CBSE Class 12

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 Multiple Choice QuestionsShort Answer Type


A number of passengers were severely burnt beyond recognition during a train accident. Name and describe a modern technique that can help hand over the dead to their relatives.

The technique that can help in the identification of victims is DNA fingerprinting which distinguishes between individuals of same species by using their DNA as sample. The chemical structure of DNA is same in everyone (99.9%) except the order of base pairs, i.e. only 0.1% of DNA makes every individual unique. DNA fingerprinting exploits the highly variable repeating sequences, i.e. VNTRs for profiling. These VNTRs are highly conserved among members of the same species. Technique
This technique has following steps:
(i) DNA Isolation DNA is extracted from the cells in a high speed centrifuge.
(ii) Amplification Many copies of the extracted DNA can be made by the use of polymerase chain reaction.
(iii) Digestion of DNA by restriction endonucleases.
(iv) Separation of DNA fragments by electrophoresis.
(v) Blotting Transfer of separated DNA fragments to synthetic membranes (like nylon or nitrocellulose).
(vi) Hybridisation, with the help of a radio labelled VNTR probe (small segments of DNA which help to detect the presence of a gene in a long DNA sequence). These probes target a specific nucleotide sequence that is complementary to them.
(vii) Autoradiography Detection of hybridised DNA fragments by autoradiography.
The presence of similarities between the victims and their relatives determines their association on the basis of which dead bodies can be identified and handed over to their families.



Draw a labelled diagrammatic sectional view of a human seminiferous tubule.



During a medical investigation, an infant was found to possess an extra chromosome 21. Decribe the symptoms the child is likely to develop later in the life.

An additional copy of chromosome number 21 (trisomy of chromosome number 21) in humans results in Down’s syndrome.Following characteristic symptoms are likely to develop in the child due to the above mentioned conditions:
(i) Short statured with small, round head.
(ii) Furrowed tongue and partially open mouth.
(iii) Broad palm with characteristics palm crease.
(iv) Slow mental, physical and psychomotor development.



Explain the role(s) of the following in biotechnology.
(a) Restriction endonuclease
(b) Gel-electrophoresis
(c) Selectable markers in pBR322

(a) Restriction Endonucleases These are the bacterial enzymes that cut dsDNA into fragments after recognising and binding to the specific nucleotide sequences, known as recognition site. These enzymes are used to form recombinant molecules of DNA, composed of DNA from different sources.

(b) Gel-Electrophoresis is the technique which allows the visualisation of separated fragments of DNA on an agarose gel matrix.
Since, the DNA fragments are negatively charged molecules, they separate and move towards the anode (+ ve) under the influence of an electric field. DNA fragments are separated on the basis of their size through the sieving effect provided by the gel.

(c) Selectable Markers in pBR322, an E. coli cloning vector has two antibiotic resistance genes, i.e. for ampicillin and tetracycline. Antibiotic resistance is used as selectable marker, i.e. it helps in identification and selection of transformants. For example, when a foreign DNA is ligated at the site of tetracycline resistance (tet r ,) gene in pBR322, the recombinant plasmid will lose tetracycline resistance due to insertional inactivation of foreign DNA, but can still be selected out from non-recombinants by placing the transformants on ampicillin containing medium. The transformants growing on ampicillin containing medium, are then transferred on tetracycline containing medium. The recombinants will grow on ampicillin containing medium but not on tetracycline one whereas non-recombinants grow on both.



(a) What precaution(s) would you recommend to a patient requiring repeated blood transfusion?
(b) If the advise is not followed by the patient there is an apprehension that the patient might contract a disease that would destroy the immune system of his/her body. Explain with the help of schematic diagram only how the immune system would get affected and destroyed.

(a) Repeated blood transfusion may result in contracting diseases like AIDS. The recipient must ensure that the donor’s blood is being screened for HIV and other pathogens.
(b) In the absence of such measures, the patient can get infected by HIV (Human Immmunodeficiency Virus) which causes AIDS. It is a threatening disorder that weakens the immune system by attacking helper T-cells in the body.
A schematic diagram showing the cycle of proliferation and effects of retrovirus (HIV) in infected person is as follows :



(a) What is inbreeding depression?
(b) Explain the importance of ‘selection’ during inbreeding in cattle.

(a) Inbreeding is the crossing of closely related animals within the same breed for 4-6 generations. The continued inbreeding however, reduces fertility and even the productivity, this is called inbreeding depression.

(a) Inbreeding is the crossing of closely related animals within the same breed for 4-6 generations. The continued inbreeding however, reduces fertility and even the productivity, this is called inbreeding depression.



(a) List the three states the annuals and biennial angiosperms have to pass through during their life cycle.
(b) List and describe any two vegetative propagules in flowering plants.

(a) The three states through which the annuals and biennial angiosperms pass during their life cycle are:
(i) Vegetative or Juvenile phase
(ii) Reproductive phase
(iii) Senescent phase

(b) Vegetative propagules are the parts/units of a plant which can be used for vegetative propagation, e.g. roots, stems, leaves, etc. Root propagules include the production of a new plant via roots, e.g. fleshy roots in case of sweet potato, tapioca and Dahlia. Vegetative propagation through leaves, many plant leaves have adventitious buds which help in the development of a new plant, e.g. Begonia, Bryophyllum, etc.



p2+2pq+q2 Explain this algebraic equation on the basis of Hardy-Weinberg’s principle.

The equation p2 + 2pq + q2 , mathematically represents the Hardy-Weinberg’s principle. It is used to
calculate the genetic variation of a population at equilibrium.
Principle It states that allele frequencies in a population are stable and remain constant from generation to generation.
In this equation,
p – frequency of allele A
q – frequency of allele a
p2 – frequency of AA (homozygous) individuals in a population
q2 – frequency of aa (homozygous) individuals
2pq – frequency of Aa (heterozygous) individuals
Also, the sum total of all the allelic frequencies is equal to 1. If the p and q allele frequencies are known, then the frequencies of the three genotypes can be calculated using the Hardy-Weinberg equation.
This equation can be used to measure the differences in frequencies of observed genotype measured from the frequencies predicated by the equation. The disturbance in genetic equilibrium results in evolution, thus the presence of any difference indicates the exent of evolutionary change.



Describe how do ‘flocs’ and ‘activated sludge’ help in sewage treatment.

Roles of ‘flocs’ and ‘activated sludge’ in sewage treatment are as follows: Flocs These are masses of bacteria held together by slime and fungal filaments to form mesh-like structures. Flocs are used during the secondary sewage treatment in the aeration tank to increase the rate of decomposition. The microbes digest a lot of organic matter, converting it into microbial biomass and releasing a lot of minerals. As a result, BOD of the sewage reduces. As the BOD of waste is reduced to 10-15% of raw sewage, it is passed into settling tanks. In these tanks, flocs are allowed to undergo sedimentation. Activated Sludge The sediment of settling tank is called activated sludge. A part of it is used as inoculum in aeration tanks. The remaining part is passed into a large tank called anaerobic sludge digester. In these tanks, anaerobic microbes are present that digest the organic mass as well as aerobic microbes of activated sludge. The remaining sludge is used as manure or compost.



Write the steps you would suggest to be undertaken to obtain a foreign-gene-product.

To obtain a foreign-gene product, following steps should be undertaken:
(i) Identification of DNA with desirable genes.
(ii) Introduction of the identified DNA into suitable host to form recombinant DNA (rDNA).
(iii) Maintenance of introduced DNA in particular host and gene cloning.
(iv) Transfer of the DNA (gene transfer) to its progeny.
(v) Selection of the recombinants from non-recombinants.
(vi) Expression of gene of interest by culturing recombinant cells.
(vii) Culturing of cells in bioreactors for large scale production of desired gene product.