List of top Questions asked in CBSE CLASS XII

(a) How is insulin different from proinsulin ?

(b) Explain all the stages of how human insulin is produced through rDNA technology.

(a) Construct a complete transcription unit with promoter and terminator on the basis of the hypothetical coding strand given below :

A T G C A T G C A T A G

(The strand is drawn on a line with arrow heads at both ends and short vertical ticks under each base, for you to complete.)

(b) Write the RNA strand transcribed from the above transcription unit along with its polarity.

Explain the three different ways in which natural selection operates. Explain with the help of graphs.

A laboratory assistant collected three water samples, namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant. But the laboratory attendant did not note which was which. The samples were labelled A, B and C and subjected to a BOD test. The BOD values of the three samples A, B and C were recorded as 30 mg/L, 8 mg/L and 400 mg/L respectively.

(a) Which sample of the water is most polluted ?

(b) Can you assign the correct label to each, assuming the river water is relatively clean ?

(i) State, what does "standing crop" of a trophic level represent.

(ii) List any two ways of measuring the standing crop of a trophic level.

Biodiversity must be conserved as it plays an important role in many ecosystem services that nature provides. Explain any two ecosystem services.

(a) Write one way by which apomictic seeds can be developed.

(b) Why do farmers prefer apomictic seeds over hybrid seeds ?

Assertion (A) : Nucleopolyhedrovirus are useful as they are good biocontrol agents.

Reason (R) : They are species-specific, narrow spectrum bioinsecticides.

Protein encoded by gene Cry-I Ab controls the infestation of which of the following ?

Connell's elegant field experiments on the rocky seacoast of Scotland, found that larger barnacles Balanus dominated the intertidal area and removed the smaller barnacles Chathamalus. This happened due to :

Assertion (A) : Cleistogamous flowers have assured seed set.

Reason (R) : Cleistogamous flowers are invariably autogamous and do not depend on any pollinating agents.

Name the extinct ancestor of humans who ate only fruits and hunted with stone weapons.

In RNAi, genes are silenced using which of the following biomolecules ?

Microbes used for biocontrol of pest butterfly caterpillars is :

Nicotine affects blood pressure and heart rate due to release of certain adrenal hormones. Following are some of the adrenal hormones :

(i) Adrenaline

(ii) Cortisol

(iii) Noradrenaline

(iv) Aldosterone

Select the correct option :

A female undergoing IVF treatment has blocked fallopian tube. The technique by which the embryo with more than 8 blastomeres will be transferred into the female for further development is :

Choose the correct statements :

(i) Polycistronic mRNA is generally found in eukaryotes.

(ii) The RNA polymerase II transcribes precursor of mRNA, the hnRNA.

(iii) The process of translation of mRNA begins when the mRNA encounters the large subunit of ribosomes.

(iv) Stop codons do not have any tRNAs.

Options :

Select the correct statement.

All genes located on the same chromosome :

Select the phase of menstrual cycle in which the remaining parts of Graafian follicle transform into the corpus luteum.

Choose the option that gives the correct number of pollen grains that will be formed after 325 microspore mother cells undergo microsporogenesis.

Medical Termination of Pregnancy (MTP) plays an important role in reproductive health.

(i) What is MTP ?

(ii) Under what conditions is it legally permitted in India ? Mention any two points.

(iii) In which stage of pregnancy is MTP considered legally and medically safe ?

(iv) Explain any two risks/concerns associated with this process.

Study the following cases :

Case I : A person bitten by a rabid dog is immediately given an injection of antitoxin for rabies.

Case II : A child is given rabies vaccine as a preventive measure.

(a) Identify the type of immunity provided in both the cases.

(b) Differentiate between the two types of immunity.

(c) What is a vaccine ? How does it help in preventing a disease ?

(i) Draw a neat sectional view of seminiferous tubule of a human testes showing spermatogenesis. Label any five stages.

(ii) Explain the hormonal regulation of spermatogenesis process in humans.