CBSE Set Qa1 Biology Sample Test Papers For Class 12th for students online

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Biology Class-Xll  (CBSE)
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Q1. What induces parthenocarpy in grapes?
Ans1.
Auxins.

Q2. What is the term used for the regions of a gene which become part of mRNA and code for the different regions of the proteins?
Ans2.
Exons.

Q3. The venous blood in the lung has a PCO2
of 46 mm Hg. Should the alveolar PCO2 exceed or be less than 46 mm Hg to result in diffusion of CO2 from the blood into the alveolus?
Ans3.
It should be less than 46 mm Hg.

Q4. Which part of the plant leaf is modified to form spines of Acacia and the sheath covering the leaf of Ficus elastica.
Ans4.
Stipules.

Q5. Name the enzyme that can break and reseal one strand of DNA.
Ans5.
Topoisomerase.

Q6. What does J-shaped growth curve of a population indicate?
Ans6.
The J-shaped curve indicates that the factors of environmental
resistance do not produce zero growth to contain the exponential growth.

Q7. What forms the cambial ring in a dicot stem during the secondary growth?
Ans7.
The cambial rings are formed by alternately forming secondary xylem.

Q8. Name the principal mineral anion in the extracellular fluid.
Ans8.
Ca2+.

Q9. A mother with O type blood group has conceived. The expected blood group of the foetus is B. Will there be any problems to the mother or the foetus? If so, specify the problems?
Ans9.
The connections between the mother and the foetus through placenta may cause some of the foetal B blood group RBCs to enter mother’s blood. This will cause production of antibodies against blood group B in the mother’s O-blood. These antibodies will then enter back into the foetal blood and may cause haemolysis there.

Q10.What are the two types of fibres of the connective tissue? Differentiate between the two.
Ans10.
Connective tissue is made of two type of fibres :
(1) White fibrous, and
(2) Yellow elastic.
White fibrous fibres are longer, highly organized and made up of collagen;
they are strong, flexible with great strength.
Yellow elastic fibres are irregularly arranged and made up of elastin;they are elastic.

Q11.A farmer observed drops of water along the margins of leaves of tomato plants growing in his well-irrigated field on a winter morning. Give reasons for this occurrence. Why had the water drops appeared only along the leaf margins?
Ans11.
Since it was cold season and the field was well irrigated, the water droplets were seen as a result of the process of guttation, which occurs in the herbaceous plants in nights. The water drops were seen only on the margins of leaf because the ‘Hydathodes’ from which guttational water loss occurs are found near the vein endings only.

Q12. Give the scientific name of the modern wheat. Why is it quoted as a typical example of rapid speciation?
Ans12.
Modern wheat is Triticum aestivum. It is quoted as a typical example
of rapid speciation because it has originated by natural interspecific crossing and diploidisation of the hybrids. The final hexaploid Triticum aestivum is very different from any of its parents: T. monococcum, T. durum, Aegilops speltoides, or A. squarrosa.

Q13. Name any two of cyanobacteria and explain how they serve as main sources of bio-fertilizers.
Ans13.
Nostoc and Anabaena are two of the cyanobacteria. Cyanobacteria are main sources of biofertilisers because they have the combination of photosynthetic and nitrogen fixing ability. They are present as free-living microorganisms in the soil and produce easily available nitrates and nitrites.

Q14. What are the two striking observations when different stages of embryos of different classes of vertebrates are studied comparatively? How does this study support evolution?
Ans14.
Two striking observations are that gill clefts and notochord appear
in the embryonic development of all vertebrates from fish to mammals. The
notochord has been replaced by the vertebral column in all the adult
vertebrates. Similarly, gills are replaced by lungs in adult amphibians,
reptiles and mammals. Such similarities in embryonic development reinforce
the idea of evolution from common ancestors.

Q15. An animal should possess at least one out of the three possible characteristics to enable it to function as a ureotelic organism. List these characteristics.
Ans15.
Ureotelism is the urinary elimination of nitrogen mainly as urea. An
animal can be ureotelic only if the animal can afford to :

    (i) excrete sufficient volumes of water,
    (ii) concentrate urea considerably in the urine,
    (iii) retain considerable amounts of urea in the blood and body fluids.
    Whale, camel, toad, and shark are all ureotelic.

Q16. Pick out the plant with whorled arrangement of leaves from the list given below and say why it is categorised so? Neem; Nerium; Nepenthes.
Ans16.
Nerium has whorled leaves phyllotaxy. It is said to be of ‘whorled leaves’ type because 3-5 leaves occur at a node.

Q17. How is elongation carried on during protein synthesis in a cell?
Ans17.
After the initiation of the protein synthesis has taken place, the
second step is the 'elongation'. A second tRNA with an apprpriate anticodon
and charged with an appropriate amino acid now forms hydrogen bonds with the second codon on the mRNA. This second aminoacyl-tRNA enters the ribosome at the A-site. A peptide bond forms between the initiation amino acid and the second amino acid. The peptide bond formation is catalysed by Peptidyl transferase. The first amino acid has got freed from the P site and is now attatched to the second tRNA as a part of the dipeptide attatched to it.
The tRNA at the A-site is pulled to the P-site along with the mRNA.
Thus the third codon comes in the A-site and the process is repeated with a
third amino acyl tRNA coming at the A-site.


Q18. What category of pathogens causes poliomyelitis? How is crippling caused in the victim of this disease? How can this disease be prevented?
Ans18.
Polio is caused by poliovirus. In CNS the virus destroys the dorsal
horn cells of the spinal cord, which control the activities of the muscles.
Without nerve impulses the muscles fail to work and shrink. Thus the limbs
become paralysed. This crippling disease can be prevented by avoiding the
ingestion of the virus via food or drink, and by vaccination of the kids in the target age group.

Q19. To which category of cells do B-cells and T-cells belong? How do they differ from each other with reference to their formation and response to antigens?
Ans19.
B-cells and T-cells are ‘Lymphocytes’.

Both develop from stem cells of liver in foetus or bone-marrow cells in adults. But B-cells mature in bone marrow whereas the T-cells mature under the influence of thymus. T-cells are responsible for cellular immunity and B-cells produce antibodies.

Once an antigen-specific B-cell is triggered by its specific antigen, it gives rise to a clone of plasma cells by quick multiplication, which produce antibody for that antigen.

T-cells are also specific for antigens and when their destined antigen is
encountered a clone of T-cells is produced. However, T-cells are of three
different types :
(i) Killer T-cells directly attack and destroy antigens, by moving to the site of invasion and producing chemicals that attract phagocytes and stimulate them to feed more aggressively on antigens.
(ii) Helper T-cells act to stimulate antibody production by B-cells.
(iii) Supressor T-cells supress the total immune reaction keeping it from
attacking the body’s own cells.

Some of suppressor cells become memory cells.

Q20. How did Urey and Miller provide the conditions of primitive earth to prove the origin of life in their experiment?
Ans20.
The primitive earth should have had :
(i) Hydrogen, nitrogen, water vapour, carbon dioxide, methane, and ammonia
in its environment.
(ii) Sources of energy such as thermal energy, radiation, and lightning. Stanley and Miller recreated the environment by keeping a mixture of H2,
CH4, NH3, and water vapour in the flask. They provided the energy by heating the liquid-containing chamber as well by electric sparks from electrodes in the gaseous chamber. The experiment was run continuously for one week and biomolecules were found to be produced abiotically in the liquid extract from the gaseous chamber.

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