CBSE Biology Class-Xll Questions

11/28/2011 CBSE

1 Describe one example of
artificial selection.

Cabbage, cauliflower and kohlrabi are derrived from a comman
ancestor, colewort by artificial selection.

2 Mention two differences in the vascular bundles of Sunflower and Maize stems

Sunflower and maize stems are different in that :

(1) Sunflower has radially arranged, fewer vascular bundles, whereas maize has
randomly distributed numerous vascular bundles.

(2) Sunflower has open (with cambium) vascular bundles, whereas maize has closed
(without cambium) vascular bundles.

3 Give any four examples of human reflexes.

(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.

4 How were exact replicas of master plate obtained in Lederberg replica plating
experiment ?

A sterile velvet disc, mounted on a wooden bloc, was gently pressed on
the master plate. Some bacteria from each colony adhered to the velvet. By
pressing this velvet on to new agar plates, Ledeberg and Lederberg obtained
exact replicas of the master plate because the few bacteria

transferred by the velvet formed colonies on the new plates.

15 In humans, genetically the sex of the child is determined by the father and
not by the mother. Explain

Human male is heteromorphic with respect to chromosome: XY. A son gets
one of its chromosomes from the father and the other from the mother. The mother
being homomorphic contributes only X. If the father contributed X, the embryo
develops into a daughter, whereas if he contributed Y, the embryo develops into
a son.

6 Describe how auxins are related with the bending of shoots towards the source
of light ?

Auxins promote elongation and growth of stem by stimulating cell walls
to stretch. Auxin is present in the tip of the coleoptile and moves away from
the illuminated side. Once the concentration of auxin increases in the
nonilluminated side, differential growth

7 Explain any two
palaeontological evidences in favour of organic evolution.

(i) Archaeopteryx provides evidences for the evolution of birds from

(ii) Fossils provide evidences of the evolution of modern day horse (Equus) from
Eohippus- Mesohippus.

8 Distinguish between bacterial transduction and bacterial transformation.


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
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.

9 Mention four aspects in which
uricotelism differs from ureotelism.


(1)Nitrogen is
mainly excreted as urea.

(2)Urea is very water-soluble and therefore
can be used as the excretory material only if the animal can afford to
have a lot of water.

(3)Urea formation requires more energy than

(4)This mode is most suitable for those
animals, which can excrete sufficient volumes of water, or can concentrate
urea considerably in the urine, or can retain considerable amounts of urea
in their blood.

(1)Nitrogen is
mainly excreted as uric acid.

(2)Uric acid is water insoluble and
therefore can be excreted without much intake of water.


(3)Uric acid formation requires much more
energy than the formation of urea.

(4)This mode is most suitable to the land
animals of drier zones and birds, which have a limited access to water.

20 Specify how a pentose
phosphate is a CO2 acceptor in the dark reaction of photosynthesis.

3 molecules of a pentose phosphate (ribulose bisphosphate) accept 3
molecules of CO2 and this carboxylation is mediated by the enzyme
RUBISCO. The 3 molecules of six-carbon intermediate, which form are short-lived
and break down into two molecules of three-carbon phosphoglyceric acid each.The
6 molecules of phosphoglyceric acid carry the rest of the dark reaction cycle.

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