List of top Questions asked in CBSE CLASS XII

Below is shown a pond that represents a self-sustaining natural ecosystem with both biotic and abiotic components interacting in a balanced food web.

Based on the diagram, answer the questions :

Mention two abiotic components of a pond ecosystem.



(i) Explain the role of stigma in pollen pistil interaction.

(ii) Describe the post pollination events leading to double fertilization in angiosperms starting with 2-celled pollen grain.

Observe the segment of mRNA given below and answers the questions given below :

(i) Explain and illustrate the steps involved to make fully processed hn RNA.

(ii) Gene encoding RNA polymerase I and RNA polymerase III have been affected by mutation.

Explain its impact on the synthesis of polypeptide giving reasons.



Below is shown a pond that represents a self-sustaining natural ecosystem with both biotic and abiotic components interacting in a balanced food web.

Based on the diagram, answer the questions :

Differentiate between the roles of producers and decomposers in such an ecosystem.



(i) Explain menstrual phase in the menstrual cycle of human female.

(ii) Why is follicular phase also called proliferative phase ?

(iii) Explain the events that occur in Graafian follicle at the time of ovulation and thereafter.

(iv) Draw a Graafian follicle and label - Antrum and secondary oocyte.

Gene manipulation is a fast emerging science. It started with the development of recombinant DNA molecules. This technology which mostly involves cutting and pasting of desired DNA fragments is based on two most important discoveries in bacteria - presence of plasmid and restriction endonucleases. The science of r-DNA technology took birth when Cohen and Boyer (1973) were able to produce a piece of gene containing foreign DNA introduced into plasmid of E.coli.

(a) Name one core technique of biotechnology that alters the genetic material.

(a) A person inhales dust and pathogens but does not always fall sick. Explain how the body protects against such infections.

(b) Name and describe any two barriers involved in this protection.

(a) What is a genetic code ?

(b) Why did scientists feel the need to propose a genetic code ?

(c) Genetic code is said to be "Universal". Why ?

Gene manipulation is a fast emerging science. It started with the development of recombinant DNA molecules. This technology which mostly involves cutting and pasting of desired DNA fragments is based on two most important discoveries in bacteria - presence of plasmid and restriction endonucleases. The science of r-DNA technology took birth when Cohen and Boyer (1973) were able to produce a piece of gene containing foreign DNA introduced into plasmid of E.coli.

(b) What is a plasmid ? Mention its importance in biotechnology.

Gene manipulation is a fast emerging science. It started with the development of recombinant DNA molecules. This technology which mostly involves cutting and pasting of desired DNA fragments is based on two most important discoveries in bacteria - presence of plasmid and restriction endonucleases. The science of r-DNA technology took birth when Cohen and Boyer (1973) were able to produce a piece of gene containing foreign DNA introduced into plasmid of E.coli.

(c) How is the action of exonuclease different from that of endonuclease ?

Gene manipulation is a fast emerging science. It started with the development of recombinant DNA molecules. This technology which mostly involves cutting and pasting of desired DNA fragments is based on two most important discoveries in bacteria - presence of plasmid and restriction endonucleases. The science of r-DNA technology took birth when Cohen and Boyer (1973) were able to produce a piece of gene containing foreign DNA introduced into plasmid of E.coli.

(c) Which gene was isolated by Cohen and Boyer and from which bacteria ?

S.L. Miller conducted an experiment in 1953 to simulate the conditions of early earth in a laboratory setup.

Answer the following questions based on the experiment :

(a) What was the aim of his experiment ?

(b) Name the gases used and the source of energy.

Draw a neat diagram of enlarged view of one microsporangium of a Young anther of an angiosperm. Label the different layers of the wall and write the functions of the outermost and innermost layer.

(a) Give the scientific name of the organism used by Morgan to study sex-linked traits.

(b) Write any four reasons why this organism made it an ideal experimental model.

There are limitations of traditional breeding techniques that led to promotion of micro-propagation ?

(a) Mention two advantages of micropropagation.

(b) Give two examples where it is commercially adopted.

Study the diagram given below :

(a) Name and explain the procedure shown in the diagram. Mention one of its advantage and one disadvantage.

(b) How is "Saheli" different from other oral pills in terms of its composition and advantage ?



Give reasons for :

(i) All copulations do not lead to pregnancy.

(ii) Meiosis I in spermatogenesis and oogenesis are markedly different.

(i) Name the organ that forms a connection between foetus and the uterus of a pregnant human female.

(ii) How does it support both nutrition and hormonal regulation ?

What factors contribute to the Amazonian rain forests in South America being the most biodiverse regions on Earth ?

The diagram below shows the sequence of amino acids in part of haemoglobin molecule.

[Diagram: haemoglobin chain Val, His, Leu, Thr, Pro, Glu, Glu; mRNA line; DNA line with triplets CAT, GTA, AAT, TGA, GGA, CTT (the T marked with *), CTC. Key: Val = valine, His = histidine, Leu = leucine, Thr = threonine, Pro = proline, Glu = glutamic acid]

(i) If the base T (marked with *) is substituted with A, in "CTT" of DNA strand how would it affect the haemoglobin chain ?

(ii) Name the condition and the effects associated with the above substitution.



After significant reduction of BOD during secondary treatment of sewage, what further processes are carried out ? Mention any one important outcome of these processes.

DNA is considered a more stable genetic material than RNA. Justify this statement with suitable reasons.

How do scientists use microinjection and gene gun in biotechnology ?

In some cases, measuring population density by counting individuals may not be feasible. Mention two other parameters that can be used and justify their use with suitable examples.

Assertion (A) : IUCN Red List helps in identifying and protecting endangered species worldwide.

Reason (R) : The list includes the species facing risk of extinction.