List of top Questions asked in CBSE CLASS XII

(b) “DNA replication is continuous and discontinuous on the two strands within the replication fork.” Explain with the help of a schematic representation.
(a) Construct a pyramid of biomass in sea with phytoplankton and fishes. Explain giving reasons about the characteristic of the constructed pyramid.
The given DNA sequence:
5’ – G$\uparrow$A A T T C – 3’
3’ – C T T A A$\uparrow$G – 5’
(a) Name the restriction enzyme that recognises the given specific sequence of bases. What are such sequences of bases referred to as?
(c) State any 3 reasons for which these types of animals are being produced.
Tropical regions harbour more species than the temperate regions. How have biologists tried to explain this in their own ways? Explain.
(b) What is a ‘vaccine’? Explain the principle on which it works.
Explain the processing of heterogeneous nuclear RNA (hnRNA) into a fully functional mRNA in eukaryotes. Where does this processing occur in the cell?
(b) Mention one advantage and one disadvantage of amniocentesis.
(a) “Farmers prefer apomictic seeds to hybrid seeds.” Justify giving two reasons.
List the events that reduce the Biochemical Oxygen Demand (BOD) of a primary effluent during sewage treatment.

Observe the population growth curve and answer the questions given below: 
 


(a) State the conditions under which growth curve ‘A’ and growth curve ‘B’ plotted in the graph are possible. 
 

If the base adenine constitutes 31% of an isolated DNA fragment, then what will be the expected percentage of the base cytosine in it? Explain how you arrived at the answer.
Assertion (A): AIDS is a syndrome caused by HIV.
Reason (R): HIV is a virus that damages the immune system with DNA as its genetic material.
Assertion (A): “Biodiversity hotspots” are the regions which possess high levels of species richness, high degree of endemism.
Reason (R): Total number of biodiversity hotspots in the world is 22 with two of these hotspots found in India.
Assertion (A): In birds, the sex of the offspring is determined by males.
Reason (R): Males are homogametic while females are heterogametic.
The human chromosome with the highest and least number of genes in them are respectively:
Match the following genes of the lac operon listed in column ‘A’ with their respective products listed in column ‘B’:
\[ \begin{array}{|c|c|} \hline \textbf{A: Gene} & \textbf{B: Products} \\ \hline \text{a. ‘i’ gene} & \text{(i) β-galactosidase} \\ \text{b. ‘z’ gene} & \text{(ii) lac permease} \\ \text{c. ‘a’ gene} & \text{(iii) repressor} \\ \text{d. ‘y’ gene} & \text{(iv) transacetylase} \\ \hline \end{array} \] Select the correct option:
The first antibiotic was discovered accidentally by A while working on B. ‘A’ and ‘B’ are:
If the sequence of nitrogen bases of the coding strand in a transcription unit is 5’ – ATGAATG – 3’, the sequence of bases in its RNA transcript would be:
The source of ‘Smack’ is:

Observe the schematic representation of assisted reproductive technology given below: 
 


Identify the most appropriate technique depicted in the above diagram. 
 

A phenomenon where a male insect mistakenly identified the patterns of a flower as the female insect partner, and tries to copulate and thereby pollinates the flower is said to be:
Which one of the following statements is correct in the context of observing DNA separation by agarose gel electrophoresis?

Study the pedigree chart of a family showing the inheritance pattern of a certain disorder. Select the option that correctly identifies the nature of the trait depicted in the pedigree chart: 
 

Identify the correct labellings in the figure of a fertilised embryo sac of an angiosperm given below: