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CBSE

Subject

Biology

Class

CBSE Class 12

CBSE Biology 2016 Exam Questions

Short Answer Type

1.

Name the calls HIV (Human Immunodeficiency Virus) gains entry into after infecting the human body. Explain the events that occur in these cells.


After infecting the human body the HIV gains entry into the macrophages.

Events that occur in the cells:

i. The virus enters into macrophages where RNA genome of virus replicates to form viral DNA with the help of enzyme reverse transcriptase.
ii. The viral DNA gets incorporated into host cells DNAand directs the infected cell to produce virus particles.
iii. The macrophage continue to produce virus and in this way acts like a HIV factory.
iv. The HIV enters into helper T-lymphocytos (TH ), replicates and produce progeny viruses. The progeny viruses released in the blood attacks other helper T-helper lymphocytes. This is repeated heading to a progressive decrease in the number of helper
v. Tlymphocytes in the body of infected person. During this period the person suffers from bouts of fever, diarrhoea and weight loss. 
vi. The decrease in the number of the T helper cells leads to infections as the immune system of the person becomes weak. 


2.

Name a free-living and a symbiotic bacterium that serve as bio-fertilizer. Why are they called so?


Free-living -  Azotobacter, Symbiotic - Rhizobium serve as bio-fertilizers.

They are called bio-fertilizers because they are organisms found in nature that enrich the nutrient quality of the soil.


3.

Explain the relationship between CFCs and Ozone in the stratosphere.


CFCs find wide use as refrigerants. CFCs discharged in the lower part of atmosphere move upward and reach stratosphere. In stratosphere, UV rays act on them reacting with the Cl atoms. Cl atoms acts as catalysts and degrades ozone releasing molecular oxygen. Cl atoms are not consumed in the reaction. Hence whatever CFC are added to stratosphere, have permanent and continuing effects on ozone level. 

bold CFCl subscript bold 3 bold space end subscript bold rightwards arrow with bold UV bold minus bold C on top bold space bold CFCl subscript bold 2 bold space bold plus bold space bold Cl
bold CFCl subscript bold 2 bold space bold rightwards arrow with bold UV bold minus bold C bold space bold space on top bold CFCl bold space bold plus bold space bold Cl
bold Cl bold space bold plus bold space bold O subscript bold 3 bold space bold rightwards arrow bold space bold ClO bold space bold plus bold space bold O subscript bold 2
bold ClO bold space bold plus bold space bold O subscript bold 3 bold space bold rightwards arrow bold Cl bold space bold plus bold space bold 2 bold O subscript bold 2


4.

What are transgenic animals? Given an example


Animals that have had their DNA manipulated to possess and express an extra (foreign) gene are known as transgenic animals.

For example - Transgenic cow like Rosie. 


5.

Mention two advantages for preferring CNG over diesel as an automobile fuel.


Two advantages for preferring CNG over diesel as an automobile fuel are:

(i) CNG is cheaper than diesel.
(ii) CNG burns efficiently as compared to diesel and causes less of air pollution. 


6.

A male honeybee has 16 chromosomes whereas its female has 32 chromosomes. Give one reason.


Male honeybees are born from unfertilised eggs by the process known as parthenogenesis where as female honeybees are born from fertilised eggs. As unfertilised eggs carries only half number of chromosome as compared to fertilised eggs, therefore male honeybee has 16 chromosomes whereas female has 32 chromosomes.


7.

Following are the features of genetic codes. What does each one indicate ? [2]
Stop codon; Unambiguous codon; Degenerate codon; Universal codon.


Stop codon - 

Unambiguous codon

Degenerate codon

Universal codon


8.

Write the probable differences in eating habits of Homo habilis and Homo erectus.


Homo habilis Homo erectus
Did not eat meat Ate meat
Brain comparatively smaller.  Larger brain. 

9.

Mention the role of 'genetic mother' in MOET.


The genetic mother helps in Superovulation. 


10.

Why are sacred groves highly protected ?


The sacred groves are highly protected because they have religious and cultural values. They also have a large number of rare and threatened plant species. 


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