Oswal Practice Papers CBSE Class 12 Biology Solutions (Practice Paper - 7)

Section-A 

1. (a) Rosaceae, Leguminoseae and Solanaceae

Explanation :    

In some members of the families Rosaceae, Leguminosae and Solanaceae pollen grains maintain viability for a month due to sporopollenin which protects pollen grains from external factors such as temperature, acids soil and sediments etc.

2. (b) The functional haploid megaspore undergoes three successive mitotic divisions.

Explanation :    

In the monosporic (Polygonum) type of embryo sac development, only one megaspore near the chalazal end remains functioning. The final three megaspores gradually deteriorate and then disappear. The functional haploid megaspore grows in size and develops into an eight-nucleate embryo sac through three sequential mitosis. Most flowering plants develop their embryo sacs in this way.

3. (a) A : 10; C : 40

Explanation :    

In double stranded DNA,
Adenine (A) pairs with Thymine (T) with two hydrogen bonds and Guanine (G) pairs with Cytosine (C) with three hydrogen bonds.
Thus, the amount of A is equal to T and the amount of G is equal to C.
So, out of 100 base pairs, if 40 are G, then C will also be 40.
As (A + T) + (G + C) = 100, so A + T = 100 – (40 + 40) = 100 – 80 = 20
and A = T = 20/2 = 10
So, nucleotides having A are 10 and nucleotides having C are 40.

4. (b) X-body

Explanation :    

In a few insects, Henking (1891) was able to observe a specific nuclear structure during spermatogenesis. He also noticed that 50% of the sperm obtained this structure after spermatogenesis, while the other 50% did not. Henking gave to this structure name as the X body, but he could not explain its significance.

5. (d) Use and disuse of organs.

Explanation :    

Five factors are known to affect Hardy-Weinberg equilibrium. These are gene migration or gene flow, genetic drift, mutation, genetic recombination and natural selection.

6.(a) Adaptive radiation

Explanation :    

The process of evolution of different species in a given areas starting from a point and radiating to other geographical areas (habitats) is called adaptive radiations.
Example : Darwinian finches and Australian marsupials

7. (a) 3’end

Explanation :    

tRNA has 4 arms.

  1. Arm with TyC loop which has Pseudo-uridine and ribothymidine nitrogen bases. It is the site for attachment to ribosomes.
  2. Arm with DHU loop has Dihydrouracil base and it is site for attachment of enzyme aminoacyl synthetase.
  3. Arm opposite to amino acid acceptor site which has an anticodon loop made up of 3' nitrogen bases which helps in identifying codon on mRNA.
  4. Arm at 3' end with a CCA sequence which is amino acid acceptor site.

8. (d) aa

Explanation :    

Individuals that are shaded are expressing the recessive phenotype and have a genotype of “aa”. In case of autosomal recessive inheritance, affected offsprings will have unaffected heterozygous parents (mostly). When the heterozygous parents mate than the recessive allete gets randomly segregated, ultimately expressing a homozygous recessive trait it the offspring, only and the disease will become evident. This is the basic rule of such kind of genetic inheritance. All the affected offsprings on tracing back in the pedigree will give a heterozygous parent.

9. (a) Baculoviruses

Explanation :    

Baculoviruses are pathogens that attack insects and other arthropods. The majority of baculoviruses used
as biological control agents are in the genus Nucleopolyhedrovirus.

10. (a) T-DNA, Crown gall

Explanation :    

The Agrobacterium tumefaciens is a plant pathogenic bacterium that carries the Ti plasmid. It can transfer a particular segment of the tumor-inducing (Ti) plasmid into the nucleus of infected cells. Crown gall disease is caused by Agrobacterium tumefaciens, a bacterium that infects plants. This bacteria causes tumors on the stem of its host.

11. (a) Insulin consists of two short polypeptide chains: chain A and chain C that are linked together by hydrogen bridges.

Explanation :    

Insulin consists of two short polypeptide chains: chain A and chain B, that are linked together by disulfide bridges.

12. (a) Stage 1: Total population numbers are low and the high birth rate is balanced by the high death rate.

Explanation :    

The stages are:

  • Stage 1: High birth rates and death rates.
  • Stage 2: Continued high birth rates and declining death rates.
  • Stage 3: Falling birth rates and death rates, finally stabilise.
  • Stage 4: Death rates and population growth slow.

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

Explanation :    

Cotyledons of the standard dicot embryo are simple structures that are basically thick and swollen because of storage of food reserves, which provide nutrients to the different parts of the embryo.

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

Explanation :    

Expression of genes is regulated by metabolic, physiological or environmental conditions. For example, if an enzyme called beta-galactosidase is synthesised by E. coli, it is used to catalyse the hydrolysis of a disaccharide, lactose, into galactose and glucose; the bacteria use them as a source of energy. Hence, if the bacteria do not have lactose around them to be utilised as an energy source, they would no longer require the synthesis of the enzyme beta-galactosidase, and the gene is switched off.

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

Explanation :    

Acquired immunity is characterised by memory. This means that when our body encounters a pathogen for the first time it produces a response called primary response, which is of low intensity. Subsequent encounter with the same pathogen elicits a highly intensified secondary or anamnestic response. This is attributed to the fact that our body appears to have memory of the first encounter.

16. (a) Both A and R true and R is the correct explanation of A.

Explanation :    

Restriction digestion using enzymes with several recognition sites, will generate several fragments and complicate the process of gene cloning. Therefore, in order to link the alien DNA (or foreign DNA) the vector needs to have very few, preferably single, recognition /cloning sites for the commonly used restriction enzymes.

Section-B

17. (i) It is responsible for the secretion of progesterone during menstrual cycle. A high level of progesterone inhibits FSH and LH and hence prevents ovulation.
(ii) It is the inner lining of the uterus. This part undergoes changes during the menstrual cycle and gets ready for the implantation of an embryo.

18. (a) Capping: While the pre-mRNA is synthesised, a 7-methylguanosine cap is added to the 5’ end of the growing transcript by a 5’ to 5’ phosphate linkage. In molecular biology, the five-prime cap (5’ cap) is a specially altered nucleotide on the 5’ end of some primary transcripts, this process is known as mRNA capping. This is a highly regulated and vital process for forming stable and mature messenger RNA that is able to undergo translation during protein synthesis. The cap protects the nascent mRNA from degradation. Moreover, initiation factors involved in protein synthesis to recognise the cap to help initiate translation by ribosomes.

(b) Tailing: An enzyme called poly (A) polymerase (PAP) is a part of the protein complex that cleaves the pre-mRNA and it immediately adds a string of approximately 200 A nucleotides, called the poly (A) tail, to the 3’ end of the just-cleaved pre-mRNA. The poly (A) tail protects the mRNA from degradation, aids in the export of the mature mRNA to the cytoplasm, and is involved in binding proteins involved in initiating translation.

19. On entering the host's body, the HIV enters macrophages, where, with the aid of the enzyme reverse transcriptase, its RNA genome multiplies to produce viral DNA. This viral DNA gets incorporated into the host cell’s DNA and directs the infected cells to produce virus particles. The macrophages continue to produce virus and, in this way, act like a HIV factory. Simultaneously, HIV enters into helper T-lymphocytes (TH), replicates and produces progeny viruses. The progeny viruses released in the blood attack other helper T-lymphocytes. This is repeated, leading to a progressive decrease in the number of helper T-lymphocytes in the body of the infected person.

A low CD4 count means that HIV has weakened the immune system. Due to the decrease in the number of helper T lymphocytes, the person starts suffering from infections that could have been otherwise overcome, such as those due to bacteria, especially Mycobacterium, viruses, fungi and even parasites like Toxoplasma. The patient becomes so immuno-deficient that he or she is unable to protect himself or herself against these
infections. With AIDS, the patient has a high risk of developing life-threatening infections or cancers.

20. (a) Severe combined immunodeficiency (SCID) is a genetic disorder caused by the absence of functional T lymphocytes (marked X). This impairs the adaptive immune system and prevents antibody production, resulting in a deficient immune response.

(b) SCID is commonly caused by an autosomal recessive condition known as adenosine deaminase (ADA) deficiency. This condition results from an inability to produce the enzyme ADA. SCID is caused by a defect in the gene for the enzyme adenosine deaminase (ADA). These patients have non-functioning T lymphocytes.

21. In the majority of terrestrial ecosystems, the biomass pyramid has a large base of primary producers and a lower trophic level on top. Autotrophic producer biomass is at an all-time high. The next trophic level, primary consumers, have a smaller biomass than producers. The next higher trophic level, secondary consumers, have a lesser biomass than primary consumers. There is a low amount of biomass at the highest trophic level. The pyramid is upright.

(a) NPP is the actual biomass produced.
GPP =12 kg Carbon/m2/year
Respiration (Carbon dioxide leaving the forest) = 5 kg Carbon/m2/year
GPP – R = NPP = 7 kg Carbon/m2/year

(b) NPP = GPP – R
So, NPP = 250 – 100= 150 J/m2/day 

Section-C

22. (a) For the follicular rupturing, the high concentration of LH is responsible.
(b) It rises and goes down between the follicular rupturing and the developing corpus luteum.
(c) The progesterone concentration goes high when the developing corpus luteum matures.

23. (a) Leydig cells are situated outside seminiferous tubules in the interstitial space. They synthesise or secrete male hormones, testosterone.
(b) Sertoli cells are lined inside the seminiferous tubules. They play a vital role in providing nutrition to the sperm.
(c) Primary spermatocytes are found in the testes in a structure known as the seminiferous tubules. They are diploid (2n) cells containing 46 chromosomes. After meiosis, two secondary spermatocytes are formed.
The accessory glands include the seminal vesicles, the prostate gland and the bulbourethral gland. The secretion of the glands includes seminal plasma that is rich in fructose, calcium and certain necessary enzymes that provide nourishment to the sperm. Secretion of bulbourethral gland helps in lubricating the penis.

24. Haemophilia is sex linked recessive disease; it is transmitted from unaffected female carrier a male child with heamophialia. Y has no allele for this. If male inherits Xh from the mother, he will be haemophilic (with the genotype XhY). If female inherits XhXh, one from the carrier mother and one from her heamophilic father, then she can be haemophilic. Simple cut will cause the increased bleeding time in haemophilic patients.

Haemophilia

This disorder is associated with the clotting capability of blood. The protein that is part of the cascade of proteins is involved in clotting of the blood. This single protein affecs haemophilic patient. A heterozygous carrier female can transmit the disorder to her sons. A female child being haemophilic is extremely rare. For such a condition to arise, the mother should be the carrier of the disease and father should be haemophilic. Only in such a case, the female child can be haemophilic.

25. (a) A: Directional
B: Stabilising
C: Disruptive
(b)

(c) Disruptive selection would favor the light and dark ones but not the gray ones.

26. (a) The primary tests for diagnosing HIV and AIDS include the ELISA Test, which stands for enzyme. linked immunosorbent assay, and is used to detect HIV infection. If an ELISA test is positive, a western blot test is usually administered to confirm the diagnosis. ELISA test is based on the principle of antigen-antibody interaction. Infection by a pathogen can be traced by studying the presence of antigen or it can be confirmed by detecting the antibodies that are synthesised against the pathogens.

(b) The most widely used techniques to diagnose cancer are:
(i) Biopsy and histopathological detection methods
(ii) Sophisticated computational techniques. (1) Radiography (X Rays) (2) CT (Computed Tomography)
(3) MRI (Magnetic Resonance Imaging).

27. (a) GEAC – Genetic Engineering Approval Committee.
(b) Objectives of GEAC as below:
(i) To make decisions regarding the validity of GM research.
(ii) Safety of introducing GMOs for public use.
(c) Golden rice is a transgenic variety of rice that contains a gene that codes for vitamin A precursor. This variety has green-yellow colored grains, is rich in Vitamin A and nutritionally very advantageous.

OR

(a) Advantages of producing GMOs.

  1. The primary benefit of GMOs is their potential to improve food security by increasing crop qualityincreasing yields and reducing the need for chemical pesticides and herbicides.
  2. GM crops exhibit desirable phenotypic traits in crop plants.
  3. The genes responsible for the production of specific proteins are inserted into GM crops. These crops then produce that specific protein.
  4. Transgenic crops synthesise new end products of a particular biochemical pathway.
  5. These crops also help in prevent the expression of existing native genes.

(b) The impact of biotechnology's continued advancement on medicine has increased. GMOs are developed and used in laboratories to produce novel medications and treatments, including antibodies, vaccines, enzymes, and a wide range of other items. For instance, the use of genetically modified microorganisms has transformed the insulin used to treat diabetes.

(c) E. coli is the most frequently used host for the production of enzymes and other proteins by recombinant DNA technology. E. coli is preferable for its relative simplicity, inexpensive and fast high-density cultivation, well-known genetics, and large number of compatible molecular tools available.

28. (a) r = Intrinsic rate of natural increase.
It is a very important parameter as required for assessing impacts of any biotic or abiotic factor on population growth.

(b) Death rate = (Number of deaths)/(Total population)=dN/dt
Death rate = 8/80 = 0.1 individuals per butterfly per week.

Section-D

29. (a) Insecticidal proteins are named Cry proteins. The toxin is coded by the gene cryIAc. There are a number of them; for example, the proteins encoded by the genes cryIAc and cryIIAb control the cotton bollworms, and cryIAb controls the corn borer.

(b) The Bt toxin protein exists, as inactive protoxins (x), but once an insect ingests the inactive toxin, it is converted into an active form (y) of the toxin.:

OR

When the insect eats the cryproteins, its own digestive enzymes activate the toxic form of the protein. Cry proteins bind to specific receptors on the intestinal walls and rupture midgut cells. Susceptible insects stop feeding within a few hours after taking their first bite, and if they have eaten enough toxin, they die within 2 or 3 days.

(c) The three primary components of the genetic package inserted into cotton DNA include:

(i) Protein gene: The Bt gene, modified for improved expression in cotton, enables the cotton plant to produce Cry-protein.
(ii) Promoter: A promoter is a DNA segment that controls the amount of Cry-protein produced and the plant parts where it is produced.
(iii) Genetic marker: A genetic marker allows researchers to identify the successful insertion of a gene into the plant’s DNA. It also assists plant breeders in identifying and developing new cotton lines with the Bt gene.

30.(a) Species 1 benefits as its growth increases co-culture with species 2.
(b) Commensalism: This is the interaction in which one species benefits and the other is neither harmed nor benefited.
(c) An orchid growing as an epiphyte on a mango branch and barnacles growing on the back of a whale gets benefit while neither the mango tree nor the whale derives any apparent benefit, are classic examples of commensalism.

OR

Amensalism: One species is harmed, whereas the other is unaffected.

Section-E

31.

Properties Spermatogenesis Oogenesis
Occurrence Seminiferous tubules of testis In ovaries
Process Production of sperm from spermatogonia Production of ovum from oogonia
Sertoli cells Found in germinal epithelium Absent
Nuclear condensation Found Absent
Food reserve Sperm contains less food reserve Ovum contains a lot of food reserves
Cytokinesis Equal Unequal
Result Forms four motile male gametes i.e., sperms Forms one non-motile female gamete i.e., ovum

OR

Spermatogenesis

32. (a) A cross of tall pea plant with violet flowers and dwarf pea plant with white flowers.

violet flowers

In the above cross we could notice that:
Ttvv are tall with white flowers
TtVv are tall with violet flowers
ttVv are dwarf plant with violet flowers, and
ttvv are dwarf and with white flowers.

(b)

  1. There is some linkage between all genes located on the same chromosome. The linkage strength depends on the percentage of the distance between the two. But linkage can be easily broken by crossing over.
  2. When genes are located on the same chromosome, then there is possibility of two situations, either a crossing over between the two genes or no crossing between two genes.
  3. Crossing over always occurs if genes are located very far from each other – 50% recombinants, 50% parental.

Example: Morgan hybridized yellow-bodied, white-eyed females to brown-bodied, red eyed males and intercrossed their F1 progeny. He found that the genes for white and yellow were very lightly linked and showed only 13% recombinant while white and miniature wing showed 37.2% recombination.

OR

(a) Coding strand: 5’ ATG GTA CTC CAG ACT TGA 3’ mRNA: 5’ AUG GUA CUC CAG ACU UGA 3’
(b) Anticodon: GAG Amino acid: Glutamate (Glu)
(c) N-ter Met Val Leu Gln Thr Stop C- ter

33. (a) (i) LAB in human intestine synthesises Vitamin B12 and checks the growth of harmful microbes in the stomach.
(ii) Lactic acid bacteria are used to produce lactic acid which is an important industrial product and also used in beverages, confectionery, dairy products etc. It has several other functions like improving the nutritional value of food, control of intestinal infections, improved digestion of lactose, control of some types of cancer, and control of serum cholesterol levels.

(b) Both types of lymphocytes and other cells of the immune system are produced in the bone marrow and T-cells are different from each other in the following aspects.

B-cells T-cells
B-cells produce antibodies. T-cells do not produce antibodies but help B-cells.
B-cells matures in bone marrow. T-cells matures in thymus.
B-cell provides humoral immunity. T-cells provides cell mediated immunity (CMI).
Only one active type. Two types of cells are there namely Helper and Killer.

OR

Detection and diagnosis:

  1. Cancer detection is based on histopathological studies and biopsy of suspected tissue. In biopsy, a piece of the suspected tissue cut into thin sections is stained and examined under microscope (histopathological studies) by a pathologist. Detection by blood and bone marrow tests for increased
    cell counts in the case of leukemia’s can also be done.
  2. Radiography (use of X-rays), CT scan (computed tomography) and MRI (magnetic resonance imaging) can also be used to detect cancers of the internal organs. Computed tomography uses and generates a three dimensional image of the internals of an object with the help of X-ray. In MRI strong magnetic fields and non-ionising radiations are used to accurately detect pathological and physiological changes in the living tissue.
  3. Antibodies against cancer-specific antigens are also used for detection of certain cancers.
  4. Techniques of molecular biology can be applied to detect genes in individuals with inherited susceptibility to certain cancers.

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