CBSE Class-12 Marking Scheme for IOP/Comptt Examination 2017 : Biology

 
Class 12 papers

CBSE Class-12 Marking Scheme for IOP/Comptt Examination 2017 :

Biology

CBSE Class-12 Marking Scheme for IOP/Comptt Examination 2017 : Biology (Delhi)

 
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July 2017
Marking Scheme - Biology (Theory)
Expected Answers/Value Points
 
SECTION – A
(Q. Nos. 1 - 5 are of one mark each)

1. Name the stage of Plasmodium that gains entry into the human when bitten by an infected female Anopheles.
Ans. Sporozoites  [1 mark]

2. List any two characters of Pea plants used by Mendel in his experiments other than height of the plant and the colour of the seed.
Ans. Flower colour / Flower position / Pod shape / Pod colour / Seed shape (Any two) = ½ + ½ [1 mark]

3. Mention the role of Restriction Enzymes in Recombinant DNA technology.
Ans. To cut DNA at specific sites / Molecular scissors (DNA) [1 mark]

4. Name the disorder caused due to the absence of one of the X-chromosomes in a human female.
Ans. Turner’s syndrome [1 mark]

5. Name the type of asexual reproduction where the parent cell ceases to exist.
Ans. Fission / Binary fission / Longitudinal fission / Multiple fission [1 mark]

SECTION-B

6. Name and state the effect of a drug that is often medically prescribed, but its overuse leads to drug dependence and drug abuse.
Ans. Morphine / barbiturates / amphetamines / benzodiazepines / lysergic acid diethyl amides (LSD) = 1
Affects the central nervous system / acts as a pain killer / acts as a sedative / treats depression / treats insomnia / creates hallucinogenic effect (Any two) = ½ + ½ [1 + 1 = 2 marks]

7. Differentiate between pericarp and perisperm.
Ans. Pericarp - wall of the fruit (which develops from the wall of ovary) = 1
Perisperm - persistent residual nucellus = 1 [1 + 1 = 2 marks]

8. “Niche is a part of a habitat.” Explain with the help of an example.
Ans. A single habitat may have different kind of organisms in it but within the habitat every organism has defined range of condition that it can tolerate , resources it utilises and plays a distinct functional role - all these together comprise its niche , for example pond is a habitat for variety of plants and animals , but in it Gambusia fish is found in its peripheral parts which is its niche. (Any other suitable example with explanation) = ½ × 4 [2 marks]

9. Name the type of immunity a baby is born with. How is it different from the one he gets from the mother’s milk after birth ?
Ans. Innate Immunity , Acquired Immunity that a baby acquires from his mother’s milk after birth is an example of passive Immunity = 1 + 1 [2 marks]

10. State the roles of AUG codon at 5' end and UAG at 3' end of a certain m-RNA during translation.
Ans. AUG codon at 5’ end = start codon (for translation) / codes for methionine = 1
UAG codon at 3’ end = stop codon (for translation) / terminate polypeptide chain= 1 [1 + 1 = 2 marks]
OR
‘Degenerate’ and ‘Universal’ are salient features of a genetic code. Explain.
Ans. Degenerate - Some aminoacids are coded by more than one codon = 1 Universal - one codon shall code for the same amino acid in all organisms (UUU would code for phenylalanine from bacteria to human beings) = 1
[1 + 1= 2 marks]

SECTION-C

11. Write the three basic facts that are highlighted in Mendel’s Law of Dominance.
Ans. (i) Characters are controlled by discrete units called factors = 1
(ii) Factors occur in pairs = 1
(iii) In a dissimilar pair of factors one member of a pair dominates (dominant) the other (recessive)= 1 [1 + 1+ 1= 3 marks]

12. (a) Name the causative agent of amoebiasis and mention its symptoms.
(b) Write how does it spread.
Ans. (a) Entamoeba histolytica = 1
Symptoms - Constipation / abdominal pain / cramps / stool with excess mucus and blood clot (Any two) = ½ + ½
(b) Spreads through housefly / drinking water contaminated with faecal matter / eating food contaminated with faecal matter (Any one) = 1 [1 + 1 + 1 = 3 marks]
 
13. (a) What is green revolution ? Mention the steps that led to it.
(b) Name the scientist whose contribution led to development of semi-dwarf wheat varieties in India.
Ans. (a) Dramatic increase in food production (wheat and rice) during the mid 1960’s is termed as Green Revolution = 1
Various plant breeding techniques / better management practices / use of agrochemicals (fertilizers and pesticides) (Any two) = ½ + ½
(b) Norman E.Borlaug = 1 [1 + 1 + 1 = 3 marks]

14. Are humming birds and fish regulators or conformers ? Give reasons in support of your answer.
Ans. Conformers = ½
Heat loss or gain is a function of surface area = ½
Since small animals have a larger surface area (relative to their volume) , they tend to lose body heat very fast when it is cold outside , they have to expend much energy , to generate body heat through metabolism = ½ × 4 = 2 / (cannot maintain a constant body temperature) [1 + 2 = 3 marks]

15. GM plants are useful in many ways. How would you convince farmers to grow GM plants on their field ? Explain giving three reasons.
Ans. Make crop more tolerant to abiotic stresses / Reduce reliance on chemical pesticides / Help to reduce post harvest loses / Increase efficiency of mineral usage / Enhance nutritional value of food (Any three) [1× 3 = 3 marks]
 
16. Name the male accessory glands in humans and write their functions.
Ans. (Paired) seminal vescicles , prostate , bulbourethral glands (paired) = ½ × 3
Functions - Secretions constitute the Seminal plasma , which is rich in fructose / calcium and certain enzymes , lubrication of penis = ½ × 3 [1½ + 1½ = 3 marks]

17. Write in what context did Darwin use the terms ‘fitness’, ‘survival’ and ‘selection’ while elaborating on the mechanism of evolution.
Ans. Fitness refers to reproductive fitness (will leave more progeny) - more survival and hence selected by nature - natural selection = 1 × 3 [3 marks]