CBSE Biology Past year paper Delhi (Set - 3) Solutions - 2011, Class 12 | Additional Study Material for NEET PDF Download

 Page 1


  
 
CBSE XII  |  Biology 
Board Paper 2011 – Delhi – Set 3 (Solution) 
 
     
CBSE 
Class XII Biology (Theory) 
Board Paper 2011 – Delhi (Set 3) 
SOLUTION 
Time: 3 hrs Total Marks: 70 
 
SECTION A 
1. Ans. AUG is the initiation codon which signals the start of translation, and UGA is the 
termination codon. When UGA comes into register with the A site, the protein-
releasing factors modify the specificity of the peptidyl transferase so that the water 
molecule is added to the peptide instead of another amino acid, causing the 
dissociation of ribosomal subunits. 
 
2.  Ans. Embryos are developed when the nucellar cells surrounding the embryo sac 
start dividing and protrude into the embryo sac. This condition of occurrence of 
more than one embryo in the seed is called polyembryony. Because orange is a 
citrus fruit, it shows the polyembryony stage, i.e. its seeds possess more than one 
embryo. Hence, when an orange seed is squeezed, many embryos of different shapes 
and sizes are seen. 
 
3. Ans. Plants are confined to the upper 200 metres strata of water because no light 
(and hence no photosynthesis) occurs beyond this depth. 
 
4. Ans. Brewer’s yeast (Saccharomyces cerevisiae) is used for fermenting malted 
cereals and fruit juices. 
 
5. Ans. Meiosis occurs in the zygote of an organism exhibiting the haplontic life cycle 
to produce haploid individuals. 
 
6. Ans. DNA is enclosed within membranes, so it is released by breaking cells. DNA 
along with other macromolecules such as RNA, proteins, polysaccharides and lipids 
are released by treating bacterial, plant, fungal or animal cells with enzymes such as 
lysozyme (bacteria), cellulose (plants) and chitinase (fungi). 
 
7. Ans. Aneuploidy is a condition of having fewer or extra chromosomes than the 
normal genome number of the species. Loss of chromosomes is called hypoploidy 
and the addition of chromosomes is called heteroploidy. 
 
8. Ans. Cryopreservation is preservation at -196°C in liquid nitrogen. It is useful in 
sperms, eggs, cells and embryonic tissues of animals. 
 
 
Page 2


  
 
CBSE XII  |  Biology 
Board Paper 2011 – Delhi – Set 3 (Solution) 
 
     
CBSE 
Class XII Biology (Theory) 
Board Paper 2011 – Delhi (Set 3) 
SOLUTION 
Time: 3 hrs Total Marks: 70 
 
SECTION A 
1. Ans. AUG is the initiation codon which signals the start of translation, and UGA is the 
termination codon. When UGA comes into register with the A site, the protein-
releasing factors modify the specificity of the peptidyl transferase so that the water 
molecule is added to the peptide instead of another amino acid, causing the 
dissociation of ribosomal subunits. 
 
2.  Ans. Embryos are developed when the nucellar cells surrounding the embryo sac 
start dividing and protrude into the embryo sac. This condition of occurrence of 
more than one embryo in the seed is called polyembryony. Because orange is a 
citrus fruit, it shows the polyembryony stage, i.e. its seeds possess more than one 
embryo. Hence, when an orange seed is squeezed, many embryos of different shapes 
and sizes are seen. 
 
3. Ans. Plants are confined to the upper 200 metres strata of water because no light 
(and hence no photosynthesis) occurs beyond this depth. 
 
4. Ans. Brewer’s yeast (Saccharomyces cerevisiae) is used for fermenting malted 
cereals and fruit juices. 
 
5. Ans. Meiosis occurs in the zygote of an organism exhibiting the haplontic life cycle 
to produce haploid individuals. 
 
6. Ans. DNA is enclosed within membranes, so it is released by breaking cells. DNA 
along with other macromolecules such as RNA, proteins, polysaccharides and lipids 
are released by treating bacterial, plant, fungal or animal cells with enzymes such as 
lysozyme (bacteria), cellulose (plants) and chitinase (fungi). 
 
7. Ans. Aneuploidy is a condition of having fewer or extra chromosomes than the 
normal genome number of the species. Loss of chromosomes is called hypoploidy 
and the addition of chromosomes is called heteroploidy. 
 
8. Ans. Cryopreservation is preservation at -196°C in liquid nitrogen. It is useful in 
sperms, eggs, cells and embryonic tissues of animals. 
 
 
  
 
CBSE XII  |  Biology 
Board Paper 2011 – Delhi – Set 3 (Solution) 
 
     
 
SECTION B 
 
9. Ans. Deoxyribonucleoside triphosphate acts as a substrate, and it provides energy 
for the polymerisation reaction as its two terminal phosphates are high-energy 
phosphates. 
 
10.  Ans. 
 
Albuminous seeds Non-albuminous seeds 
(i) The seeds in which the 
cotyledons are thin and 
membranous and the food is 
stored in the endosperm are 
called endospermic or 
albuminous seeds. 
(ii) Examples: Wheat, maize 
(i) The seeds which are without 
endosperms are called non-
endospermic or non-albuminous 
seeds. 
 
(ii) Examples: Pea, gram 
 
 
11.  Ans. A hereditary disease can be corrected by gene therapy. Treatment of a genetic 
disorder by manipulating genes is called gene therapy. The first clinical gene 
therapy was given in 1990 to a 4-year-old girl with adenosine deaminase (ADA) 
deficiency. 
              Gene therapy of ADA deficiency: 
(i) The patient lacks functional T-lymphocytes and therefore fails to fight the 
infecting pathogens.  
(ii) Lymphocytes are extracted from the patient’s bone marrow and a normal 
functional copy of genes coding for ADA is introduced into these lymphocytes 
with the help of a retrovirus.  
(iii) The cells so treated are introduced into the patient’s bone marrow.  
(iv) The lymphocytes produced by these cells contain functional ADA gene and 
reactivate the victim’s immune system for life. 
 
12.  Ans. When the Nile Perch, a large predator fish, was introduced in Lake Victoria, it 
started feeding on the native cichlid fish. As a result, the cichlid fish became extinct 
and the Nile Perch, not finding any food, died too. 
 
13.  Ans. The Ti plasmid (tumour-inducing plasmid) of Agrobacterium tumefaciens has 
been modified to be used as a cloning vector. The Ti plasmid integrates as a segment 
of its DNA, TDNA, into the chromosomal DNA of its host plant cells which causes 
tumour. As gene transfer occurs without any human effort, the bacterium is known 
as the ‘natural genetic engineer’ of plants. Ti plasmids are used as vectors to transfer 
Page 3


  
 
CBSE XII  |  Biology 
Board Paper 2011 – Delhi – Set 3 (Solution) 
 
     
CBSE 
Class XII Biology (Theory) 
Board Paper 2011 – Delhi (Set 3) 
SOLUTION 
Time: 3 hrs Total Marks: 70 
 
SECTION A 
1. Ans. AUG is the initiation codon which signals the start of translation, and UGA is the 
termination codon. When UGA comes into register with the A site, the protein-
releasing factors modify the specificity of the peptidyl transferase so that the water 
molecule is added to the peptide instead of another amino acid, causing the 
dissociation of ribosomal subunits. 
 
2.  Ans. Embryos are developed when the nucellar cells surrounding the embryo sac 
start dividing and protrude into the embryo sac. This condition of occurrence of 
more than one embryo in the seed is called polyembryony. Because orange is a 
citrus fruit, it shows the polyembryony stage, i.e. its seeds possess more than one 
embryo. Hence, when an orange seed is squeezed, many embryos of different shapes 
and sizes are seen. 
 
3. Ans. Plants are confined to the upper 200 metres strata of water because no light 
(and hence no photosynthesis) occurs beyond this depth. 
 
4. Ans. Brewer’s yeast (Saccharomyces cerevisiae) is used for fermenting malted 
cereals and fruit juices. 
 
5. Ans. Meiosis occurs in the zygote of an organism exhibiting the haplontic life cycle 
to produce haploid individuals. 
 
6. Ans. DNA is enclosed within membranes, so it is released by breaking cells. DNA 
along with other macromolecules such as RNA, proteins, polysaccharides and lipids 
are released by treating bacterial, plant, fungal or animal cells with enzymes such as 
lysozyme (bacteria), cellulose (plants) and chitinase (fungi). 
 
7. Ans. Aneuploidy is a condition of having fewer or extra chromosomes than the 
normal genome number of the species. Loss of chromosomes is called hypoploidy 
and the addition of chromosomes is called heteroploidy. 
 
8. Ans. Cryopreservation is preservation at -196°C in liquid nitrogen. It is useful in 
sperms, eggs, cells and embryonic tissues of animals. 
 
 
  
 
CBSE XII  |  Biology 
Board Paper 2011 – Delhi – Set 3 (Solution) 
 
     
 
SECTION B 
 
9. Ans. Deoxyribonucleoside triphosphate acts as a substrate, and it provides energy 
for the polymerisation reaction as its two terminal phosphates are high-energy 
phosphates. 
 
10.  Ans. 
 
Albuminous seeds Non-albuminous seeds 
(i) The seeds in which the 
cotyledons are thin and 
membranous and the food is 
stored in the endosperm are 
called endospermic or 
albuminous seeds. 
(ii) Examples: Wheat, maize 
(i) The seeds which are without 
endosperms are called non-
endospermic or non-albuminous 
seeds. 
 
(ii) Examples: Pea, gram 
 
 
11.  Ans. A hereditary disease can be corrected by gene therapy. Treatment of a genetic 
disorder by manipulating genes is called gene therapy. The first clinical gene 
therapy was given in 1990 to a 4-year-old girl with adenosine deaminase (ADA) 
deficiency. 
              Gene therapy of ADA deficiency: 
(i) The patient lacks functional T-lymphocytes and therefore fails to fight the 
infecting pathogens.  
(ii) Lymphocytes are extracted from the patient’s bone marrow and a normal 
functional copy of genes coding for ADA is introduced into these lymphocytes 
with the help of a retrovirus.  
(iii) The cells so treated are introduced into the patient’s bone marrow.  
(iv) The lymphocytes produced by these cells contain functional ADA gene and 
reactivate the victim’s immune system for life. 
 
12.  Ans. When the Nile Perch, a large predator fish, was introduced in Lake Victoria, it 
started feeding on the native cichlid fish. As a result, the cichlid fish became extinct 
and the Nile Perch, not finding any food, died too. 
 
13.  Ans. The Ti plasmid (tumour-inducing plasmid) of Agrobacterium tumefaciens has 
been modified to be used as a cloning vector. The Ti plasmid integrates as a segment 
of its DNA, TDNA, into the chromosomal DNA of its host plant cells which causes 
tumour. As gene transfer occurs without any human effort, the bacterium is known 
as the ‘natural genetic engineer’ of plants. Ti plasmids are used as vectors to transfer 
  
 
CBSE XII  |  Biology 
Board Paper 2011 – Delhi – Set 3 (Solution) 
 
     
the foreign gene of interest into the target plant cells so these are used in genetic 
transformation in plants. 
 
14.  Ans. Discharge of domestic sewage into rivers results in the rise of BOD because 
decomposer organisms consume a large amount of oxygen to decompose organic 
matter. When the amount of organic matter reduces, the amount of dissolved 
oxygen again increases. 
 
15.  Ans. The two types of immune systems in the human body are 
(a) Humoral or antibody-mediated immune system: This immune system operates 
by the production of humoral antibodies, so it is named humoral or antibody-
mediated immune system. The humoral antibodies in blood and lymph react 
with specific foreign substances (antigens) and promote their destruction. 
(b) Cell-mediated immune system: This immune system operates by the production 
of lymphocytes and macrophages. Hence, it is called cell-mediated. The T-
lymphocytes attack the pathogenic microorganisms which have entered the 
host’s body or grafts such as a transplanted kidney. It also protects the body 
from its own cells which have become cancerous.  
OR 
 
Elephantiasis is caused by the filarial worm Wuchereria (Wuchereria bancrofti 
and Wuchereria malayi). It usually affects the lymphatic vessels of the lower limbs  
                   and genital organs.  
                   Ringworm is caused by the fungi Microsporum, Trichophyton and  
                   Epidermophyton. It affects the skin, nails and scalp. 
 
16. Ans. Primary lymphoid organs are those organs where T lymphocytes and B     
lymphocytes mature and acquire their antigen-specific receptors. They include bone 
marrow and thymus. 
Secondary lymphoid organs are spleen, tonsils, Peyer’s patch of the small intestine 
and appendix. The lymphocytes interact with the antigen and proliferate to form a 
clone. 
 
17. Ans. CuT increases the phagocytosis of sperms in the uterus and copper ions 
released decrease sperm motility and prevent fertilisation. So, it is considered a 
good contraceptive device to space children. 
 
18. Ans. RNA interference (RNAi) is a system within living cells which helps control the 
activity of specific genes and is used as a method of cellular defence against 
parasites. This method involves silencing of a specific mRNA. The introduction of 
DNA produces both sense and antisense RNA in the host cells. These two RNAs, 
being complementary to each other, form double-stranded RNA which binds to and 
prevents the translation of mRNA. 
Page 4


  
 
CBSE XII  |  Biology 
Board Paper 2011 – Delhi – Set 3 (Solution) 
 
     
CBSE 
Class XII Biology (Theory) 
Board Paper 2011 – Delhi (Set 3) 
SOLUTION 
Time: 3 hrs Total Marks: 70 
 
SECTION A 
1. Ans. AUG is the initiation codon which signals the start of translation, and UGA is the 
termination codon. When UGA comes into register with the A site, the protein-
releasing factors modify the specificity of the peptidyl transferase so that the water 
molecule is added to the peptide instead of another amino acid, causing the 
dissociation of ribosomal subunits. 
 
2.  Ans. Embryos are developed when the nucellar cells surrounding the embryo sac 
start dividing and protrude into the embryo sac. This condition of occurrence of 
more than one embryo in the seed is called polyembryony. Because orange is a 
citrus fruit, it shows the polyembryony stage, i.e. its seeds possess more than one 
embryo. Hence, when an orange seed is squeezed, many embryos of different shapes 
and sizes are seen. 
 
3. Ans. Plants are confined to the upper 200 metres strata of water because no light 
(and hence no photosynthesis) occurs beyond this depth. 
 
4. Ans. Brewer’s yeast (Saccharomyces cerevisiae) is used for fermenting malted 
cereals and fruit juices. 
 
5. Ans. Meiosis occurs in the zygote of an organism exhibiting the haplontic life cycle 
to produce haploid individuals. 
 
6. Ans. DNA is enclosed within membranes, so it is released by breaking cells. DNA 
along with other macromolecules such as RNA, proteins, polysaccharides and lipids 
are released by treating bacterial, plant, fungal or animal cells with enzymes such as 
lysozyme (bacteria), cellulose (plants) and chitinase (fungi). 
 
7. Ans. Aneuploidy is a condition of having fewer or extra chromosomes than the 
normal genome number of the species. Loss of chromosomes is called hypoploidy 
and the addition of chromosomes is called heteroploidy. 
 
8. Ans. Cryopreservation is preservation at -196°C in liquid nitrogen. It is useful in 
sperms, eggs, cells and embryonic tissues of animals. 
 
 
  
 
CBSE XII  |  Biology 
Board Paper 2011 – Delhi – Set 3 (Solution) 
 
     
 
SECTION B 
 
9. Ans. Deoxyribonucleoside triphosphate acts as a substrate, and it provides energy 
for the polymerisation reaction as its two terminal phosphates are high-energy 
phosphates. 
 
10.  Ans. 
 
Albuminous seeds Non-albuminous seeds 
(i) The seeds in which the 
cotyledons are thin and 
membranous and the food is 
stored in the endosperm are 
called endospermic or 
albuminous seeds. 
(ii) Examples: Wheat, maize 
(i) The seeds which are without 
endosperms are called non-
endospermic or non-albuminous 
seeds. 
 
(ii) Examples: Pea, gram 
 
 
11.  Ans. A hereditary disease can be corrected by gene therapy. Treatment of a genetic 
disorder by manipulating genes is called gene therapy. The first clinical gene 
therapy was given in 1990 to a 4-year-old girl with adenosine deaminase (ADA) 
deficiency. 
              Gene therapy of ADA deficiency: 
(i) The patient lacks functional T-lymphocytes and therefore fails to fight the 
infecting pathogens.  
(ii) Lymphocytes are extracted from the patient’s bone marrow and a normal 
functional copy of genes coding for ADA is introduced into these lymphocytes 
with the help of a retrovirus.  
(iii) The cells so treated are introduced into the patient’s bone marrow.  
(iv) The lymphocytes produced by these cells contain functional ADA gene and 
reactivate the victim’s immune system for life. 
 
12.  Ans. When the Nile Perch, a large predator fish, was introduced in Lake Victoria, it 
started feeding on the native cichlid fish. As a result, the cichlid fish became extinct 
and the Nile Perch, not finding any food, died too. 
 
13.  Ans. The Ti plasmid (tumour-inducing plasmid) of Agrobacterium tumefaciens has 
been modified to be used as a cloning vector. The Ti plasmid integrates as a segment 
of its DNA, TDNA, into the chromosomal DNA of its host plant cells which causes 
tumour. As gene transfer occurs without any human effort, the bacterium is known 
as the ‘natural genetic engineer’ of plants. Ti plasmids are used as vectors to transfer 
  
 
CBSE XII  |  Biology 
Board Paper 2011 – Delhi – Set 3 (Solution) 
 
     
the foreign gene of interest into the target plant cells so these are used in genetic 
transformation in plants. 
 
14.  Ans. Discharge of domestic sewage into rivers results in the rise of BOD because 
decomposer organisms consume a large amount of oxygen to decompose organic 
matter. When the amount of organic matter reduces, the amount of dissolved 
oxygen again increases. 
 
15.  Ans. The two types of immune systems in the human body are 
(a) Humoral or antibody-mediated immune system: This immune system operates 
by the production of humoral antibodies, so it is named humoral or antibody-
mediated immune system. The humoral antibodies in blood and lymph react 
with specific foreign substances (antigens) and promote their destruction. 
(b) Cell-mediated immune system: This immune system operates by the production 
of lymphocytes and macrophages. Hence, it is called cell-mediated. The T-
lymphocytes attack the pathogenic microorganisms which have entered the 
host’s body or grafts such as a transplanted kidney. It also protects the body 
from its own cells which have become cancerous.  
OR 
 
Elephantiasis is caused by the filarial worm Wuchereria (Wuchereria bancrofti 
and Wuchereria malayi). It usually affects the lymphatic vessels of the lower limbs  
                   and genital organs.  
                   Ringworm is caused by the fungi Microsporum, Trichophyton and  
                   Epidermophyton. It affects the skin, nails and scalp. 
 
16. Ans. Primary lymphoid organs are those organs where T lymphocytes and B     
lymphocytes mature and acquire their antigen-specific receptors. They include bone 
marrow and thymus. 
Secondary lymphoid organs are spleen, tonsils, Peyer’s patch of the small intestine 
and appendix. The lymphocytes interact with the antigen and proliferate to form a 
clone. 
 
17. Ans. CuT increases the phagocytosis of sperms in the uterus and copper ions 
released decrease sperm motility and prevent fertilisation. So, it is considered a 
good contraceptive device to space children. 
 
18. Ans. RNA interference (RNAi) is a system within living cells which helps control the 
activity of specific genes and is used as a method of cellular defence against 
parasites. This method involves silencing of a specific mRNA. The introduction of 
DNA produces both sense and antisense RNA in the host cells. These two RNAs, 
being complementary to each other, form double-stranded RNA which binds to and 
prevents the translation of mRNA. 
  
 
CBSE XII  |  Biology 
Board Paper 2011 – Delhi – Set 3 (Solution) 
 
     
SECTION C 
19. Ans.    
(i) A – AATTC (Recognition site of the restriction endonuclease); B – Ori (Origin of 
replication). 
(ii) A and C depict a palindromic nucleotide sequence. It is the sequence of base 
pairs which read the same when the orientation of reading is the same. 
(iii) PCR stands for polymerase chain reaction. 
Importance of PCR in biotechnology: 
a. It is used to detect HIV in suspected AIDS patients. 
b. It is used to detect gene mutations in persons suspected with cancer. 
 
20.  Ans. 
(i) Parasitism 
(ii) Mutualism 
(iii) Commensalism 
(iv) Mutualism 
(v) Commensalism 
(vi) Competition 
 
21. Ans. These are the lagging and leading strands of the replication fork.  
(i) During DNA replication, the DNA-directed RNA polymerase synthesises the 
primer strands of RNA for leading and lagging strands. The synthesis of the 
continuous strand of DNA is formed in the 5'–3' direction on the 3'–5' DNA 
template because of the addition of deoxyribonucleotides at the 3' end of primer 
RNA. This process occurs in the presence of DNA polymerase and ATP. Because 
one new strand is formed in a continuous stretch in the 5'–3' direction, this 
strand is called the leading strand. 
(ii) In the second parental strand, the enzyme primase forms the RNA primer. The 
enzyme DNA polymerase synthesises DNA in the form of short stretches once 
again in the 5'–3' direction starting from an RNA primer. These DNA short 
segments are called Okazaki fragments which are joined by the enzyme DNA 
ligase. This strand is called the lagging strand. 
 
 
 
 
 
 
 
 
 
 
 
Page 5


  
 
CBSE XII  |  Biology 
Board Paper 2011 – Delhi – Set 3 (Solution) 
 
     
CBSE 
Class XII Biology (Theory) 
Board Paper 2011 – Delhi (Set 3) 
SOLUTION 
Time: 3 hrs Total Marks: 70 
 
SECTION A 
1. Ans. AUG is the initiation codon which signals the start of translation, and UGA is the 
termination codon. When UGA comes into register with the A site, the protein-
releasing factors modify the specificity of the peptidyl transferase so that the water 
molecule is added to the peptide instead of another amino acid, causing the 
dissociation of ribosomal subunits. 
 
2.  Ans. Embryos are developed when the nucellar cells surrounding the embryo sac 
start dividing and protrude into the embryo sac. This condition of occurrence of 
more than one embryo in the seed is called polyembryony. Because orange is a 
citrus fruit, it shows the polyembryony stage, i.e. its seeds possess more than one 
embryo. Hence, when an orange seed is squeezed, many embryos of different shapes 
and sizes are seen. 
 
3. Ans. Plants are confined to the upper 200 metres strata of water because no light 
(and hence no photosynthesis) occurs beyond this depth. 
 
4. Ans. Brewer’s yeast (Saccharomyces cerevisiae) is used for fermenting malted 
cereals and fruit juices. 
 
5. Ans. Meiosis occurs in the zygote of an organism exhibiting the haplontic life cycle 
to produce haploid individuals. 
 
6. Ans. DNA is enclosed within membranes, so it is released by breaking cells. DNA 
along with other macromolecules such as RNA, proteins, polysaccharides and lipids 
are released by treating bacterial, plant, fungal or animal cells with enzymes such as 
lysozyme (bacteria), cellulose (plants) and chitinase (fungi). 
 
7. Ans. Aneuploidy is a condition of having fewer or extra chromosomes than the 
normal genome number of the species. Loss of chromosomes is called hypoploidy 
and the addition of chromosomes is called heteroploidy. 
 
8. Ans. Cryopreservation is preservation at -196°C in liquid nitrogen. It is useful in 
sperms, eggs, cells and embryonic tissues of animals. 
 
 
  
 
CBSE XII  |  Biology 
Board Paper 2011 – Delhi – Set 3 (Solution) 
 
     
 
SECTION B 
 
9. Ans. Deoxyribonucleoside triphosphate acts as a substrate, and it provides energy 
for the polymerisation reaction as its two terminal phosphates are high-energy 
phosphates. 
 
10.  Ans. 
 
Albuminous seeds Non-albuminous seeds 
(i) The seeds in which the 
cotyledons are thin and 
membranous and the food is 
stored in the endosperm are 
called endospermic or 
albuminous seeds. 
(ii) Examples: Wheat, maize 
(i) The seeds which are without 
endosperms are called non-
endospermic or non-albuminous 
seeds. 
 
(ii) Examples: Pea, gram 
 
 
11.  Ans. A hereditary disease can be corrected by gene therapy. Treatment of a genetic 
disorder by manipulating genes is called gene therapy. The first clinical gene 
therapy was given in 1990 to a 4-year-old girl with adenosine deaminase (ADA) 
deficiency. 
              Gene therapy of ADA deficiency: 
(i) The patient lacks functional T-lymphocytes and therefore fails to fight the 
infecting pathogens.  
(ii) Lymphocytes are extracted from the patient’s bone marrow and a normal 
functional copy of genes coding for ADA is introduced into these lymphocytes 
with the help of a retrovirus.  
(iii) The cells so treated are introduced into the patient’s bone marrow.  
(iv) The lymphocytes produced by these cells contain functional ADA gene and 
reactivate the victim’s immune system for life. 
 
12.  Ans. When the Nile Perch, a large predator fish, was introduced in Lake Victoria, it 
started feeding on the native cichlid fish. As a result, the cichlid fish became extinct 
and the Nile Perch, not finding any food, died too. 
 
13.  Ans. The Ti plasmid (tumour-inducing plasmid) of Agrobacterium tumefaciens has 
been modified to be used as a cloning vector. The Ti plasmid integrates as a segment 
of its DNA, TDNA, into the chromosomal DNA of its host plant cells which causes 
tumour. As gene transfer occurs without any human effort, the bacterium is known 
as the ‘natural genetic engineer’ of plants. Ti plasmids are used as vectors to transfer 
  
 
CBSE XII  |  Biology 
Board Paper 2011 – Delhi – Set 3 (Solution) 
 
     
the foreign gene of interest into the target plant cells so these are used in genetic 
transformation in plants. 
 
14.  Ans. Discharge of domestic sewage into rivers results in the rise of BOD because 
decomposer organisms consume a large amount of oxygen to decompose organic 
matter. When the amount of organic matter reduces, the amount of dissolved 
oxygen again increases. 
 
15.  Ans. The two types of immune systems in the human body are 
(a) Humoral or antibody-mediated immune system: This immune system operates 
by the production of humoral antibodies, so it is named humoral or antibody-
mediated immune system. The humoral antibodies in blood and lymph react 
with specific foreign substances (antigens) and promote their destruction. 
(b) Cell-mediated immune system: This immune system operates by the production 
of lymphocytes and macrophages. Hence, it is called cell-mediated. The T-
lymphocytes attack the pathogenic microorganisms which have entered the 
host’s body or grafts such as a transplanted kidney. It also protects the body 
from its own cells which have become cancerous.  
OR 
 
Elephantiasis is caused by the filarial worm Wuchereria (Wuchereria bancrofti 
and Wuchereria malayi). It usually affects the lymphatic vessels of the lower limbs  
                   and genital organs.  
                   Ringworm is caused by the fungi Microsporum, Trichophyton and  
                   Epidermophyton. It affects the skin, nails and scalp. 
 
16. Ans. Primary lymphoid organs are those organs where T lymphocytes and B     
lymphocytes mature and acquire their antigen-specific receptors. They include bone 
marrow and thymus. 
Secondary lymphoid organs are spleen, tonsils, Peyer’s patch of the small intestine 
and appendix. The lymphocytes interact with the antigen and proliferate to form a 
clone. 
 
17. Ans. CuT increases the phagocytosis of sperms in the uterus and copper ions 
released decrease sperm motility and prevent fertilisation. So, it is considered a 
good contraceptive device to space children. 
 
18. Ans. RNA interference (RNAi) is a system within living cells which helps control the 
activity of specific genes and is used as a method of cellular defence against 
parasites. This method involves silencing of a specific mRNA. The introduction of 
DNA produces both sense and antisense RNA in the host cells. These two RNAs, 
being complementary to each other, form double-stranded RNA which binds to and 
prevents the translation of mRNA. 
  
 
CBSE XII  |  Biology 
Board Paper 2011 – Delhi – Set 3 (Solution) 
 
     
SECTION C 
19. Ans.    
(i) A – AATTC (Recognition site of the restriction endonuclease); B – Ori (Origin of 
replication). 
(ii) A and C depict a palindromic nucleotide sequence. It is the sequence of base 
pairs which read the same when the orientation of reading is the same. 
(iii) PCR stands for polymerase chain reaction. 
Importance of PCR in biotechnology: 
a. It is used to detect HIV in suspected AIDS patients. 
b. It is used to detect gene mutations in persons suspected with cancer. 
 
20.  Ans. 
(i) Parasitism 
(ii) Mutualism 
(iii) Commensalism 
(iv) Mutualism 
(v) Commensalism 
(vi) Competition 
 
21. Ans. These are the lagging and leading strands of the replication fork.  
(i) During DNA replication, the DNA-directed RNA polymerase synthesises the 
primer strands of RNA for leading and lagging strands. The synthesis of the 
continuous strand of DNA is formed in the 5'–3' direction on the 3'–5' DNA 
template because of the addition of deoxyribonucleotides at the 3' end of primer 
RNA. This process occurs in the presence of DNA polymerase and ATP. Because 
one new strand is formed in a continuous stretch in the 5'–3' direction, this 
strand is called the leading strand. 
(ii) In the second parental strand, the enzyme primase forms the RNA primer. The 
enzyme DNA polymerase synthesises DNA in the form of short stretches once 
again in the 5'–3' direction starting from an RNA primer. These DNA short 
segments are called Okazaki fragments which are joined by the enzyme DNA 
ligase. This strand is called the lagging strand. 
 
 
 
 
 
 
 
 
 
 
 
  
 
CBSE XII  |  Biology 
Board Paper 2011 – Delhi – Set 3 (Solution) 
 
     
22.   Ans. 
(a)   
 
(b) Sporopollenin is considered the most resistant organic material because it can 
withstand high temperatures, strong acids and alkalis and cannot be degraded 
by any enzyme. 
 
23.  Ans. 
Dominance Co-dominance Incomplete dominance 
When two different 
factors or a pair of 
contrasting forms of a 
character are present in 
an organism, only one 
expresses itself in the F 1 
generation and is termed 
dominant, while the other 
remains unexpressed and 
is called the recessive 
factor. 
When both alleles are 
present together in a 
heterozygous organism 
and express their traits 
independently instead of 
showing a dominant–
recessive relationship, 
they are called co-
dominant alleles and the 
phenomenon is called co-
dominance. 
Incomplete dominance is 
the phenomenon where 
none of the two 
contrasting genes or 
factors are dominant, 
and the expression of the 
character in a F 1 hybrid 
individual is of the 
intermediate type.