Marking Schemes

Who are the markers of answer scripts?

 How do they mark the answer scripts? 

How can it be ensured that marking is fair, objective and reliable? 

Questions of these types naturally arise in the  minds of candidates appearing in the public examination. These questions are equally pertinent to the teachers who are not adequately exposed to the CBSE system of marking. 

Answer Scripts marking is a specialised job. It is assigned to teachers-PGTs for Class XII and

TGTs for Class X who are in direct touch with the subject and have a minimum of 3 years experience

of teaching the subject at that level. Appointment of examiners is made in accordance with the welldefined norms. Markers examine scripts with the help of detailed guidelines called the 'Marking Schemes'.

 To Students :

--- Study each question carefully, comprehend them and write down the main points of the

answer and note down their difficulties for clarification.

--- Examine a question in conjunction with the Marking Scheme and find out the proximity of

the answer to that suggested in the Marking Scheme.

--- We will feel motivated if this publication is commented upon by practitioners in the context

of its impact on their teaching learning strategies. Contribution of the experts and the officials

of the Board in bringing out this document is gratefully acknowledged

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 if 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.

QUESTION PAPER CODE 57/1/1

EXPECTED ANSWERS/VALUE POINTS

     SECTION A

Q.Nos. 1 - 8 are of one mark each.

1. A bilobed, dithecous anther has 100 microspore mother cells per microsporangium. How

many male gametophytes this anther can produce ?

Ans. 1600

[1 mark]

2. Mention two functions of the codon AUG .

Ans. Initiator Codon, codes for Methionine = ½ + ½

[1 mark]

3. Name the scientist who disproved spontaneous generation theory.

Ans. Louis Pasteur (If only Pasteur = ½)

[1 mark]

4. What is it that prevents a child to suffer from a disease he/she is vaccinated against ? Give

one reason.

Ans. Antibodies produced , kills pathogen / destroys antigen

// provides immunity against pathogen = ½ + ½

[1 mark]

5. Why is the enzyme cellulase used for isolating genetic material from plant cells but not for

animal cells ?

Ans. Animal cells do not have cellulose cell walls; (hence do not need the enzyme) , plant cell have

cellulose in their cell wall; (hence need enzyme to break it down) = ½ + ½

[1 mark]

6. Name a molecular diagnostic technique to detect the presence of a pathogen in its early

stage of infection.

Ans. Recombinant DNA technology / Polymerase Chain Reaction (PCR) / ELISA  Any one = 1

[1 mark]

7. If 8 individuals in a laboratory population of 80 fruitflies died in a week, then what would be

the death rate for population for the said period ?

Ans. 0.1 , individuals per fruitfly per week = ½ + ½

[1 mark]

8. Mention one positive and one negative application of amniocentesis .

Ans. To detect genetic disorder, may lead to (illegal) female foeticide  = ½ + ½

[1 mark]

SECTION B

Q. Nos. 9 - 18 are of two marks each

9. A moss plant produces a large number of antherozoids but relatively only a few egg cells.

Why ?

Ans. Eggs are stationary / non-motile within the plant body /  antherozoids carried by water ,  lost in

transit  = ½ × 4

[ 2 marks]

10. Mention the reasons for difference in ploidy of zygote and primary endosperm nucleus in

an angiosperm.

Ans. Zygote formed by the fusion of male gamete (n) and egg cell (n) , hence diploid (2n) = ½ + ½

Primary endosperm nucleus formed by the fusion of two polar nuclei / secondary nucleus (n + n) and

male gamete (n) , hence triploid  (3n) = ½ +  ½

[1 + 1 = 2 marks]

11. How does an electrostatic precipitator work to remove particulate pollutants released from

the thermal power plants ?

Ans. Electrode wires at several thousand volts produce a corona that releases electrons , electrons attach

to dust particles , (dust particles) become negatively charged , are attracted to collecting plates , low

velocity of air help settling of dust  =  ½ × 4

[2 marks]

12. Name the type of food chains responsible for the flow of larger fraction of energy in

an aquatic and a terrestrial ecosystem respectively. Mention one difference between the

two food chains.

Ans. Aquatic ecosystem - Grazing Food Chain / GFC = ½

Terrestrial ecosystem - Detritus Food Chain / DFC = ½

Difference : GFC begins with phytoplanktons / producers whereas DFC begins with dead organisms

/ detritus = 1

[1 + 1 = 2 marks]

13. How does a test-cross help in identifying the genotype of the organism ? Explain.

Ans. Cross between an organism with unknown genotype (expressing dominant trait) , and an organism

with recessive trait (homozygous) = ½ +  ½

If all offsprings show dominant trait the organism is homozygous (dominant) , if half organisms show

dominant and half recessive then the organism is heterozygous (dominant) = ½  + ½

[1 + 1 = 2 marks]2 1 9

14. Name the host and the site where the following occur in the life-cycle of a malarial parasite:

(a) Formation of gametocytes

(b) Fusion of gametocytes

Ans. (a) Human , RBC = ½ + ½

(b) Female Anopheles (mosquito) , intestine = ½ + ½

[1 + 1 = 2 marks]

15. Honey collection improves when beehives are kept in crop-fields during flowering season.

Explain.

Ans. Bees visit flowers for collecting nectar / pollen (for honey) , more the number of flowers (during

flowering season) larger is the nectar / pollen collection (hence more honey production) = 1 + 1

[2 marks]

OR

How does addition of a small amount of curd to fresh milk help in formation of curd? Mention

a nutritional quality that gets added to the curd.

Inoculum or starter contains (millions) of LAB which multiply at a suitable temperature , produce

acids / lactic acid that coagulates , and partially digests the milk proteins thus converting milk to curd,

it also improves its nutritional quality (of milk) by increasing vitamin B12 

 = ½ × 4

[2 marks]

16. Why  is  the  introduction  of  genetically  engineered  lymphocytes  into  an  ADA deficiency

patient not a permanent cure ? Suggest a possible permanent cure.

Ans. Lymphocytes not immortal / short lived , hence patient requires periodic infusion of such genetically

engineered lymphocytes , however if a gene producing ADA is isolated from marrow cells , and

introduced into the cells at early embryonic stages it could be a permanent cure = ½ × 4

[2 marks]

17. How does the floral pattern of Mediterranean orchid Ophrys  guarantee cross pollination?

Ans. One petal of Ophrys resembles the female of a bee, male bees are attracted to it / perceives it as

female , pseudocopulates , same male bee pseudocopulates with another flower of Ophrys and

pollination is completed = ½ × 4

[2 marks]

18. In the biosphere immense biological diversity exists at all levels of biological organisation.

Explain any two levels of biodiversity.

Ans. (i) Genetic diversity: A single species might show high diversity at the genetic level over its

distributional range. The genetic variation shown by the medicinal plant Rauwolfia vomitoria2 2 0

growing in different Himalayan ranges might be in terms of the potency and concentration of

the active chemical (reserpine) that the plant produces. India has more than 50000 genetically

different strains of rice and 1000 varieties of mango / any other example explained

(ii) Species diversity: The diversity at the species level. For example, the Western Ghats have a

greater amphibian species diversity than the Eastern Ghats.

(iii) Ecological diversity: At the ecosystem level, India, for instance, with its deserts / rain forests

/ mangroves / coral reefs / wetlands / estuaries / and alpine meadows has a greater ecosystem

diversity than a Scandinavian country like Norway  (Any two) = 1 + 1

[2 marks]