List of top Biology Questions asked in CBSE CLASS XII

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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(a) Why does DNA replication occur within a replication fork and not in its entire length simultaneously?
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(a) Why must a cell be made ‘competent’ in biotechnology experiments? How does calcium ion help in doing so?
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(a) State the conditions for growth curve ‘A’ and growth curve ‘B’.
(b) Explain the arrows and the result.
Sequence:
5’ – G$\uparrow$A A T T C – 3’
3’ – C T T A A$\uparrow$G – 5’
(a) Name the restriction enzyme and type of sequence.
(a) How is the grazing food chain different from the detritus food chain?
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If the cells in the leaves of a maize plant contain 10 chromosomes each, write the number of chromosomes in its endosperm and zygote. Name and explain the process by which an endosperm and a zygote are formed in maize.
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(b) What does ‘K’ represent?
(b) “The detritus food chain may be connected to the grazing food chain at some levels in an ecosystem.” Give an example in support of the statement.
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Assertion (A): The sugar-phosphate backbone of two chains in DNA double helix shows anti-parallel polarity.
Reason (R): The phosphor-diester bonds in one strand go from a 3’ carbon of one nucleotide to a 5’ carbon of adjacent nucleotide, whereas those in complementary strand go vice versa.
Single step large mutation leading to speciation is also called:
How many base pairs will be there in 20 nucleosomes in a DNA double helix?
Write the symptoms of malaria in human and explain what causes these symptoms.
Describe the different steps in the sexual mode of reproduction in the life cycle of a malarial parasite from the time of its initiation till where it is completed and ready to start a fresh cycle.
Draw a schematic diagram of an antibody molecule and label any 4 parts. Mention their chemical nature. Name the cells which produce them.

 Growth curve ‘B’ shows a different pattern from that of growth curve ‘A’. Justify giving one reason. 
 

Work out a cross to explain how normal parents may have a sickle-cell anaemic child.
Describe the life cycle of HIV from the time of its entry into the human body till full-blown AIDS sets in.
Describe the steps followed in this technique.
Describe the development of placenta during pregnancy in a human female.

Explain the role pf placenta
 

State the objective of adopting artificial hybridisation programme in plants.
Explain the genetic basis that results in the expression of this disorder.
Why is sickle-cell anaemia, a human blood disorder, so named?