CBSE Class 12

Pre Boards

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Sample Papers

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 Multiple Choice QuestionsShort Answer Type

1. Why do children cured by enzyme-replacement therapy for adenosine deaminase deficiency need periodic treatment?

Children cured by enzyme-replacement therapy for adenosine deaminase deficiency need periodic treatment because these cells are not immortal.


What is a cistron?

Cistron is a segment of DNA that codes for a certain polypeptide or protein.

3. List two advantages of the use of unleaded petrol in automobiles as fuel.

Following are the two advantages of using unleaded petrol as fuel in automobiles:

i. Unleaded petrol does not have lead compounds therefore does not inactivate the catalytic converter.

ii. It does not emit harmful compounds, thus causes less pollution and helps in preventing health diseases like bronchitis, asthma and lung diseases.


(a) Select the homologous structures from the combinations given below :
(i) Forelimbs of whales and bats
(ii) Tuber of potato and sweet potato
(iii) Eyes of octopus and mammals
(iv) Thorns of Bougainvillea and tendrils of Cucurbita

(b) State the kind of evolution they represent.

 (a) Homologous organs or structures are those that are similar in anatomy or structure and have a common origin, but perform different functions.
From the given options, following are homologous structures:

i. Forelimbs of whales and bats are similar in structure but perform different functions of swimming and flying, respectively.

ii. Thorns of Bougainvillea and tendrils of Cucurbita are both modifications of a stem arising from axillary bud but perform different functions of protection (thorns) and climbing (tendrils), respectively.

(b) The evolution represented by homologous organs or structures is divergent evolution as they have a common origin but have diverged (became dissimilar) with time into different structures performing different functions.



(a) Why are the plants raised through micro-propagation termed as somaclones?
(b) Mention two advantages of this technique.

(a) The plants raised through micropropagation are called somaclones because they are genetically identical to the original plant from which they were grown.

(b) The advantages of micro-propagation are as follows:
(i) It helps in the propagation of a large number of plants in a short span of time.
(ii) It helps in recovery of healthy plants from diseased ones. It helps in producing plants that are disease and pest resistant.


6. What is mutualism? Mention any two examples where the organisms involved are commercially exploited in agriculture.

Mutualism is the interaction between the organisms of two species in which both organisms are benefited.
Examples of the organisms involved that are commercially exploited in agriculture are as follows:
(i) Rhizobium  which is a mutual relationship between bacteria and leguminous plants is used as a biofertiliser. These enrich the nutrient quality of the soil by fixing atmospheric nitrogen into organic forms..
(ii) Mycorrhiza which is an association of the soil fungus and the roots of higher plants. Farmers use Mycorrhiza commercially in agriculture as it improves the soil quality and plant growth and development, reduces soil erosion, help the plants absorb phosphorous from the soil. Plants having such associations show  resistance to root-borne pathogens, tolerance to salinity and drought.


How many chromosomes do drones of honeybee possess? Name the type of cell division involved in the production of sperms by them.

Drones of honey bees are haploid and possess 16 chromosomes.
Drones produce sperms by mitosis. 



Explain the different steps involved during primary treatment phase of sewage.

The primary phase of sewage treatment involves physical removal of particles by filtration and sedimentation.
Different steps involved during the primary treatment are as follows:

i. Initially, floating debris of sewage water is removed by sequential filtration.

ii. Then, grit (soil and small pebbles) are removed by sedimentation.

Effluent (that is the supernatant formed when all the solids settle from the primary sludge) is then taken for secondary treatment.

9. Retroviruses have no DNA. However, the DNA of the infected host cell does possess viral DNA. How is it possible?

Retroviruses convert their RNA into DNA by the method of reverse transcription, thus it is possible that the DNA of the infected host possesses DNA even when retroviruses do not have their own DNA.



Why do moss plants produce very large number of male gametes? Provide one reason. What are these gametes called?

Mosses are bryophytes and they need water for fertilisation. During the transfer of male gametes, many of them are destroyed or lost. Thus, moss plants produce very large number of male gametes to compensate for the loss during transport.

These male gametes are called antherozoids.