List of top Biology Questions asked in CBSE CLASS XII

(iv) Why could parasite not survive in a transgenic tobacco plant?
(iv) Why is insertional inactivation preferred over the use of selectable markers in cloning vectors?
(ii) State the role of ‘rop’ gene.
(ii) How does geitonogamy differ from xenogamy?
(i) Name the nematode (scientific name) that infects the roots of tobacco plant and reduces its yield.
(iii) A cloning vector does not have a selectable marker. How will it affect the process of cloning?
(iii) Name the type of flowers that are invariably autogamous.
(ii) Name the vector that is used to introduce nematode-specific genes into the host plant (tobacco).
(i) Describe the events of spermatogenesis with the help of a schematic diagram.
(i) Show the development of a megaspore mother cell up to the formation of a mature embryo sac in flowering plants with the help of labelled diagrams only.
(ii) In England after industrialisation, the population of dark-winged moths were more favoured than white-winged moths. Explain.
(ii) Explain the role of hormones in spermatogenesis.
(iii) Anthropogenic action can enhance the rate of evolution. Explain with the help of an example.
(i) Why did Hershey and Chase use $^{35$S and $^{32}$P in their experiment? Explain.}
(ii) State the importance of (1) blending and (2) centrifugation in their experiment.
(iii) Write the conclusion they arrived at the end of their experiment.
Why does menstruation only occur if the released ovum is not fertilised? Explain.
(c) Name the hormone and its source organ responsible for the events occurring during the proliferative phase of the menstrual cycle. Explain the event.
(b) For successful pregnancy, at what phase of the menstrual cycle an early embryo (up to 3 blastomeres) should be implanted in the Uterus (IUT) of a human female who has opted for Assisted Reproductive Technology (ART)? Support your answer with a reason.
In a human female, the reproductive phase starts on the onset of puberty and ceases around middle age. Study the graph given below regarding the menstrual cycle and answer the questions that follow:
\begin{figure}[H] % The [H] forces the figure to appear exactly here \centering \includegraphics[width=\textwidth]{Screenshot 2025-01-16 174432.png} % Replace with the actual path to your image \caption{Hormonal regulation and events during the menstrual cycle.} \label{fig:menstrual_cycle} \end{figure} (a) Name the hormones and their source organ, which are responsible for the menstrual cycle at puberty.
(a) Tropical regions harbour more species than temperate regions. How have biologists tried to explain this in their own ways? Explain.
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Expression of different genes for different traits may show dominance, incomplete dominance, or co-dominance. Write about expression of such genes with examples for one of the above modes.
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(b) “DNA replication is continuous and discontinuous on the two strands within the replication fork.” Explain with the help of a schematic representation.
(ii) The ecological pyramids may have an ‘upright’ or an ‘inverted’ shape. Justify with the help of suitable examples.
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(i) What does an ecological pyramid represent?
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