MARKING SCHEME

Sample Question Paper 2

Class XII

Biology – 2012-13

1. (i) the pollen grains are sticky. 1

2. (p+q)

2 = p2 + 2pq + q2 = 1 1

3. First exposure to the infection works as vaccination,  the immune system of the

body gets familiar with the nature of microorganisms and specific antibodies can

be produced against infection.  ½+½=1

4. Ethidium bromide (EtBr) 1

5. Centrifugation and centrifuge            ½+½=1

6. Normal Endonuclease : makes cut at random position within a DNA ½

Restriction endonuclease: makes cut only at specific position within a DNA ½

7. – Depletion of dissolved oxygen in water        ½

– Nutrient enrichment ½

8. Each trophic level has a certain mass of living material at a particular time called

as the standing crop. ½

The standing crop is measured as the mass of living organisms (biomass) or the

number in a unit area. The biomass of a species is expressed in terms of fresh or

dry weight. ½

9. Amniocentasis – A foetal sex diagnostic test based on the chromosomal pattern in

the amniotic fluid surrounding the developing embryo. 121

It is justified to ban the process for diagnosis of sex to legally check the increasing

female foeticides.

X Y

10.X

X

Phenotypes : 50% daughter normal (XX)

50% daughter carrier (XX)

50% son normal (XY)

50% son haemophilic (X Y) ½ ×4=2

11. Original stock of seed eating finches migrated to different habitats (of Galapagos

Islands), adapted to different feeding methods, by altered beak structure, evolved

into different types of finches. ½ ×4=2

12. Testcross , Cross purple flower plant with a (homozygous) recessive plant  with

white flowers, if all the flowers of the progeny of the above are purple , the plant

is homozygous dominant  ½ ×4=2

13. Lymphocytes are not immortal but are 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

X X X Y

XX X Y

14. PCR, because it is a thermostable DNA polymerase enzyme, gets isolated  from

bacteria Thermus aquaticus from hot water springs, does not get denatured at high

temperature during PCR and works as normal DNA polymerase enzyme. ½ ×4=2

15. Detrivore feeds on dead plants and animals / detritus  ½

Example : Earthworm / Nematodes  ½

Decomposer breaks down complex ( organic ) matter into simpler ( inorganic )

matter ½+½

Example : Fungus / Bacteria.

16. DDT neither excreted nor metabolized and keeps accumulating in the food chain,

interferes with Calcium metabolism, decline in bird  population / due to thinning

of egg shell / premature breaking of eggs, bio-magnification. ½ ×4=2

17. Pioneer Community (Species) – Community that invades a bare area                       1

Climax community – Last sustainable community that is in near equilibrium with

the environment                                                                                                                 1

18. a) Salmonella typhi

b) Streptococcus pneumoniae

c) Wuchereria bancrofti and Wuchereria malayi

d) Entamoeba histolytica ½×4=2

OR

a) Acquired Immuno Deficiency Syndrome

b) Enzyme Linked Immuno Sorbent Assay                                                     1×1=2

19. (i) Regeneration of endometrium.

(ii) Uterus gets highly vascularised, ready for embryo implantation

(iii) Disintegration of the endometrium leading to menstruation  1×3=3

20. IVF – In Vitro Fertilization

ZIFT – Zygote Intra Fellopian Transfer

IUI – Intra – Uterine Insemination

MTP – Medical Termination of Pregnency   ½×4=2

21. Since there are only four bases which code for twenty aminoacids, the code should

be madeup of three bases i.e. (4 x 4 x 4) = 64 codons ; a number more than the

required.  1

If the codon consists of four letters, only (4 x 4) only  sixteen codes are possible,

which is less than the required. 1

• As the ribosome moves on mRNA the codons are read in a contiguous

fashion 1

22. DNA sequences which are repeated many a times, show a high degree of

polymorphism, and form a bulk of DNA in a genome, called as satellite DNA  1½

DNA from every tissue from an individual, shows  the same degree of

polymorphism and  is heritable,  hence very useful in DNA finger printing  1½

23. • Microbes naturally present in the sewage are employed in the secondary

treatment of the sewage.

• The effluent from the primary treatment is passed into large aeration tanks.

• This allows the rapid growth of aerobic microbes into flocs which consume the

organic matter of the sewage and reduce the BOD.

• Then the  effluent is passed into a settling tank, where the flocs are allowed to

sediment forming the activated sludge.

• Major parts of this activated sludge is pumped into anaerobic sludge digesters,

where the anaerobic bacteria digest microbes in the activated sludge.

• During this digestion bacteria produce a mixture of gases like methane,

hydrogen sulphide and carbon dioxide, which form the biogas and can be used

as a source of energy. The effluent is generally released into rivers and streams.

24. BOD – Biological Oxygen Demand, it refers to the amount of oxygen that would be

consumed if all the organic matter in one litre of water were oxidized by Bacteria.

The BOD test measures the rate of uptake of oxygen by microorganisms in a

sample of water  and thus, indirectly BOD is a measure of the organic matter

present in the water. The greater is a measure of BOD of waste water; more is its

polluting potential.

25. (i) Origin of replication / ori  site  – From here the replication starts (and any

piece of DNA when linked, can be made to replicate within the host cell)

(ii) At least two  Selectable markers – Helps in identifying and eliminating non

transformants

(iii) Unique Restriction sites – The foreign DNA links to this region of the plasmid

1×3=3

26. Population interaction:

Species A Species B Name of interaction

+ + Mutualism

– – Competition

+ – Predation

+ – Parasitism

+ 0 Amensalism

(i) + Beneficial- Bothe species benefit in Mutualism e.g. lichens

(ii)  – Detrimental – Both species loose in competition e.g. (detrimental)

– In Predation & Parasitism the predator and the parasite benefits but it is

detrimental to the other species (host and prey respectively)

(iii)  0 Neutral – In commensalism one species is benefits but the other is neither

harmed nor benefitted.  3

27. Plants: Ephemeral mode (complete life cycle in short period) / Deep tap roots /

Deciduous leaves / Waxy cuticle / sunken stomata / Succulence to

store water / C4 Pathway of Photosynthesis. (Any 3) 1½

Animals: No sweating / uricotelic / deposition of fat in sub epidermal

layer / burrowing nature / thick skin / body covered with

scales (Any three) 1½

OR

Ans. (i) Particulate matter will pollute the air  1

(ii) Use of CNG / Phasing out of old vehicles / Use of unleaded  petrol /

Use of low sulphur fuel / Use of catalytic converters / Application of

stringent pollution level norms (Any four) 2

28. (a)3

(b)  Fallopian tube  1

zygote undergoes, cleavage to form, blastomeres & develop into 8-16

cell stage- the morula ½+½

OR

(b) Egg cell – Forms zygote (2n)

Polar nuclei – Primary endosperm nucleus (3n)

Antipodals – Degenerates ½×3=1½

29. (a) DNA synthesis ½

(b) i) Semi conservative

ii) Semi discontinuous

iii) Unidirectional

(c) DNA polymerase III – adds nucleotides

DNA polymerase I – fills the gaps

RNA primase – brings primer

Helicase – breaks hydrogen bond

Toisomerase – Causes unwinding

Gyrase – relaxes supercoiling

DNA ligase – joins Okazaki fragments

      OR

Inheritance of flower colour in Pisum sativum

Pea Plant

Parent VV x vv

(violet)        (white)

Gamete V  v

F1         V v (violet) 1½

This inheritance pattern follows law of dominance, that is between the two alleles

of the flower colour (gene) the dominant allele is expressed( phenotypically), over

the recessive    1

Any 3 enzymes and their

function

1 x 3 = 3128

In the case of  Antirrhinum(snap dragon) the dominant flower colour is red. white

in recessive ½

Antirrhinum

Parent RR x rr

(Red)        (white)

Gamete R  r

F1                                                   R r (Pink)

This is the case of incomplete dominance, it is because the allele for red colour is

not completely dominant over its recessive allele, law of dominance not exhibited

1

30. (a) I will support  Aditya’s concern for the health of his family. Both

Active as well as Passive smoking is injurious to health as smoking is

associated with increased incidence of cancers of lungs, urinary

bladder, throat and oral cavity, bronchitis, emphysema / coronary

heart disease / gastric ulcer etc.          2

b) (i) By printing statutory warning on cigarette packets.

(ii) As  advertisement in mass media such as Television, Newspaper,

Internet etc.

(iii) Designating Non smoking Zones in public areas such as

Restaurants, Airports etc.         3

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