CBSE Class-12 Exam 2017 : Marking Scheme, Biology


 
Class 12 papers

CBSE Class-12 Exam 2017 : Marking Scheme

Question Paper, Biology

CBSE Class-12 Exam 2017 :  Biology (Delhi)

Senior School Certificate Examination
April 2017
Marking Scheme - Biology (Theory)
Expected Answers/Value Points
 
General Instructions :
 
The Marking Scheme and mechanics of marking
1 In the marking scheme the marking points are separated by commas, one oblique line (/) indicates acceptable alternative, two obliques (//) indicate complete acceptable alternative set of marking points.
2. Any words/phrases given within brackets do not have marks.
3. Allow spelling mistakes unless the misspelt word has another biological meaning. Ignore plurals unless otherwise stated in the marking scheme.
4. In any question exclusively on diagram no marks on any description. But in questions on descriptions, same value points may be marked on the diagrams as a subsitute.
5. All awarded marks are to be written in the left hand margin at the end of the question or its part.
6. Place a tick () in red directly on the key/operative term or idea provided it is in correct context. Place “Half-tick” ½ wherever there is ½ mark in the marking scheme. (Do not place tick indiscriminately just to show that you have read the answer).
7. If no marks are awarded to any part or question put a cross (×) at incorrect value portion and mark it zero (in words only).
8. Add up ticks or the half ticks for a part of the question, do the calculation if any, and write the part total or the question total in the left hand margin.
9. Add part totals of the question and write the question total at the end. Count all the ticks for the entire question as a recheck and draw a circle around the question total to confirm correct addition.
10. If parts have been attempted at different places do the totalling at the end of the part attempted last.
11. If any extra part is attempted or any question is reattempted, score out the last one and write “extra”.
12. In questions where only a certain number of items are asked evaluate only that many numbers in sequence as is asked ignoring all the extra ones even if otherwise correct.
13. Transcribe the marks on the cover page. Add up question totals. Recheck the script total by adding up circled marks in the script.
14. Points/answer given in brackets in marking scheme are not so important and may be ignored for marking.
 

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CBSE Class-12 Exam 2017 :  Biology (Foreign)

General Instructions :
 
The Marking Scheme and mechanics of marking
1 In the marking scheme the marking points are separated by commas, one oblique line (/) indicates acceptable alternative, two obliques (//) indicate complete acceptable alternative set of marking points.
2. Any words/phrases given within brackets do not have marks.
3. Allow spelling mistakes unless the misspelt word has another biological meaning. Ignore plurals unless otherwise stated in the marking scheme.
4. In any question exclusively on diagram no marks on any description. But in questions on descriptions, same value points may be marked on the diagrams as a subsitute.
5. All awarded marks are to be written in the left hand margin at the end of the question or its part.
6. Place a tick () in red directly on the key/operative term or idea provided it is in correct context. Place “Half-tick” ½ wherever there is ½ mark in the marking scheme. (Do not place tick indiscriminately just to show that you have read the answer).
7. If no marks are awarded to any part or question put a cross (×) at incorrect value portion and mark it zero (in words only).
8. Add up ticks or the half ticks for a part of the question, do the calculation if any, and write the part total or the question total in the left hand margin.
9. Add part totals of the question and write the question total at the end. Count all the ticks for the entire question as a recheck and draw a circle around the question total to confirm correct addition.
10. If parts have been attempted at different places do the totalling at the end of the part attempted last.
11. If any extra part is attempted or any question is reattempted, score out the last one and write “extra”.
12. In questions where only a certain number of items are asked evaluate only that many numbers in sequence as is asked ignoring all the extra ones even if otherwise correct.
13. Transcribe the marks on the cover page. Add up question totals. Recheck the script total by adding up circled marks in the script.
14. Points/answer given in brackets in marking scheme are not so important and may be ignored for marking.
 
SECTION – A
Q. Nos. 1 - 5 are of one marks each
1. Name the vegetative propagules in (i) Potato, and (ii) Pistia.
Ans. i) Eye / Eye buds = ½ ii) Offset = ½ [1 Mark]
2. Mention the combination(s) of sex chromosomes in a male and a female bird.
Ans. Male-ZZ = ½ , Female-ZW = ½ [1 Mark]
3. A region of a coding DNA strand has the following nucleotide sequence : -ATGCWhat
shall be the nucleotide sequence in (i) sister DNA segment it replicates, and (ii) m-RNA polynucleotide it transcribes ?
Ans. i) -TACG- = ½ , ii) -AUGC - = ½ [1 Mark]
4. List the type of cry genes that provide resistance to corn plants and cotton plants respectively against lepidopterans.
Ans. cryIAc/ cryIIAb- cotton = ½ cryIAb -corn = ½ [1 Mark]
5. Very small animals are rarely found in polar regions. Give two reasons.
Ans. Small animals have larger surface area relative to their volume// loose heat very fast, due to small size, expend much energy to generate body heat through metabolism. [ 1 Mark]

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CBSE Class-12 Exam 2017 :  Biology  (Outside)

SECTION – A
Q. Nos. 1 - 5 are of one marks each
1. Name the type of cross that would help to find the genotype of a pea plant bearing violet flowers.
Ans Test cross =1 [1 Mark]
2. State two postulates of Oparin and Haldane with reference to origin of life.
Ans (i) First form of life could have come from pre-existing non-living organic molecules / RNA & Protein = ½
(ii) Formation of life was preceded by chemical evolution / formation of diverse organic molecules from inorganic constituents = ½ [1 Mark]
3. A herd of cattle is showing reduced fertility and productivity. Provide one reason and one suggestion to overcome this problem.
Ans Reason: Inbreeding depression / continuous inbreeding = ½ Suggestion: Should be mated with unrelated superior cattle of the same breed / out - breeding / out - crossing = ½ [1 Mark]
4. What are Cry genes ? In which organism are they present ?
Ans. The genes which code for Bt toxin / Cry proteins / toxic proteins , Bacillus thuringiensis = ½ + ½ [1 Mark]
5. An electrostatic precipitator in a thermal power plant is not able to generate high voltage of several thousands. Write the ecological implication because of it.
Ans Air Pollution // particulate matter / dust particles released in the air. [1 Mark]
 
SECTION - B
Q Nos. 6-10 are of two marks each
 
6. A pollen grain in angiosperm at the time of dehiscence from an anther could be 2-celled or
3-celled. Explain. How are the cells placed within the pollen grain when shed at a 2-celled stage ?
Ans • In 2-celled stage the mature pollen grain contains a generative and vegetative cell, whereas in 3-
celled stage one vegetative cell and two male gametes are present = ½ + ½
• The generative cell floats in the cytoplasm of vegetative cell =1
[2 Marks]
7. Differentiate between the genetic codes given below :
(a) Unambiguous and Universal
(b) Degenerate and Initiator
 

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Courtesy: CBSE