List of top Biology Questions asked in CBSE CLASS XII

(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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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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(a) Why must a cell be made ‘competent’ in biotechnology experiments? How does calcium ion help in doing so?
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(b) Explain the arrows and the result.
(a) State the conditions for growth curve ‘A’ and growth curve ‘B’.
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.
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.
How many base pairs will be there in 20 nucleosomes in a DNA double helix?
Single step large mutation leading to speciation is also called:
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?
Explain the charging of an adaptor molecule. Why does this molecule need to be charged?
Write the initiating codon for translation, its anticodon, and the amino acid it codes for.
Identify the polarity from ‘X’ to ‘X\(^\prime\)’ in the mRNA segment shown. Mention how many more amino acids can be added to the polypeptide that is being translated and why.