Biology Unsolved Sample Paper Solutions CBSE Class 12

Section-A

1. (c) Testis

Explanation: The testes are the primary male reproductive organ and are responsible for testosterone and sperm production.

2. (d) 6 months after parturition

Explanation: Lactational amenorrhoea (absence of menstruation) method is based on the fact that cycle does not occur during the period of intense lactation following parturition. This method has been reported to be effective only up to a maximum period of 6 months following parturition.

3. (c) A-(ii), B-(iii), C-(i)

Explanation: Johannsen coined the term gene. Mendel introduced law of independent assortment. T.H. Morgan performed experiments on Drosophila.

4. (c) Chemical synthesis of gene

Explanation: Har Gobind Khorana was awarded Nobel Prize in Physiology or Medicine for their interpretation of the genetic code and its function in protein synthesis.

5. (a) Lactate fermentation

Explanation: In lactate fermentation CO2 is not give off while in aerobic respiration and alcoholic fermentation CO2 is produced.
6. (c) Vitamin B12

Explanation: A small amount of curd added to the fresh milk as inoculum or starter contains millions of LAB, which at suitable temperature multiply, thus converting milk to curd, which also improves its nutritional quality by increasing vitamin-B12.

7. (d) (i)-c, (ii)-a, (iii)-d, (iv)-b

  • Explanation: 1. Thylacine was a carnivorous marsupial which went extinct from Australia. It is also called Tasmanian wolf.
  • 2. Dodo was a flightless bird which was endemic to Mauritius.
  • 3. Quagga is a species of Zebra, which went extinct from Africa.
  • 4. Steller’s sea cow inhabited the shallow waters around the uninhabited commander islands between Russia and Alaska.

8. (c) Both (a) and (b)

Explanation: RNAi takes place in all eukaryotic organisms as a method of cellular defence. This method involves silencing of a specific mRNA due to a complementary dsRNA molecule that binds to and prevents translation of the mRNA (silencing). The source of this complementary RNA could be from an infection by viruses having RNA genomes or mobile genetic elements (transposons) that replicate via RNA intermediate.

9. (b) Lithosere

Explanation: Lichens are the pioneer vegetation of Lithosere succession as Lichens are the first organisms to colonize bare rock.

10. (d) (C)

Explanation: In forest ecosystem, it is a partly upright or spindle shaped because producers are lesser in number and support a greater number of herbivores and which in turn support a fewer number of carnivores. After that the number goes down at each successive level.

11. (a) Baking

Explanation: Saccharomyces cerevisiae is used primarily for backing and alcoholic fermentation.

12. (c) Toxin from Plasmodium species

Explanation: Haemozoin is a toxin released by Plasmodium species, which is responsible for the chills and high fever recurring every three to four days. The rupture of RBCs during life cycle or plasmodium is associated with release of a toxic substance, heaemozoin.

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

Explanation: In the artificial hybridisation programme selection of parents is done first. In case of female parent producing unisexual female flowers, emasculation step is not needed. During emasculation in bisexual female flower anthers are removed from flower bud before the anther dehisces. Thus assertion is false but reason is true.

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

Explanation: The promoter is the sequence where the RNA polymerase binds and begins transcription. The operator region is a segment of DNA adjacent to the promoter elements in most operons. A repressor protein binds to the operator and makes the promoter regions of prokaryotic DNA inaccessible for RNA polymerase to transcribe the genes. Thus both assertion and reason are true and reason is the correct explanation of assertion.

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

Explanation: Streptococcus pneumoniae causes pneumonia and H. influenza type b (Hib) causes bacteremia, pneumonia, epiglottis, and acute bacterial meningitis. These are respiratory disorders and are transmitted through droplet infection. A healthy person acquires the infection by inhaling the droplets/aerosols released by an infected person. Thus both assertion and reason are true but reason is not the correct explanation of the assertion.

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

Explanation: Biofertilizers are eco-friendly as they don’t hamper the microbial flora and pH of soil which chemical fertilizers can harm microbes present in the soil and alter the pH which will not be suitable for the growth of plants. Thus both assertion and reason are true and reason is the correct explanation of assertion.

Section-B

17. (a) The primary sporogenous tissue function as microspore mother cells. Each undergoes meiosis and forms four haploid microspores. The four microspores formed from a microspore mother cell are usually arranged in tetrad.

(b) In the flowering plant, the largest number of haploid cells occur in microsporangia. It is also called as pollen sac. It is present within the anther. It undergoes meiosis to produce microspores. Each microsporangium has 100 microspore mother cells which form 400 microspores by meiosis. Thus, the correct answer is ‘Microsporangia’.

18. Through test cross, the genotype of plant with dominant phenotype Purple flower (PP) can be determined. In this cross, the unknown dominant genotype is made to cross with recessive genotype (ww).

diagram3

Conclusion: If the progeny shows 1 : 1 ratio of purple and white flowers, then the purple flower is hybrid with PW genotype.

But, if the progeny obtained consists of all purple flowers, then the genotype of purple flower is PP.

diagram4

19. The order of arrangement in a decreasing manner as according to the importance is Penicillium, Biogas, Citric Acid and Curd. Penicillin is the most important product for the welfare of human society. It is an antibiotic, which is used for controlling various bacterial diseases. The second most important product is  biogas. It is an eco-friendly source of energy. The third important product is citric acid, which is used as a food preservative. The least important product is curd, a food item obtained by the action of Lactobacillus bacteria on milk.

20. (a) Rt-PCR is a diagnostic test for Covid-19. It stands for Real-Time Polymerase Chain Reaction.

(b) In Rt-PCR multiple copies of gene (for DNA) of interest is synthesised in vitro using two sets of primers and enzyme DNA polymerase. The process of replication is repeated many times so that segment of DNA gets amplified to billion time which ease the detection of viruses.

20. (a) Rt-PCR is a diagnostic test for Covid-19. It stands for Real-Time Polymerase Chain Reaction.

(b) In Rt-PCR multiple copies of gene (for DNA) of interest is synthesised in vitro using two sets of primers and enzyme DNA polymerase. The process of replication is repeated many times so that segment of DNA gets amplified to billion time which ease the detection of viruses.

21. (a) N-population density at the given time, r-intrinsic rate of natural increase in population and k-carrying capacity.

Logistic growth model is given by dN/dt = rN (1 – N/k) where N is no. of individual at time t, K is carrying capacity r is intrinsic growth rate.

(b) Exponential growth curve represents the human population growth at present because the number of human beings are increasing rapidly. Such a curve is not sustainable because at one point the human population would reach a place where there would not be finite resources for every one.

OR

(a)

area

(b) A German naturalist and a geographer named Alexander Von Humboldt quoted some observations during his exploration in the forests of South America.

These observations were:

  • (i) Within a region, species richness increased with an increase in area but only up to certain limit.
  • (ii) When plotted on graph the species area relation shows rectangular hyperbola.
  • (iii) There are wide range of factors that determine the slope of curve namely, predation-prey dynamics, immigration, extinction and clustering of the species.

Section-C

  • 22. (a) The nucleus of primary endosperm cell gets divided through free nuclear division. The nucleus divides to form hundred and twenty eight nuclei to form cellular endosperm through cytokinesis.
  • (b) Tender coconut is considered a healthy source of nutrition because the liquid endosperm of the coconut (Coconut water) is abundantly rich in minerals, vitamins, fats, proteins and carbohydrates. These nutrients act as the energy resource for the developing embryo.
  • (c) In pea seed, the endosperm is completely consumed during the development of an embryo. Hence due to absence of an embryo, the seeds of pea are non- endospermic in nature. On the other hand, castor seeds are endospermic as the endosperm is still present even after the development of an embryo.
  • 23. (a) Follicular Phase: Also known as proliferative phase, during this stage the FSH is released by the pituitary glands that helps in the growth of ovarian follicles. It also causes maturation of primary oocytes in the follicle. The follicular cells secrete estrogen which is responsible for the thickness of endometrium and becomes more vascular and glandular. The phase lasts for 10 to 14 days.
  • (b) Luteal Phase of Menstrual Cycle: It helps in development of Corpus Luteum which secretes progesterone.
  • (c) It collects the ovum that is released by an ovary.
  • 24. (a) It represents the maturation of pro-insulin into insulin.
  • (b) Insulin contains two short polypeptide chains that is Chain A and Chain B that are linked together with di-sulphide bridges.
  • (c) It is useful for monitoring residual beta cell function in the human body under insulin therapy.

25. (a) Given data,

The number of adenine (A) containing nucleotides = 240

Since, the Number of adenine (A) = Number of thymine (T)

The thymine (T) = 240……….. (According to Chargaff’s rule that is A + G = C + T)

Now, calculating guanine (G) = G + C = 1000 – (A + T)

= G + C = 1000 – 480

= 520 (G = 260, C = 260)

Therefore, C + T = 240 + 260 = 500

(b) Diagrammatic representation of portion of DNA fragment,

Diagrammaticrepresentation

26. Darwinian Theory of evolution states that, evolution took place through natural selection. The number of life forms depends upon the life span of the organisms and their ability to reproduce. Another aspect of natural selection is the survival of the fittest, where nature itself selects the individuals that are fit to adapt to the prevailing environment. Darwin also, observed that, variation is inheritable, and the species fit to survive the most produces more offspring. Hence, the characteristic of population changes consequently to the evolution of new forms of life.

Example of such selection is the antibiotic resistance in bacteria. When a bacterial population was grown on an agar medium containing antibiotic called Penicillin, the colonies that were sensitive to the Penicillin died. While one or few bacterial colonies that were resistant to Penicillin survived. This is due to, such bacteria had undergone some change or mutation which resulted in the evolution of a gene that made them resistant to Penicillin drug. Therefore the resistant bacteria got multiplied rapidly, as compared to non-resistant bacteria, thereby, increasing their population. Hence, the advantage of an organism over the other helps in the struggle for existence.

The two key concepts of the theory are:

  • (i) Natural selection: According to the concept, nature selects the organism which is most fit and able to adapt.
  • (ii) Branching descent: As according to the concept, various species have come into existence from a common ancestor.
  • Characteristics of Neanderthal man:
  • (i) They used hides so as to protect their body and buried the dead ones.
  • (ii) They were short but were very strong with outward curved thigh bones.
  • (iii) The cranial capacity of the Neanderthal man was 1400 cc.

OR

(a) Test cross is used to indentify the genotype of a tall pea plant bearing white flowers.

(b) A cross of tall pea plant with violet flowers and dwarf pea plant with white 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

27. (a) The advantage of beginning gene therapy prior to birth is that the body would not reject it as it has not yet recognised self.

(b) Some of the risks of gene replacements have to do with the viral vectors used to deliever the new gene into the cell. Risks vary depending on the specific gene therapy, the viral vector used and the disease being treated.

28. (a) GMOs are Genetically Modified Organisms. These are created by inserting the genes from the DNA of a species with desired characteristics into the genes of an unrelated plant or animal. A hybrid is produced when the pollen of one plant is used to fertilize some unrelated plant species. Also when the animals of two different species breed, a hybrid is formed. On the contrary, GMOs are created by incorporating the genes with desired characteristics from a plant or animal of one species into the DNA of a different species.

(b) Biopiracy is the commercial exploitation of naturally occurring biochemical or genetic material by obtaining patents that restrict its future use without paying any compensation to the native community.

Section-D

29. (a) The chromosomal theory of inheritance was given by two scientists named Walter Sutton and Theodore Boveri in 1902. When Mendelian work was rediscovered, the structure called chromosome was discovered. This gave rise to a major breakthrough in the history of genetics. Sutton and Boveri came to a conclusion that the pairing and separation of pair of chromosomes would lead to the segregation of a pair of factors that carry.

(b) There are following evidences that prove the chromosomal theory:

  • (i) The behaviour of the chromosomes during meiosis can be correlated with the behaviour of Mendelian hypothetical evidences.
  • (ii) The number of chromosome is same in each cell of an organism. But this number varies from organism to organism.
  • (iii) The size of a nucleus is similar in sperm as well as ovum. This nucleus contains such structures which are easily identifiable during the process of cell division. With the above points Sutton combined the knowledge of chromosomal segregation with Mendelian principles which gave rise to the new theory called chromosomal theory of Inheritance.

(c) Thomas Morgan and his colleagues worked on Drosophila melanogaster to provide an experimental verification for an idea of chromosomal theory of inheritance. They worked on this species and gave the basis for variation that is produced due to sexual reproduction. So the work of Morgan carries importance in genetics.

OR

(c) 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.

  • 30. (a) Chemically the structure of a coat shown in the diagram is viral protein coat.
  • (b) Enzyme B is Reverse Transcriptase through which the viral DNA is produced. X is the viral RNA introduced into human cell and C is the viral DNA.
  • (c) Macrophage of the host cell D.

OR

(c) The two different cells the new virus ‘E’ subsequently attacks are macrophage and helper T-lymphocytes.

Section-E

31. The female reproductive system consists of following given parts that are situated in the pelvic region:
(a) Pair of ovaries, (b) Pair of Oviducts, (c) Uterus, (d) Cervix, (e) Vagina, (f) External genitalia.

diagram1

Explanation:

(a) Ovaries: Ovaries are the part of primary female sex organs producing ovum, a female gamete. They are located one on each side in the lower abdomen. It is about 2-4 cm in the length and is connected to pelvic wall and uterus.

(b) The Oviducts (fallopian tube): They form female accessory duct along with uterus and vagina. Each fallopian tube is about 10-12 cm in length and extends from the periphery of each ovary to the uterus. Infundibulum is a part closer to the ovary and is funnel shaped structure possessing finger like projections called fimbriae. The Infundibulum extends in a wider part called ampulla. The last part of oviduct is an isthmus which joins to uterus.

(c) Uterus: It is also known as womb. It is singular and is inverted pear shaped. It is supported by ligaments attached to the pelvic wall. The uterine wall has three layers:

  • 1. External thin layer called perimetrium.
  • 2. Middle thick layer of smooth muscles called myometrium.
  • 3. Inner glandular layer called endometrium which undergoes cyclic changes during menstrual cycle.

(d) Cervix and cervical canal: The uterus opens in the vagina through narrow structure called cervix. The cavity of cervix is a cervical canal. The cervical canal along with vagina forms a birth canal.

(f) External genitalia: This includes:

  • 1. Mons pubis: A cushion of fatty tissue that is covered by the skin and pubic hair.
  • 2. Labia majora: These are fleshy folds of tissue which extend down from mons pubis and surround the vaginal opening.
  • 3. Labia minora: This is a paired fold of tissue under labia majora.
  • 4. Hymen: The opening of vagina is covered by hymen.
  • 5. Clitoris: It is tiny finger like structures located at upper junction of two labia minora above urethral opening.

These all structures complete a female reproductive system along with mammary glands for nourishing young foetus after its birth.

OR

There are couples all over the world who cannot have their own child due to infertility of either of the parents. Such parents may be helped by assisted techniques as given below.

  • (a) ZIFT and GIFT—In such techniques zygote or developing embryos may be placed into the fallopian tube, assisting the women to have pregnancy and their own child.
  • (b) Test tube Baby—This is the method in which the in vitro fertilisation is carried out, the zygote is placed in the fallopian tube of a normal female and grown in her womb. This was first experimented in the sheep. The first test tube sheep was named as “Dolly”.
  • (c) Intra Cytoplasmic Sperm Injections—Where sperms are directly injected into the ovum to form an embryo.
  • (d) Artificial Insemination—It is the method by which the sperms from the healthy donor are transferred into the uterus through vagina. This method is adopted when the male partner is infertile.
  • 32. (a) Such properties are:
  • (i) The genetic code is a triplet code, for example UUU, AUG etc. they form the triplet.
  • (ii) One codon codes for only one amino acid and hence is unambiguous and specific in nature.
  • (iii) AUG has a dual function as it codes for methionine and also acts as an initiation codon.
  • (iv) UAA is a termination codon.

(b) Unambiguous code means that one codon codes for only one amino acid. For example AUG codes for only methionine. The universal code means that the codon and its corresponding amino acid are same in all the organisms. UUU and UUC code for phenylalanine in both humans as well as the bacteria. The degenerate code means that some amino acids are coded by more than one codon.

OR

(a) The gases present in flask used by S.L. Miller in his experimental setup were CH4 (methane), NH3 (ammonia), H2O (water vapours), H2 (hydrogen).

(b) S.L. Miller created an environment in a laboratory similar to the one that existed before life originated. In a closed flask containing hydrogen, electric discharge was created using electrodes. The conditions were similar to those in primitive atmosphere.

(c) Stanley L. Miller and Hrold C. Urey wanted to investigate the molecular origin of life. They simulated the conditions thought at the time to be present on the early Earth and tested the chemical origin of life under those conditions. The Miller-Urey experiment was immediately recognised as an important breakthrough in the study of the origin of life. It was received as confirmation that several of the key molecules of life could have been synthesised on the primitive Earth.

33. (a) The balance of the ecology is due to the balance between the biotic and abiotic factors that govern the
ecology.

These factors are:

(i) Biotic Factors:

1. Autotrophs

2. Heterotrophs

(ii) Abiotic Factors:

1. Temperature

2. Water

3. Decomposers

4. None of these

3. Light

4. Soil

(b) Temperature is the most important abiotic factor that maintains the balance in the ecosystem and related processes. Temperature regulates the kinetics of the enzymes present in the body of an organism, thereby controlling the metabolic activities of the organism. All related physiological features are governed by the temperature. But certain organisms that tolerate the extreme temperatures have been found. These are extremophiles. The organisms that tolerate wide range
of temperatures are called eurythermal and those who tolerate narrow range of temperatures are called stenothermal. They are found to be present in the underwater sea volcanic vents and hot springs as well as under freeze ice cap of polar regions respectively. Therefore the thermal variation decides the extent of a geographically habitual area.

(c) Certain animal species maintain homeostasis escaping the time period of unfavourable stress. Such animals are unable to migrate to another habitat. So to undergo a partially inactive state is called hibernation. Aquatic frog species named American bullfrog can undergo hibernation in unfavourable environmental state. Similarly, another frog species American toad can survive in freezing conditions. It stops breathing and medically termed as dead in freezing condition. Also, the polar bear suspends itself by hibernating. It gains fat and energy only when it gets out of hibernation.

OR

(a) The pioneer species in the hydric succession is phytoplankton. The hydric succession progresses from hydric to mesic conditions. The primary succession starts in the water. The phytoplankton is successively replaced by free floating angiosperms. As the period passes, there is appearance of rooted hydrophytes.

The secondary succession depends upon the soil and climatic conditions. Further the rooted hydrophytes are succeeded by sedges, grasses and finally the climax community appears that are trees. (Hydric condition) Phytoplankton (Pioneer species)

diagram2

(b) Climax attains much earlier as the soil already exists. The secondary succession extensively depends upon the soil, availability of water and the environmental factors that are prevailing over an area. Major bio-geochemical cycles do take place in soil. This gives a chance for secondary succession.
Hence due to the availability of soil the climax can be reached at much faster rate after secondary succession commences.

(c) It is the type of succession taking place in wetter areas, where the succession series progress from hydric to mesic conditions. The pioneer species of hydrarch succession is the species of primary succession, example: small phytoplankton.

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