Sample Model Papers For Biology CBSE Board for students online

Latest Yuva Education Is Providing Biology Sample Model Paper For Class XII for students online. All these are just samples for prepration for exams only. These are not actual papers.
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Model Paper 1


Max Marks: 70
Time allowed : 3 hrs

Q1. At what stage is the embryo implanted in the uterus of the mother in case of a test tube baby?(1 mark)

32-celled Morula.

Q.2 Write the full form of TRNA.(1 mark)

Ans2. Transfer Ribosenucleic Acid

Q3. Which of the following diseases could be avoided by analysing the pedigree of parents -
Klinefelter's syndrome, Haemophilia, Amoebiasis.(1 mark)

Ans3. Haemophilia.

Q4. 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?(1 mark)

It should be less than 46 mm Hg.

Q5. What forms the cambial ring in a dicot stem during the secondary growth?(1 mark)

Ans5. The cambial rings are formed by alternately forming secondary xylem.

Q 6. When is a tumour referred to as malignant? (1 mark)

Ans6. A tumour is called malignant, if it has the capacity to undergo metastasis.

Q 7. What are the two functions of DNA polymerase? (1 mark)

Ans7. DNA Polymerase is responsible for
(i) binding the deoxynucleotide bases after they have arranged on the DNA
(ii) proofreading the formed sequence for errors.

Q.8 Name any two structures in our body, which bear ciliated epithilium.(1 mark)

Ans8. (i) Kidney tubules, and
(ii) Fallopian tubes

Section B

Q9. Why is the cleavage in mammals referred to as simple holoblastic?(2 marks)

Ans9.Cleavage in mammals is referred to as simple holoblastic because in the absence of any yolk in the mammalian egg, it divides completely. This means the cleavage furrow divides the egg or blastoderms completely.

Q.10. Amongst pea tendrils, opuntia spines, lemon thorns and cucurbit tendril, which ones are homologous structures? Give reasons.(2 marks)

Pea tendrils are homologous to Opuntia spines because both are modifications of leaves; Lemon thorns are homologous to Cucurbit tendril because both are modified stems.

Q.11 How is growth measured in animals?(2 marks)

Ans 11. In animals two main patterns of the growth occur:
(A) Isometric growth:
- when an organ grows at the same mean rate as the rest of the body.

- Isometric growth is seen in fish an certain insects.

(B) Allometric growth:
- an organ grows at the different rate from the rest of the body.

This produces the change in size of the organism, which is accompanied by a change in the shape of the organism, eg in human beings.

Q.12 Give any four examples of human reflexes (2 marks)

Ans12. (1) Salivation on the sight or smell of a tasty food,
(2) constriction of pupils on the illumination with bright light,
(3) Blinking of eye lids on the approach of an object close to the eyes,
(4) Jerk in the legs when a pin pricks sole.

Q 13. What was Emerson's experiment? What is Emerson's enhancement effect?(2 marks)

Ans. 13 Emerson's experiment: When the quantum yield (number of oxygen evolved per quanta of light absorbed) is plotted with respect to the wave length of the light used for photosynthesis, the plot indicated that beyond 680nm there is a drop in the rate of photosynthesis. This drop is known as 'red drop' or 'Emerson effect' i.e. far red light (690-700nm) is inefficient for photosynthesis. If, along with 690-700nm light a light of shorter wavelength i.e. 660nm is provided, the rate of photosynthesis shows a rise again (Emerson enhancement effect).

Q 14. Differentiate between morula and blastula of mammals. (2 marks)

Ans14. Morula is a solid ball of cells and looks like a mulberry. Often zona pellucida remains intact around the morula.
Blastula is like a hollow tennis ball where a cavity called 'Blastocoel' forms due to rearrangement of blastomeres. The zona pellucida envelope disintegrates as the blastula develops.

Q 15. What is the biological significance of Azolla pinnata in agriculture? (2 marks)

Ans15.Azolla pinnata is a small, fast-growing fern that occurs floating on water. Anabaena azollae, a cyanobacterium lives in cavities of Azolla leaves. It fixes nitrogen from air and excretes the nitrogenous compounds into the leaf cavity of the fern. Thus Azolla pinnata serves as an excellent biofertilizer when grown in association with rice.

Q 16. 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.(2 marks)

Ans16.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.

Q17. How a sickle-cell carrier has an advantage over the rest of the human population in a Malaria ridden area? (2 marks)

A defective form of haemoglobin is found in the RBCs of the Sickle-cell carrier. They have sickle-shaped RBCs, which are highly inefficient in binding oxygen. However, a sickle shaped RBC effectively kills the malaria parasite too, which harbors in it. Thus individuals homozygous for Sickle-cell trait have a better survivability in the event of malaria epidemics and thus natural selection has not eliminated this trait in those geographical locations in the world, where there is a prevalence of malaria.

Q 18. Explain the structural and functional significance of fovea in the human eye.(2 marks)

Ans18.Fovea is a depressed area of the retina, which contains only cones and no rods. Ability for vision is highest in the fovea. When eyes are fixed on an object, its image is focussed on the fovea and is consequently seen most accurately.

Section C

Q.19 Distinguish between bacterial transduction and bacterial transformation. (3 marks)

Ans19. Transduction: Transfer of the DNA from one bacterium to the other, mediated through a bacteriophage. The carried DNA bestows some of the properties of the donor to the acceptor bacterium.
Transformation: Transfer of the DNA from one bacterium to the other through the solution phase, after which the acceptor bacterium gets the properties of the donor bacterium.

Q20. Define Cancer. How are cancers broadly classified pathologically? Name one major cancer in women and one major cancer in men in India.(3 marks)

Pathologically, cancers may be broadly classified into three major catagories:
(i) Carcinomas, which are malignant growths of the epithelial (ectodermal) tissues.
(ii) Sarcomas, which are malignant growths of the mesodermal tissues such as bone, muscle,
lymph nodes etc.
(iii) Leukaemias, which are unchecked proliferation of blood cells, and there precursors in the bone marrow.
In India the major form of cancer in women is the uterine-cervical cancer and it is the cancer of the mouth and throat in men.

Q21. Human skin colour is believed to be controlled by at least three separate genes:
What do you call this type of inheritance?(3 marks)
i Suppose the genes are designated as A, B, and C and darkness is dominant over fairness, what shall be the genotypes of the darkest, fairest and intermediate skin colour?
ii What will be the total number of allele combinations in the gametes of a person
heterozygous,all the three genes.

This inheritance is called Quantitative Inheritance and individual genes show Incomplete Dominance.
(ii) The genotype of the darkest will be AABBCC, that of the intermediate
will be AaBbCc; and that of the fairest will be aabbcc.
(iii) The number of possible allele combinations in the gametes is eight for
such a three-gene combination (23)

Q22. If abiotic origin of life were in progress on a planet other than the earth, what should be the conditions there? (3 marks)

Ans 22.
(i)If abiotic origin of life is in progress on a planet other than the earth, the conditions there should be same as the one prevalent on the primitive earth. Thus the following conditions would be required:
Solids, liquids, and gases coexist.
(ii)The atmosphere should be reducing.
(iii)Hydrogen, nitrogen, water vapour, carbon dioxide, methane, and ammonia should be present. Sources of energy such as thermal energy, radiation, and lightning should be present.

Q 23. An mRNA strand has a series of codons out of which three are mentioned below.
(i) AUG, (ii) UUU and (iii) UAG
(a) What will these codons be translated into?
(b) What are the DNA codons that would have transcribed these RNA codons? (3 marks)


(a) (i) AUG and (ii) UUU will be translated. (iii) UAG is a stop codon which will not be translated.

RNA codon       DNA codon


Q24. Describe the role of red blood cells in the transport of oxygen and carbon dioxide by blood.(3 marks)

Blood transports oxygen with the help of haemoglobin in RBCs and carbon dioxide with the help of cytoplasm of red blood cells. Oxygen diffuses into the RBCs and combines loosely with the Fe2+ ions of hemoglobin to form oxyhaemoglobin. A lower PO2 and higher PCO2 in the tissues causes dissociation of oxyhaemoglobin to deoxyhaemoglobin (reduced haemoglobin) and molecular oxygen.
Carbon dioxide diffuses into RBCs and reacts with water in the cytoplasm, to form Carbonic acid:
CO2 + H2O = H2CO3, The reaction is catalysed by zinc containing Carbonic Anhydrase.
In RBCs carbonic acid forms bicarbonate:
H2CO3 H++HCO3-
Some of the bicarbonate remains in the RBCs but most of it comes out in plasma. Some of the CO2 reacts with globin of deoxyhaemoglobin to form Carbaminohaemoglobin in RBCs.

Q 25. Answer the following with reference to the anatomy of dicot stem:

(i) Where exactly are the cambial cells located in the vascular bundles?
(ii) What is the name given to such a bundle?
(iii) How are the xylem vessels arranged?
(iv) What type of cells constitute the pith? (3 marks)

Ans25. (i) The cambial cells are located in the vascular bundles of the young dicotyledonous stem, between xylem and phloem.
(ii) It is called Fascicular Cambium.
(iii) Xylem vessels are arranged towards the center, protoxylem innermost and metaxylem next.
(iv) Parenchymatous cells constitute the pith.

Q26. Give one reason why lysozyme is considered an enzyme and not a hormone? How does it defend the body? Name any two secretions in humans, which contain lysozyme.(3 marks)

Lysozyme is not a hormone because it does not satisfy the following criteria
required for a protein to become a hormone:
(i) It is not secreted in response to any change in the internal or external environment of the body,
(ii) It is not transported by blood,
(iii)It has no specific target organ of its own,
(iv) It does not stimulate or inhibit specific biological processes in the targeted organs to modify their activity.
Lysozyme destroys the cell wall of many bacteria and thus prevents many infections.

Q27. Sometimes natural selection is not operative if the abnormal allele is somewhat advantageous to the individual. Explain this mechanism with a suitable example. (3 marks)

Ans27.Sometimes natural selection is not operative if the abnormal allele
is advantageous to the individual because the abnormality offers better
survivability due to its advantages over the normality. Sickle cell anaemia is a good example of this phenomenon, where a defective form of haemoglobin is found in the RBCs. Individual homozygous for this trait die at an early age. Even those homozygous for this trait have sickle-shaped RBCs, which are highly inefficient in binding oxygen. However, natural selection has not eliminated this trait in those geographical locations in the world, where there is a prevalence of malaria. This has happened because a sickle shaped RBC effectively kills the malaria parasite, which harbours in it. Thus individuals homozygous for Sickle-cell trait have a better survivability in the event of malaria epidemics and are favoured during the natural selection despite of their abnormal trait.

Section D

Question Nos. 28 - 30 are long answer questions carrying 5 marks each.
It is appropriate to answer them in about 80 - 120 word each.

Q 28. In the case of snapdragon (Antirrhinum majus) a plant with red flowers was crossed with another plant with white flowers. Trace the inheritance of flower colour up to the F2 generation indicating the genotypesand phenotypes at each level. What special feature do you note in the genotypic ratios in F2 generation? (5 marks)

When homozygous red (RR) Antirrhinum majus is crossed with homozygous white (rr), the F1 heterozygous (Rr) bears pink flowers. These heterozygous pink- flowered A. majus give homozygous red (RR), heterozygous pink (Rr) and homozygous white (rr) in a ratio of 1:2:1in their F2 generation when underwent self pollination. This is an example of 'Incomplete Dominance'. The phenotypic and genotypic ratios are the same In case of Incomplete Dominance except for the fact that unlike in Complete Dominance, the homozygote and heterozygote for the dominant allele differ in their phenotypes.

Q 29. Name the hormone that regulates each of the following and mention the source of it:
(i) Urinary elimination of water
(ii) Storage of glucose as glycogen
(iii) Sodium and potassium ion metabolism
(iv) Basal metabolic rate
(v) Descent of testes into the scrotum (5 marks)


Hormone Source
(i) Vasopressin (ADH) Posterior pituitary
(ii) Insulin Islets of Langerhans in the Pancreas
Adrenal cortex
(iv) Thyroid hormones Thyroid gland
(v) Follicle Stimulating Hormone (FSH) Anterior Pituitary

Q30. What is a synapse? How is the nerve impulse transmitted across a synapse? (5 marks)

Synapse is the junction between two neurons, across which the impulse has to pass from one neuron to the next. Synapses are formed between the axon terminals of one neuron and the dendrites, the cell body or even sometimes the axon of one or more neurons. There is no actual continuity between neurons at the synapse. The axon of a neuron divides near its termination into many branches, each branch losing its myelin sheath before termination. These axon terminals of a neuron then end in expanded feet-like forms on the dendrites or the cell body of another neuron; there is a narrow fluid-filled space, called Synaptic cleft, separating the membranes of the two neurons at the synapse. The axon terminal contains many membrane-bound vesicles, called Synaptic Vesicle, in its cytoplasm. Within these vesicles, chemical substances, such as adrenaline and acetylcholine remain stored. When a nerve impulse passes the axon terminal, its synaptic vesicles release their stored chemicals to the synaptic cleft. These diffuse through the cleft to reach the membrane of the next neuron, and stimulating the latter. This causes the nerve impulse to be transmitted along the next neuron.


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