH of the gut which solubilise the crystals, the activated toxin binds to the surface of its midgut epithelial cells, create pores that cause cell swelling and lysis, and eventually cause death of the insect

OR

a) Caused due to the deletion of the gene for adenosine deaminase

b) lymphocytes from the blood of the patient are grown in a culture medium outside the body, a functional ADA cDNA (using a retroviral vector) is then introduced into theselymphocytes which are subsequently returned to the patient, patient requires periodic infusion of such genetically engineered lymphocytes

c) if the gene isolated from bone marrow cells producing ADA, is introduced into cells at early embryonic stages it could be a permanent cure

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Question 14: (Marks 3)

(a) Differentiate between Intrauterine insemination and Intrauterine transfer.

(b) Mention one positive and one negative application of amniocentesis.

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Answer :

a) IUI - the semen collected either from the husband or a healthy donor is artificially introduced either into the vagina or into the uterus of the female

IUT - embryos with more than 8 blastomeres transferred into the uterus

b) Positive application: used to test for the presence of certain genetic disorders / Down’s syndrome / Klinefelter’s syndrome / Haemophilia / Sickle-cell anemia

Negative application : used to detect sex of the foetus and leads to female foeticides

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Question 15: (Marks 3)

Explain the solutions found by Ahmed Khan, a Bengaluru based plastic sack manu facturer, after realising the problems created by plastic wastes.

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Answer :

Polyblend a fine powder was made from recycled modified plastic , this was mixed with the bitumen to lay roads which proved to enhance the bitumen’s water repellant properties , and helped to increase road life by the factor of three

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Question 16: (Marks 3)

Mention the chemical nature of an antibody and name the type of cells they are produced by. Write the difference between active and passive immune responses on the basis of antibodies.

OR

Name the cells that act as HIV factory in humans when infected by HIV. Explain the events that occur in these infected cells.

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Answer :

Made up of proteins / peptide , B-lymphocytes / B-cells

Active immunity - due to exposure to antigens / pathogens / vaccination / immunisation leads to production of antibodies by the individual, slow process

Passive immunity - Ready-made antibodies are directly given to protect the body of an individual against foreign agents, fast process/ provide immediate immunity

OR

Macrophages

Once the virus enters the human body the virus / viral genome infects macrophages where its RNA genome replicates , to form viral DNA , with the help of the enzyme reverse transcriptase, this viral DNA gets incorporated into host cell’s DNA , and directs the infected cells to produce virus particles

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Question 17: (Marks 3)

(a) Why is the collection of white winged moths and dark winged moths made in England between 1850 – 1920 considered a good example of natural selection ?

(b) ‘‘Evolution is based on chance events in nature and chance mutations in organ isms.’’ Justify the statement.

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Answer :

a) During Pre-industrialisation white-winged moths survived due to white coloured lichens on trees, During post-industrialisation white-winged moths did not survive due to predation / predators could spot the moth against contrasting back ground, then the dark-winged or melanised moths survived, this showed that in a mixed population of white and dark winged moths those who can adapt better will survive

b) excess use of herbicides /pesticides and antibiotics has resulted in selection of resistant varieties that developed due to chance mutation (in much lesser time scale)

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Question 18: (Marks 3)

(a) Compare the mechanism of sex determination in humans with that of honey bees, with respect to chromosome number.

(b) How is the gamete formation comparable in the above two cases ?

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Answer :

a) In honeybee union of a sperm and an egg develops as a female (queen or worker), and an unfertilised egg develops as a male (drone) by means of parthenogenesis, the females are diploid / having 32 chromosomes and males are haploid / having 16 chromosomes , in humansboth male and female individuals have 23 pair of chromosomes / diploid / female 44+ XX and males 44+ XY

b) In humans gametes are formed by meiosis, in honeybee female gametes are formed by meiosis and male gametes by mitosis

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Question 19: (Marks 3)

Differentiate between the pattern of inheritance in humans of the blood diseases, haemophilia and thalassemia.

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Answer :

(both corresponding points to be written for credit)

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Question 20: (Marks 3)

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Answer :

i) statin

ii) cholesterol lowering agent

iii) Penicillium notatum

iv) Penicillin

v) Trichoderma polysporum

iv) Immunosuppressive agent

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Question 21: (Marks 3)

(a) Write the scientific name of methanogen bacteria. Where are these bacteria generally found ? Explain their role in biogas production.

(b) Name the components of biogas.

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Answer :

a) Methanobacterium

found in the anaerobic sludge / rumen of cattle , they grow anaerobically on cellulosic material and cause its breakdown (into methane CO2 and H2 )

b) methane , CO2 , H2

SECTION - D

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Question 22: (Marks 3)

Given below is the diagram representing the observations made for separating DNA fragments by Gel electrophoresis technique. Observe the illustration and answer the questions that follow :

(a) Why are the DNA fragments seen to be moving in the direction A → B ?

(b) Write the medium used on which DNA fragments separate.

(c) Mention how the separated DNA fragments can be visualised for further technical use.

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Answer :

a) Because the DNA fragments are negatively charged

b) Agarose gel

c) After staining DNA with ethidium bromide, followed by exposure to UV rays

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Question 23: (Marks 3)

Study the transverse section of human ovary given below and answer the questions that follow :

(a) Name the hormone that helps in the growth of A → B → C.

(b) Name the hormone secreted by A and B.

(c) State the role of the hormone produced by D

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Answer :

a) Gonadotropins // FSH and LH

b) Estrogen

c) Maintenance of uterine endometrium

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Question 24: (Marks 3)

Indiscriminate use of chemicals, pesticides and weedicides by humans are polluting our water bodies, which in turn are harming the living organisms. Study the flow chart and answer the questions based on it.

(a) Why does the concentration of DDT seem to be considerably highin the top consumer ?

(b) How would the organisms at the highest level be affected ?

(c) Name the phenomenon observed.

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Answer :

a) DDT cannot be metabolised or excreted so gets accumulated by an organism , increase in concentration at successive trophic levels

b) Disturb calcium metabolism in fish eating birds / causes thinning of egg shell , premature breaking of eggs leading to population decline

c) Biomagnification

SECTION - E

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Question 25: (Marks 5)

(a) According to ecologists, tropical regions in the world account for greater biological diversity. Justify.

(b) Why are habitat loss and alien species invasion considered as the causes of biodiversity loss ? Explain with the help of an example of each.

OR

(a) What is an ecological succession ?

(b) Differentiate between primary and secondary succession. Why is secondary succession faster than primary succession ? Explain with suitable examples.

(c) What are pioneer species ? Give examples of pioneer species in Xerarch and Hydrarch successions respectively.

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Answer :

a) (i) have remained relatively undisturbed for millions of years / had a long evolutionary time for species diversification

(ii) environment less seasonal / more constant and predictable / such constant environment promotes niche specialization

(iii) more solar energy available in the tropics contributes to higher productivity and greater diversity

b) Habitat loss

Amazon rain forest is being cut for cultivating soyabeans / degradation of habitat by pollution / human activities leading to clearing of forests for commercial or tourism purpose =1 (any other relevant example)

Alien species invasion -

The Nile perch introduced into Lake Victoria in East Africa led eventually to the extinction of an ecologically unique assemblage of more than 200 species of cichlid fish in the lake / Recent illegal introduction of the African catfish Clarias gariepinus for aquaculture purposes is posing a threat to the indigenous catfishes in our rivers / carrot grass / lantana / water hyacinth causes threat to our indigenous species

(any other relevant example)

OR

a) The gradual and fairly predictable change in the species composition of a given area is called ecological succession

Secondary succession is faster since some soil or sediment is already present

eg. abondoned lands/ burnt or cut forests/ lands that have been flooded

c) The species that invade a bare area are called pioneer species

Xerarch- lichens , Hydrarch- phytoplanktons

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Question 26: (Marks 5)

(a) Name the type of DNA that forms the basis of DNA fingerprinting and mention two features of this DNA.

(b) Write the steps carried out in the process of DNA fingerprinting technique, and mention its application.

OR

Explain the role of different genes in a lac operon, when in a ‘Switched On’ state.

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Answer :

a) Satellite DNA / repetitive DNA

These sequences normally do not code for any proteins, these sequence show high degree of polymorphism

b) (i) isolation of DNA,

(ii) digestion of DNA by restriction endonucleases,

(iii) separation of DNA fragments by electrophoresis,

(iv) transferring (blotting) of separated DNA fragments to synthetic membranes such as nitrocellulose or nylon,

(v) hybridisation using labelled VNTR probe,

(vi) detection of hybridised DNA fragments by autoradiography

Application - Forensic science / determining population and genetic diversities / paternity test

OR

The regulator gene produces repressor, inactivated in the presence of inducer, that is lactose, RNA polymerase then gets access to the promoter gene and transcription proceeds

z gene codes for beta-galactosidase (ß-gal), responsible for the hydrolysis of the disaccharide lactose into galactose and glucose

y gene codes for permease , which increases permeability of the cell to ß-galactosides / lactose

a gene encodes enzyme transacetylase

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Question 27: (Marks 5)

(a) Draw a diagram of a fully developed embryo sac of an angiosperm. Label its chalazal end and any other five parts within the embryo sac.

(b) Why does the development of an endosperm precede that of the embryo in angiosperm ?

(c) Number of chromosomes in an onion plant cell is 16. Name the cells of the embryo sac having 16 and 24 chromosomes formed after fertilisation.

OR

Describe the events that occur after fertilisation of an ovum till implantation in a human female.

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Answer :

Chalazal end = ½, and any other five parts

b) the cells get filled with reserve food materials, used for providing the nutrition of developing embryo / for assured nutrition of developing embryo

c) cells with 16 chromosome- zygote, cells with 24 chromosome- endosperm

OR

The mitotic division called cleavage , starts as the zygote moves towards the uterus and forms 2-4 - 8 -16 daughter cells called blastomeres , and forms the first embryonic stage morula, the morula continues to divide and transforms into blastocyst as it moves further into the uterus, the blastomeres in the blastocyst are arranged into an outer layer called trophoblast, and an inner group of cells attached to trophoblast called the inner cell mass, the trophoblast layer then gets attached to the endometrium, and the inner cell mass gets differentiated as the embryo, after attachment the uterine cells divide rapidly and covers the blastocyst, that gets embedded in the endometrium of the uterus (this is called implantation )