Std. XII (CBSE) − Biology : Solution to Prelim Test (Full Length Test)
SECTION − A1. The anterior portion of head of the sperm is covered by a cap−like structure called acrosome. Function :
Enzymes present in acrosome help in fertilization of the ovum. [1 M]
2. The DNA polymerase used in PCR is called as Taq DNA polymerase. Its source organism is a
thermophilic bacterium Thermus aquaticus. The characterific feature of Taq DNA polymerase is that it is heat stable and remains active at high temperature induced denaturation of DNA required during PCR.
[1 M]
3. Allergy is inappropriate immune response of a person to harmless substances coming in contact with the
body or entering the body from the environment, or in food, or in medicine. The substances which cause allergic reaction are called allergens. They are generally weak antigens. [1 M]
4. Wuchereria bancrofti. [1 M]
5. IgE antibody. [1 M]
SECTION − B
6. In blood group 3 alleles IA, IB and li control a single trait blood group so it is an example of blood
grouping.
In AB blood group IA and IB are codominant so, it is an example of co-dominance. [2 M]
7. Mutualism is seen between them because both are equally benefitted. Female wasp lays eggs in
fruits and uses developing seeds within the fruit for nourishing its larvae. In return, the wasp pollinates the fig inflorescence, while searching for suitable egg laying site. [2 M]
8. [2 M]
Incomplete dominance Co−dominance
1. In this type of inheritance both the genes of an allelomorphic pair express themselves partially.
1. In this type, both the genes of an allelomorphic pair express themselves equally in F1 hybrid.
2. One allele cannot suppress the expression of the other completely.
2. The alleles in the pair are able to express themselves independently even if present together in F1 hybrid.
3. The phenotype of hybrids is intermediate between the phenotypes of parents.
3. The phenotypes of hybrids equally express the phenotypes of parents (independtly). 4. e.g. In four o’clock plant, when two
pure varieties / white and Red, are crossed, F1 hybrids that are produced
are pink.
4. e.g. coat colour in cattle, one with red coat and the other with white coat. When they are crossed F1 hybrids have roan colour
(mixture of red and white).
9. 1. When ultraviolet (UV) radiation falls on the cells of living organisms it is absorbed in the DNA and
proteins present in the nucleus.
2. The high energy of UV radiations break the chemical bonds within these molecules. 3. This results in damage to the skin cells and cause skin cancers of various types.
4. High doses of UV−B radiations also cause inflammation of cornea called slow blindness and
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10. i) In band ‘d’, DNA fragments are smaller than those on band ‘c’. The fragments separate according to
their size through the sieving effect provided by the gel. So, the smaller fragments move farther away than the larger ones.
ii) End towards, ‘b’.
iii) Gel containing DNA fragments is stained with eithidium bromide and exposed to UV radiation.
Orange colour bands of DNA are visible. [2 M]
OR
i) Entamoeba histolytica is a protozoan parasite in the large intestine of human, which causes
amoebiasis (amoebic dysentery) [1/2 M]
ii) Diagnostic symptoms
a) Abdominal pain and cramps
b) Stools with excess mucous and blood clots. [1 M]
iii) The disease is transmitted by houseflies. They act as mechanical carrier, transmit the parasite from faeces of infected persons of food products, thereby contaminating them. [1/2 M]
11. The replication where daughter DNA molecule is a hybrid containing one old DNA strand and one old
DNA strand. Since every daughter DNA molecule retains one of the old strands, replication is said to be semi conservative.
Since every daughter DNA molecule retains one of the old strands, replication is said to be semi conservative.
Mathew Meselson and Franklin stahl (1958) proved it experimentally. [2 M]
12. (i) Operon : can be defined as group of genes with related functions or it is segment of genetic material
(DNA) which functions as regulated unit.
(iii) Repressor : It is a small protein formed by regulator gene which binds to operate gene and prevents passage of RNA polymerase and thereby transcription. [2 M]
SECTION − C 13. i) Strategies to prevent self−pollination
(a) Pollen grain release and stigma receptivity both are not synchronized. Either the anther matures
first or the stigma.
(b) Anther and stigma are placed at different positions so that the pollen cannot come in contact with
the stigma of same flower.
(c) A genetic mechanism called self−incompatibility, which prevents self−pollination from fertilising the ovule either by inhibiting pollen germination or by retarding the growth of pollen
tube.
(d) By producing unisexual flowers. Plants are dioecious and continued self−pollination may cause
inbreeding depression.
ii) Geitonogamy is referred to as genetically autogamy because the pollen grains come from the same
plant, though from a different flower. [3 M]
14. (i) Fimbriae are the feathery finger−like projections present at the end of Fallopian tubes and it
collects the ovum after it is released from the ovary into the Fallopian tube.
(ii) If the egg is fertilized, Corpus luteum secretes progesterone to maintain pregnancy and relaxin
towards the end of pregnancy.
(iii) Oxytocin is a hormone secreted by the posterior pituitary and it stimulates the contraction of
uterine muscles during child birth (parturition). [3 M]
15. Lac operon consists of
(i) an operator, which control all structural gene as a unit (ii) a regulatory gene (i gene)
(iii) three structural genes (z, y, a) which codes for enzymes
The regulatory gene codes for a repressor protein and repressor has high affinity for the operator region and prevent RNA polymerase from transcribing the structural gene i.e., the lac operon is switched off or inactive.
Lac operon in presence of inducer
When inducer (lactose) is added it binds with repressor protein and inactivate it. This allow RNA polymerase access to promoter and transcription proceeds. [3 M] 16. (i) Morphine is obtained from the latex of Papaver somniferum. It is a depressant, which slows down
the body functions.
(ii) Cocaine is obtained from Erythroxylom coca. It is a stimulant and produces a sense of euphoria and
increased energy.
(iii) Marijuana is obtained from the inflorescence of Cannabis sativa. It affects cardiovascular system of
the body. [3 M]
17. The principle of vaccination is based on the property of memory of the immune system. In vaccination, a
preparation of antigenic proteins of pathogen or inactivated / live but weakened pathogen is introduced into the body. The antigens generate primary immune response by producing antibodies.
Vaccines also generate memory B cells and memory T−cells when vaccinated person is attacked by same pathogens, the existing memory B cells and memory T−cells recognise the antigen quickly and over whelm the invaders with manire production of lymphocytes and antibodies. [3 M] 18. Vectorless gene transfer is a method to introduce recombinant DNA into recipient cells of host without
involving carrier molecule. [1 M]
Two methods of vectorless gene transfer are : [2 M]
i) Microinjection : It is the introduction of foreign gene into plant cell or animal cell by using microneedles or micropipettes.
ii) Electroporation : In this method, electrical impulses induce transient pores in the plant cell membrane through which the DNA molecules are incorporated into the plant cells.
19. (i) ‘a’ depicts conformers. [1/2 M]
(ii) The other line ‘b’ depicts regulators. [1/2 M]
(iii) Differences between conformers and regulators
Conformers Regulators
These cannot maintain a constant integral environment and change according to the ambient atmospheric conditions.
These organisms maintain a constant internal environment despite changes in the environment. They show a narrow range of distribution. They show a much wider range of distribution.
[1M]
(iv) Humans are regulators. [1M]
20. (i) BOD : Biochemical Oxygen Demand [1 M]
(ii) In the river near a sugar factory, lot of organic matter is added. Microorganisms degrade it in the water body. Due to maximum consumption of oxygen by microbes during degradation process, dissolved oxygen gets reduced. This leads to mortality of fish and other aquatic life. [1 M]
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(iii) When organic matter will decrease and microbes do not require oxygen to decompose it, the BOD will decrease. As a result, aquatic life will again start flourishing. [1 M]
21. Downs syndrome is due to Trisomy of chromosome 21. [½ M]
Turners syndrome is due to monosomy of X chromosome (XO). [½ M]
Difference between the two is, in Down syndrome autosomes are involved and total number of
chromosome is 47. [½ M]
In turners syndrome sex chromosome (X−chromes) is involved. Total number of chromosome is 45.[½ M]
22. i) a − Protein coat [½ M]
ii) b − Reverse transcriptase
c − It is viral DNA [1 M]
iii) Macrophage (animal cell) [½ M]
iv) Macrophages and helper T−cells. [1 M]
23. a) No. 4 is camer female (x4 x) No. 5 is Haemophiliac boy (xh y) and No. 6 is Normal boy (xy) [1 M]
b) Normal male (xy) carrier Female (x4 x)
Hence the chances of first child to be haemophiliae is 25% (1/4th) [1 M] 24. Two different types of parent strands function as template strands.
On the template strand with 3 → 5 polarity, the new strand is synthesized as a continuous strand. The enzyme DNA polymerase can carry out polymerization of the nucleotides only in 5 → 3 direction. This is called continuous synthesis and the strand is called leading strand on the other template strand with 5 → 3 polarity, the new strand is synthesised from the point of replication fork, also in 5 → 3 direction. But, in short fragments, they are later joined by DNA ligases to form a strand called lagging strand.
[3 M] OR
i) Parasitism ii) Mutualism iii) Commensalism
iv) Mutualism v) Commensalism vi) Competition [3 M]
SECTION − D
25. Primary endosperm nucleus (PEN) is the product of second fertilization, in Angiosperms and develops
into endosperm.
Endosperm is the nutritive tissue for the nourishment of developing embryo.
It is a post fertilization product and is triploid (3n). Development of embryo and endosperm takes place simultaneously. As the development of embryo takes place close to each other, proper nutrition of embryo and endosperm takes place close to each other, proper nutrition of embryo is ensured.
In seeds like Pea, Bean endosperm is totally consumed during embryo development, hence mature seeds do not possess endosperm and therefore called non endospermic seeds.
In other Angiospermic seeds like Castor, Wheat, Maize, the endosperm persists in the mature seeds and it continues to support the growth of embryo during germination − such seeds are called endospermic seeds. Depending upon the mode of development of endosperm, three types of endosperm are recognized. They are as follows :
(a) Nuclear endosperm (b) Cellular endosperm (c) Helobial endosperm
xh x xx xh x xy Carrier Female (boy) Normal Female (boy) Haemophliae Male (boy) Normal Male (boy)
(a) Nuclear endosperm :
In this type the primary endosperm nucleus undergoes free nuclear division (Karyokinesis) and nuclear divisions are not followed by cytokinesis. As a result, large number of triploid nuclei, which are freely suspended in common cytoplasm of embryo sac are formed. (See figure (a)).
Later on, wall formation occurs around these nuclei in a centripetal manner to form cellular mass. It is the most common type of endosperm. In coconut, cell wall formation of endosperm is never found complete. e.g. Maize, sunflower etc.
(b) Cellular Type : e.g. Datura, Pentunia etc.
It is less common and seen mostly in dicotyledons. In this type the primary endosperm nucleus undergoes nuclear division, immediately followed by cytokinesis. Therefore, endosperm development occurs in cellular form right from the beginning (See figure (b)).
(c) Helobial Endsoperm :
It is an intermediate type which is common in Helobial series of monocots and hence named accordingly.
In this type, the first division of primary endosperm nucleus is followed by wall formation. As a result, the central cell is divided into a large micropylar and a small chalazal chamber. In both the chambers further development of endosperm occurs a nuclear type (see figure (c)). Hence, it is called, intermediate between Nuclear and Cellular type. E.g. Vallisneria.
[5 M]
OR
Sperms are microscopic, elongated, haploid motile male gametes measuring about 0.055 mm in length. • Sperms are produced by the process of spermatogenesis.
• Sperms can remain viable for 72 hours but can fertilize the ovum in the first 12 to 14 hours only. • Typically, a sperm shows 4 parts :
1. Head : It is a flat and oval region consisting of a large nucleus and an acrosome. Acrosome secretes hydrolytic enzymes like hyaluronidase which helps in penetration of the egg during fertilization.
2. Neck : It is a very short region having two centrioles. The proximal centriole plays a role in first cleavage of zygote. The distal centriole give rise to the axial filament of the sperm.
3. Middle piece : It serves as a powerhouse for sperm. It has many mitochondria spirally coiled around the axial filament. Mitochondria provide energy for the movement of the sperm in the female genital tract. Sperm swims at a speed of 1.5 to 3 mm per minute. 4. Tail : It is a long, slender, tapering structure formed of
cytoplasm, Spermatozoa swim about by the lashing movement of the tail.
Development of endosperm
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26. i) The differences between leading strand and lagging strand of DNA are as follows :
Leading strand Lagging strand
i) It is a replicated strand of DNA which grows continuously without any gap.
Lagging strand is a replicated strand of DNA which is formed in short segments called Okazaki fragments. Its growth is discontinuous. ii) It does not require DNA ligase for its
growth.
DNA ligase is required for joining Okazaki fragments.
iii) Only a single RNA primer is required. Starting of each Okazaki fragment requires a new RNA primer.
ii) The differences between euchromatin and heterochromatin are as follows :
Euchromatin Heterochromatin
i) These are the regions where chromatin is loosely packed.
These are the regions where chromatin is densely packed.
ii) It stains lighter. It stains darker.
iii) This is transcriptionally more active. It is transcriptionally active or inert. iii) Histones are rich in basic amino acids lysine and arginine, hence they are positively charged.
iv) Splicing [5 M]
OR
i) Gene therapy is collection of methods that allow correction of gene defect that has been diagnosed in child or an embryo.
Gene therapy may be germ line gene therapy or somatic gene therapy.
It was used in 1990 on 4yr old girl having enzyme adenosine deaminase deficiency (ADA
deficiency) [2½ M]
ii) Bioreactors are used for processing large volume of culture for obtaining the product of interest in large quantities.
In bioreactors, raw materials are biologically converted into specific products eg. enzymes.
Commonly used bioreactor is stirred tank reactor. It is cylindrical with curved base Bio reactor has
agitator system, an [2½ M]
27. i) 1. Sewage :
The waste water which is generated in cities and towns in called sewage. The sewage generally contains human excreta, domestic wastes and microbes.
2. Sewage treatment :
There are three types of sewage treatment, viz., primary treatment, secondary treatment and tertiary treatment.
(i) Primary treatment :
The primary treatment involves the removal the removal of large pieces of floating debris, oily substances, etc. through processes like sedimentation and filtration. The primary treatment therefore is a physical process.
(ii) Secondary treatment :
a) The secondary treatment is a biological treatment.
b) In this, the primary effluents are transferred to the large aeration tanks where they are constantly agitated mechanically and air is pumped into them.
(iii) Owing to this the useful aerobic microbes grow into flocs.
(iv) The flocs contain bacteria and fungal hyphae which consume the major part of the organic matter present in the effluent. This reduces the BOD of the effluent. This reduces the BOD of the effluent.
3. Tertiary treatment :
(i) The secondary treatment of sewage is followed by the tertiary treatment.
(ii) In this, the waste water whose BOD is reduced during secondary treatment is then passed into a settling tank in which the bacterial flocs are allowed to sediment. This sediment is known as activated sludge.
(iii) A small portion of this activated sludge is passed into the aeration tank while its major ortion is pumped into the large tanks called anaerobic sludge digesters.
(iv) In large tanks, anaerobic bacteria grow and digest the bacteria and fungi of the sludge. During the process of digestion gases like methane, CO2 and H2S are produced.
ii) Antibiotic Microbial source
1. Penicillin Penicillium chrysogenum 2. Streptomycin Streptomyces griseus
[5 M]
OR
Gene amplification is any duplication of a region of DNA that contains a gene.
The polymerase Chain Reaction (PCR) is a scientific technique in molecular biology to amplify a single or a few copies of a piece of DNA, across several orders of magnitude, generating thousands to billions of copies of a particular DNA sequence.
It was first developed by Kary Mullis (1983) and he was awarded Noble Prize in chemistry for his work on PCR.
PCR is very specific, the targeted DNA sequence can be less than one part in a million of the total DNA sample − a single gene can be copied by using PCR.
It is named as PCR from DNA polymerase which is a enzyme that carries out DNA replication in a cell and it is a chain reaction as DNA polymerase carries out replication over and over again until there are millions of copies of the targeted DNA.
Basic requirement of PCR :
(i) A DNA fragment to be copied.
(ii) Two primers: they are needed to start the reaction.
(iii) A thermostable DNA polymerase enzyme that can withstand temperature upto 94° C − Taq polymerase isolated from bacterium Thermus aquaticus.
(iv) 4 types of dNTP’s (deoxyribonucleotides):
Most of the PCR methods use Thermal cycling (alternate heating and cooling DNA sample) and each cycle has following three steps:
(a) Heat denaturation: Temperature required 92°C to 94°C which breaks the hydrogen bonds of DNA sample to get single stranded DNA.
(b) Annealing: Sequence specific primers are linked to the DNA segment by lowering the temperature to 55°C.
(c) Extension: of the DNA strand is done by addition of dNTP’s behind the primers with the help of DNA polymerase at 72° to 75° C.
Applications of PCR technique:
It is indispensable technique used in medical and biological research labs.
It is used in DNA cloning, gene amplification functional analysis of genes, diagnosis of hereditary diseases. Identification of genetic fingerprints the detection and diagnosis of infectious diseases and
diagnosis of cancer. [5 M]
