Oswal Practice Papers CBSE Class 12 Biology Solutions (Practice Paper - 10)

Section-A

1. (d) Both (b) and (c)

Explanation :

Micropyle is the small minute pore which is differentiated from the surface of the egg. It is formed by the projection of integuments into which the male gamete through pollen tube enters into the egg of the ovule. It is usually located at the top of the seed or ovule.

2. (c) 22

Explanation :

Endosperm is triploid (11 × 3 = 33) and antipodal cells are haploid (11 × 1 = 11). So, the diploid number of chromosomes will be, 22 (11 × 2 = 22).

3. (c) 1 : 1 : 1 : 1

Explanation :

The given cross is a test cross. The ratio for a test cross is always 1:1 for a monohybrid cross and 1:1:1:1 for a dihybrid cross.

4. (b) DNA dependent DNA polymerase catalyses polymerisation only in one direction (5’ → 3’)

Explanation :

DNA dependent DNA polymerase catalyses polymerization only in one direction (5’→ 3’). Consequently the replication on template with polarity 3’ to 5’ is continuous while the other template with polarity 5’ to 3’ is discontinuous and is synthesized in Okazaki fragments which later join by DNA ligase.

5. (d) 100 per hour

Explanation :

Population of Paramecium shows 100 per hour growth, i.e. two individuals are produced by one.

6. (b) Cancer

Explanation :

Cancer is the most fatal non-infectious disease. In 2017, 9.6 million people were estimated to have died from various forms of cancer. While AIDS is the most fatal infectious disease affecting nearly 36.9 million people world-wide. Diabetes and obesity are also non-infectious diseases but not the most fatal.

7. (c) X: Lactic acid

Explanation :

Bacteria (usually Streptococcus thermophilus and Lactobacillus bulgaricus) are used to ferment milk. Lactic acid is produced which helps to coagulate the milk.

8. (d) II and IV

Explanation :

Influenza is an infectious disease caused by influenza viruses. Cancer is a non-infectious disease. Smallpox is a serious, highly contagious disease. It is caused by the Variola Virus. Allergy is the exaggerated response of the immune system to certain antigens. It is a non-infectious response.

9. (a) Only I and II are true
10. (c) Cobra

Explanation :

Frog is prey to Cobra. Neither rabbit, parrot nor wolf eats frog.

11. (c) Amphibians

Explanation :

Amphibians have the highest percentage presently, 32% of all amphibian species in the world face the threat of extinction. Other than these, 23% of all mammal species and 12% of all bird species are also facing the risk.

12. (a) ‘a’: Antigen binding site, ‘b’: Light Chain,‘c’: Heavy chain

Explanation :

Each antibody molecule has four peptide chains, two small, called light chains and two, longer called heavy chains. Hence, is represented as H₂L₂.

13. (c) A is true but R is false.

Explanation :

In monosporic type of development of embryo sac, only one megaspore situated towards chalazal end remains functional. This type of embryo sac development occurs in a majority of flowering plants and the common example is Polygonum. Thus assertion is true, but reason is false.

14. (b) Both A and R are true and R is not the correct explanation of A.

Explanation :

A single mRNA strand is capable of forming different polypeptide chains because it has different reading frame, (the way through which reading of mRNA done by tRNA). Thus both assertion and reason are true but reason is not the correct explanation of the assertion.

15. (b) Both A and R are true and R is not the correct explanation of A.

Explanation :

In the structure of Immunoglobulin, two heavy and light chains are present and connected by disulphilde linkages. Thus both assertion and reason are true but reason is not the correct explanation of the assertion.

16. (d) A is false but R is true.

Explanation :

Biofertilizers are eco-friendly as they don’t hamper the microbial flora and pH of soil while chemical fertilizers can harm microbes present in the soil and alter the pH which will not be suitable for the growth of plants. Thus bio-fertilizers are preferred over chemical fertilizers. Thus assertion is false but
reason is true.

Section-B

17. (a) Endosperm accumulates the food reserves and functions as the nutritive tissue for the developing embryo. It develops from the triploid primary endospermal cell by repeated cell divisions. Its development starts shortly before the development of the embryo.

(b) Endosperm is formed by three methods:

Nuclear Type : In this, primary endosperm nucleus divides repeatedly by mitosis and number of free nuclei arrange themselves at the periphery of embryo sac, leaving a central vacuole. The nuclear division is not accompanied by the cell wall formation. But later on, protoplast collects around each nucleus and then walls are formed. So the endosperm becomes cellular at the maturity e.g., wheat, rice, maize and sunflower.
Cellular Type : In this, the repeated mitotic division of primary endosperm nucleus is followed by cytogenesis around each nucleus, so the endosperm becomes cellular from the beginning. e.g., Datura and Petunia.
Helobial Type : In this, the first mitotic division of primary endosperm nucleus is followed by incomplete cytokinesis and the endosperm is formed both by cellular and nuclear type. E.g. Eremurus.

18. To explain the concept, we can take an example of body colour Grey (G) and Albino (g). As per the question a homozygous female (gg) having albino colour is crossed with heterozygous male (Gg) having Grey colour. By using punnett square the following conclusions can be derived,

Conclusion—F₁ generation will show 2 : 2 Albino : Grey as the phenotypic ratio.

19. The river decomposers are most active at site B. The number of bacteria will suddenly rise near the sewage pumping station. They are effective decomposers and can breakdown complex molecules into simpler forms in sewage.

20. (i) This diagrammatic representation depicts various trophic levels that depend on others in an ecosystem.
(ii) ‘A’ denotes Producers, example. plants, ‘B’ denotes Primary consumers, example: sheep, deer, goat etc. ‘C’ denotes Secondary consumers, example: lion, cheetah etc. and ‘D’ denotes Decomposers that ultimately consume the dead plant and animals to form humus.

21. (a) Saccharomyces cerevisiae → ethanol
(b) Monascus purpureus → statin.

Patient	Disease	Causative agent	Mode of infection
Patient 1	Ascariasis	Ascaris	A healthy person acquires this infection through contaminated water, vegetables, fruits, etc.
Patient 2	Ringworm infection	Fungus (Microsporum,Trichophyton)	Ringworms are generally acquired from soil or by using towels, clothes or even the comb of infected individuals
Patient 3	Typhoid	Salmonella typhi	Contaminated food and water
Patient 4	Malaria	Plasmodium	Bite from female Anopheles mosquito
Patient 5	Amoebic dysentery	Entamoeba histolytica	Contaminated food

Section-C

22. (a) Double fertilization:

One male gamete fuses with the egg cell in the embryo sac to form zygote (2n) by the process of syngamy.
The other male gamete fuses with the two polar nuceli to form PEN (Primary Endospern Nucleus) (3n) by the process of triple fusion.
Both syngamy and triple fusion together are known as double fertilisations.

(b) The endosperm contains reserve food material which is used for nutrition of developing embryo.

(c) A typical dicot embryo consists of an embryonal axis and two cotyledons. The portion of the embryonal axis above the level of cotyledons is the epicotyl which terminates with the plumule. The portion below the level of the cotyledons is the hypocotyl that terminates at its lower end with the
radical which is covered by a root cap.

23. (a)

A diploid egg is formed without reductional division which directly develops into embryo without fertilisation.
Some diploid cells of the nucellus start dividing and develop into a embryo.

(b) Advantages:

No segregation of characters in hybrid progeny.
Apomictic hybrid can be used to grow crop year after year.
It is economical as ordinary hybrid seeds that are costly.

Disadvantages:

They lack ability to adapt to changing environment.
Cannot control deleterious genetic mutation.

(c) Hybrid seeds are costly as farmers have to purchase seeds year after year. Production of hybrid seeds is a highly technical and expensive method. It requires intensive labour and is not easily feasible.

24. (a) Bt-cotton has been genetically modified by the insertion of one or more genes from a common soil bacterium, Bacillus thuringiensis. These genes encode for the production of insecticidal proteins and thus, genetically transformed plants produce one or more toxins as they grow.

(b) Yes, Bt-toxins are highly specific. The toxins produced by Bt-cotton and coin are toxic to a selected number of arthropod species because cotton is primarily a fiber crop, the contamination of food with toxins from cotton is highly unlikely.

(c) Bacillus thruingiensis forms protein crystals during particular phase of growth. These crystals contain toxic insecticidal protein which exist as inactive protoxins but once insect ingest inactive toxin it is converted into an active form of toxin due to alkaline pH of the gut which solubilize crystals.
Activated toxin binds surface of midgut epithelial cells and excrete pore.

25. (a) The incidence of Down syndrome is correlated with maternal age; older women are more likely to become pregnant with foetuses carrying the trisomy 21 genotype.

(b) Short stature, round head, mental retardation etc.

26. 26. (a) The genetic material should be able to carry out replication or generate a replica.
(b) It should be chemically or structurally stable.
(c) It should provide scope for slow mutation.
(d) It should be able to express itself as characters.

Out of the two, clearly, DNA is more stable because of the following factors:
(a) Presence of H and not OH at 2’ position.
(b) Presence of thymine instead of uracil.
(c) It is less reactive.
(d) It is structurally more stable because of its double stranded structure with hydrogen bonding.
(e) DNA is slower to mutate than RNA.
(f) Complementary strands of DNA further resist changes by evolving a process of repair.

(a) Sickle cell anaemia is an autosomal recessive trait disease than can be transmitted from parents to the offspring when both the partners are carrier for the gene. The disease is controlled by a single pair of allele, Hb^A and HB^S. Out of three possible genotypes only homozygous individuals for HbS
(Hb^S Hb^S) show the diseased phenotype while heterozygous (Hb^AHb^S) individuals are carrier of the disease.

(b)

The affected child is male.

27. (a) Rosie was the first transgenic cow and it was produced in 1997. The gene for human protein a-lactalbumin was introduced in this cow. Thus, the cow could produce protein-enriched milk.

(b) Rosie cow was considered different from a normal cow as it produced human protein enriched milk. The milk contained ‘human alpha-lactalbumin’ and was nutritionally a more balanced product for human babies than a normal cow milk.

28. (a) Those plants, bacteria, fungi or animals whose genes have been altered by manipulation are called Genetically Modified Organisms (GMOs).

(b) Advantages of GMO to a farmer:

Tolerance to abiotic stresses like cold, drought, salt, heat, etc.
Reduce reliance on chemical pesticides.
Reduced post harvest losses.
Increased efficiency of mineral usage by plants.
Enhanced nutritional value.
To create tailor-made plants.

Section-D

29. (a) F₂ generation :

Gametes	Ry (male)	rY (male)	Ry (male)	ry (male)
RY (Female)	RRYY Round Yellow	RrYY Round Yellow	RRYy Round Yellow	RrYy Round Yellow
rY (Female)	RrYY Round Yellow	rrYY Wrinkled Yellow	RrYy Round Yellow	rrYy Wrinkled Yellow
Ry (Female)	RRYy Round Yellow	RrYy Round Yellow	RRyy Round Green	Rryy Round green
ry (Female)	RrYy Round Yellow	rrYy Wrinkled Yellow	Rryy Round Green	rryy Wrinkled Green

(b) Conclusion of the cross is:
Round Yellow = 9 Round Green = 3 q Wrinkled yellow = 3 q Wrinkled green = 1 Hence, the phenotypic Ratio is 9: 3: 3: 1.
(c) The result of the genetic cross indicates probability of all genotypes and phenotypes along with their ratio.

The genotype is genetic structure. The genotype ratio of Punnet Square is 1:2:2:4:1:2:1:1:2 ( i.e., RRYY- 1, RRYy-2, RrYY-2, RrYy-4, RRyy-1, Rryy-2, rrYY-1,rryy - 1 and rrYy-2).

30. (a) The graph shows the increasing demand for organic food, worldwide farming and also shows the area under global organic farming increased from 43.7 million hectares in 2014 to 64.2 million hectares in 2017.

(b) Biofertilizers are defined as preparations containing living cells or latent cells of efficient strains of microorganisms that help crop plants for the uptake of nutrients by their interactions in the rhizosphere.

(c) Organic farming beneficial to us because it involves the use of biofertilizers and biopesticides which increase the nutrient quality of the crop and controls any kind of pest and pathogen.

(c) We should prefer biofertilizers instead of chemical fertilizers because Biofertilizers are the microorganisms that add to the nutrient quality of the soil. Bacteria, fungi, and algae are some of the beneficial microorganisms that help in improving the fertility of the soil. They are also cost effective
and eco-friendly.

Section-E

31.

Method	Scientific term	Mechanism of action
Fertility awareness	Periodic abstinence	Requires a good understanding of the fertile and infertile phases of the woman’s menstrual cycle. Abstain from sexual intecourse during fertile period.
Breastfeeding	Lactational amenorrhoea method	In this case, there is a delay in ovulation caused by the action of prolactin, in turn, inhibits the secretion of follicle stimulating hormone (FSH) and luteinizing hormone (LH), and as a result ovulation does not occur.
Withdrawal or pulling out	Coitus interruptus	Coitus interruptus prevents fertilisation by stopping contact between spermatozoa in the sperm and the ovum or egg.

1. Scrotum or Scrotal sac
2. Testicular lobule
3. An Acrosome
4. Zona pellucida
5. 28-29 days

32. (a) The above mentioned test shows the maternal marker that has been passed to the child is 6 and the alleged father matches the child, since one of his markers is indeed 7.

(b) DNA finger printing: DNA finger printing is a technique that shows the genetic makeup of living things. It is a method of finding the difference between the satellite DNA regions in the genome. Alec Jeffreys developed this technique in which he used satellite DNAs also called VNTRs (Variable
Number of Tandem Repeats) as a probe because it showed the high level of polymorphism. Following are the steps involved in DNA fingerprinting:

Isolating the DNA.
↓
Digesting the DNA with the help of restriction endonuclease enzymes.
↓
Separating the digested fragments as per the fragment size by the process of electrophoresis.
↓
Blotting the separated fragments onto synthetic membranes like nylon.
↓
Hybridising the fragments using labelled VNTR probes.
↓
Analysing the hybrid fragments using autoradiography.

(a) The graphs above show the number of the male (a) and female (b) with haemophilia in different age intervals. The possibility of a female becoming a haemophilic is extremely rare.

(b) Morgan’s work is founded on the basis of the observations during experiments on fruit flies that is Drosophila melanogaster. He structured the chromosomal theory of linkage. He defined linkage as the co-existence of two or more genes in the same chromosome and performed dihybrid crosses in Drosophila to show that linked genes are inherited together and are located on X-chromosome. His experiments have also justified that tightly linked genes show very low recombination while loosely linked genes show higher recombination.

33. The PCR technique begins with denaturation of desired DNA into two strands, each acting as a template. For each strand, separate set of primers are used. DNA polymerase extends the primers using nucleotides provided in the reaction. If the process of replication of DNA is repeated many times, the segment of DNA can be amplified to approximately billion times. Such repeated amplification is achieved by the use of a thermostable DNA polymerase which is isolated from a bacterium Thermus aquaticus.

Denaturation—Double helical DNA is denatured by providing high temperature (95-degree Celsius). DNA polymerase does not get degraded in such high temperature. The DNA polymerase used in this reaction is thermostable and is isolated form the thermophilic bacteria, Thermus aquaticus (Taq).

Annealing: It is the step in which primers are annealed to single stranded DNA templates. Two sets of primers are used. The temperatures of the reaction mixture is lowered to 50-60°C for some seconds to allow annealing of primers. DNA polymerase extends the primer in 5’ to 3’direction.

Extension—Replication of DNA occurs in vitro. This cycle is repeated several times to generate up to 1 billion identical copies of the DNA.

(a) A depicts the algal or phytoplankton population whereas B depicts aquatic animals. The growth pattern of A is because of excessive lodging of nutrients from the adjacent agricultural land leaching the nutrients here.

(b) The result of above graph will be eutrophication, decrease in BOD, unpleasant odour and death of aquatic ecosystem.

(c) Above graph formation occurs when the water body becomes overly enriched with nutrients. Various factors like excessive use of fertilisers, untreated sewage, usage of detergents containing phosphorous and industrial discharge of waste may contribute to eutrophication.