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CBSE Class 12th Biology Board Exam 2020: Check 50 Important Questions and Answers

CBSE Class 12th Biology Board Exam 2020: Check 50 Important Questions and Answers

Check important questions & answers for CBSE Class 12th Biology Board Exam 2020. Our experts have pointed out these questions after the analysis of previous years’ papers, latest syllabus & sample paper. All the questions given in this article are important for the preparation of the CBSE board exam 2020 of Class 12 Biology subject. 

CBSE Class 12th Biology Examination 1 Mark Important Questions: 

Question 1- Which cells are responsible for the synthesis of Androgens? 

Answer:  Leydig cells are responsible for the synthesis of Androgens. 

Question 2- Procedure that is misused for foeticide but is responsible for carrying out the procedure for testing genetic disorders is: 

  1. Lactational amenorrhea
  2. Artificial insemination
  3. Amniocentesis 
  4. Parturition

Answer:  Amniocentesis 

Question 3- Several times it has been seen that when transplantation of tissues or organs is done to save patients, the transplantation fails due to the rejection of tissues or organs in the patient’s body. Which immune response is responsible for this rejection? 

Answer: Cell-mediated immune response is responsible for this rejection. 

Question 4-  Choose the correct reasons for Rheumatoid arthritis for the following: 

  1. Lymphocytes become more active.
  2. The body attacks self cells.
  3. More antibodies are produced in the body.
  4. The ability to differentiate pathogens or foreign molecules from self cells is lost.

Choose the correct options:

i) a and b

ii) c and d

iii) a and c

iv) b and d 

Answer: iv) b and d 

Question 5- Label the enzymes P & Q responsible for carrying out the following process: 

Answer: ‘P’ enzyme is Restriction Endonuclease and ‘Q’ enzyme is Ligase. 

Question 6- A biotechnologist wanted to create a colony of E.coli possessing the plasmid pBR322, sensitive to Tetracycline. Which one of the following restriction sites would he use to ligate a foreign DNA?

  1. Sal I
  2. Pvu I
  3. EcoRI
  4. Hind III

Answer: Sal I 

Question 7- What is the most important cause of biodiversity loss?  

Answer: Habitat loss and fragmentation is the most important cause of biodiversity loss. 

Question 8- How many pollen grains and ovules are likely to be formed in the anther and the ovary of an angiosperm bearing 25 microspore mother cells and 25 megaspore mother cells respectively? 

Answer: 100 pollen grains and 25 ovules. 

Question 9- An embryo A develops from the synergids and the embryo B develops from the nucellus (in polyembryony). State the ploidy of both the embryos A and B. 

Answer: A-Haploid; B-Diploid. 

Question 10- What is the scientific name of the source organism through which the first antibiotic was produced? 

Answer: Penicillium notatum

Question 11- What is the technique through which Gene expression can be controlled with the help of an RNA molecule? 

Answer: RNA interference 

CBSE Class 12th Biology Examination 2 Marks Important Questions: 

Question 12- How does an encysted Amoeba reproduce on the return of favourable conditions? 

Answer: Encysted Amoeba divides by multiple fission / produces amoeba or pseudopodiospores / cyst wall bursts out / spores are liberated to grow as amoebae (sporulation).  

Question 13- Answer the following: 

a. Define Gemmules and Conidia. 

b. Name the organisms for both of them in which they are formed. 

Answer: a. Gemmules are internal buds involved in asexual reproduction. It is an asexually reproduced mass of cells, that is capable of developing into a new organism. Conidia are asexual spores that are formed at the end or on the side of the conidiophore, a specialized hyphal structure that produces the conidium.

b. Gemmules: Sponges; Conidia: Penicillium. 

Question 14- How IUD’s acts as a contraceptive? Mention any two copper-related IUD’s. 

Answer: IUD’s acts as a contraceptive as the Cu ions released suppress sperm motility as well as the fertilizing capacity. 

Two copper-related IUD’s are: 

CuT and Cu7. 

Question 15- Why the inheritance pattern in human beings can’t be studied in human beings as it is done in the pea plant? Is there any alternate method to study the inheritance in human beings? If yes, state.   

Answer: Control crosses cannot be performed in human beings. Yes, there is an alternate method. It is known as Pedigree analysis (the study of the traits in several generations of a family). 

Question 16- State the technology used by the scientist to recover virus-free sugarcane plants for crop breeding experiments from the diseased sugarcanes.

Answer:  The technology used by the scientist is Tissue culture. In this, Meristem apical or axillary is excised. The explant is grown in a test tube under sterile condition/special nutrient medium. 

Question 17- Explain the events that occur in the host cell on the introduction of a nematode-resistant gene into the tobacco plant by using Agrobacterium vectors.

Answer:  RNA interference. Silencing of a specific mRNA due to a complementary RNA. dsRNA/Introduction of DNA was such that it produced both sense/ and anti-sense RNA in the host cells/these two RNAs formed dsRNA that initiated RNAi.  

Question 18- Draw a labelled pyramid of biomass starting with phytoplankton up to three trophic levels. State whether the pyramid is inverted or upright. 

Answer:

The pyramid is inverted because the biomass of fishes is much more than that of the zooplankton and phytoplankton. 

Question 19- The below-mentioned figure shows the molecule present in a mammal’s body. Label the parts marked as ‘a’ and ‘b’ along with the type of cells which produce these molecules. 

Answer: ‘a’ is Antigen binding site and ‘b’ is Light chain.  The cells which produce these cells are B-lymphocytes (B-cells) and Biofortification.  

Question 20- There is an alarming increase in lifestyle diseases in India in addition to the large scale malnutrition. Suggest a process by which both these problems can be addressed. Give examples. 

Answer: a) Enhancing food quality with respect to protein –  Maize hybrids that had twice the amount of the amino acids, lysine and tryptophan, compared to existing maize hybrids were developed. Wheat variety, Atlas 66, having a high protein content, has been used as a donor for improving cultivated wheat. Protein-enriched beans – broad, lablab, French and garden peas. 

b) Vitamin Enriched – Vitamin A Enriched Carrots, Spinach, Pumpkin. Vitamin C enriched bitter gourd, mustard, tomato. 

c) Enrichment of Micro Nutrient And Mineral Content- Iron and calcium-enriched spinach and bathua.  

Question 21- Why does the lac operon shut down sometime after the addition of lactose in the medium where E.coli was growing? Why low-level expression of the lac operon is always required?

Answer: After the addition of lactose, a complete breakdown of lactose to glucose and galactose takes place. Therefore, there is no more lactose to bind to the repressor protein and the lac operon shuts down. A very low level of expression of lac operon has to be present in the cell all the time, otherwise, lactose cannot enter the cells. 

Question 22- a) While cloning vectors, which of the two will be preferred by biotechnologists – bacteriophages or plasmids. Justify your answer.

b) Name the first transgenic cow developed and state the improvement in the quality of the product produced by it. 

Answer: a) Bacteriophages, as it has very high copy numbers of their genome within the bacterial cells whereas some plasmids may have only one or two copies per cell and others may have 15-100 copies per cell. 

b) Rosie, it produced human protein-enriched milk (2.4 Gms per litre). 

Question 23- Explain the impact of the removal of thymus gland on the immune system of a human body. 

Answer: The thymus provides micro-environments for the development and maturation of T lymphocytes. The T-cells themselves do not secrete antibodies but, they help B cells produce them. Therefore, immunity will be reduced.

Question 24- A farmer maintained beehives in his Brassica field during its flowering season. How will he be benefitted?

Answer: Keeping beehives in crop fields during the flowering period increases pollination efficiency and improves the yield– crop yield and honey yield. 

Question 25- How do automobiles are fitted with catalytic converters to reduce air pollution? Suggest the best fuel for such vehicles.

Answer: Catalytic converters have expensive metals like platinum–palladium and rhodium as catalysts. As the exhaust emission passes through the catalytic converter, unburnt hydrocarbons are converted into carbon dioxide and water. Carbon monoxide and nitric oxides are changed to carbon dioxide and nitrogen gas respectively. Unleaded petrol is the best fuel for such vehicles. 

Question 26- State the Mendelian principle which can be derived from a dihybrid cross and not from monohybrid cross.

Answer: From the dihybrid cross, law of independent assortment can be derived which states that, when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters. 

Question 27- Comment upon the mode of pollination in Vallisneria and Eichhornia which have emergent flowers. 

Answer: In Eichhornia the flowers emerge above the level of water and are pollinated by insects or wind. In Vallisneria, the female flower reaches the surface of the water by the long stalk and the male flowers or pollen grains are released on to the surface of the water. They are carried passively by water currents some of them eventually reach the female flowers and the stigma. 

CBSE Class 12th Biology Examination 3 Marks Important Questions: 

Question 28- Draw a labelled diagram of L.S of a pistil showing the passage of growing of pollen tube up to its destination.

Answer: 

Question 29- How does the gain or loss of chromosome(s) take place in humans? Describe one example each of chromosomal disorder along with the symptoms involving an autosome and a sex chromosome.

Answer: Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s) ( aneuploidy).  

Autosomes:- 

Down’s Syndrome: The cause is the presence of an additional copy of chromosome number 21 (trisomy of 21). 

The affected individual is:

  1. short-statured with a small round head
  2. furrowed tongue and partially open mouth
  3. Palm is broad with characteristic palm crease
  4. Physical, psychomotor and mental development is retarded. 

Sex chromosomes:-

Klinefelter’s Syndrome: This is caused due to the presence of an additional copy of X-chromosome resulting in a karyotype of 47, XXY. 

Such an individual has overall masculine development. Feminine development is also expressed by the development of breast/ Gynaecomastia. Such individuals are sterile.  

Question 30- 

Answer: a) i. point mutation/ single base substitution 

point mutation/ single base deletion 

b) i. 4 aminoacids 

4 aminoacids

Question 31- “Apomixes is a form of asexual reproduction that mimics sexual reproduction in plants.” Explain. 

Answer: In some species, the diploid egg cell is formed without reduction division and develops into the embryo without fertilization. In many Citrus and Mango varieties, some of the nucellar cells surrounding the embryo sac start dividing, protrudes into the embryo sac and develops into the embryos. In such species, each ovule contains many embryos. 

Question 32- 

a) State the hypothesis which S.L. Miller tried to prove in the laboratory with the help of the set up given above. 

b) Name the organic compound observed by him in the liquid water after running the above experiment.

c) A scientist simulated a similar setup and added CH4, NH3 and water vapour at 800 ℃. Which important component is missing in his experiment?

Answer: a) Chemical evolution – The first form of life originated from pre-existing non-living organic molecules. 

b) Amino acids

c) H2 

Question 33- How can inbreeding be both advantageous and disadvantageous in cattle breeding programme? 

Answer: Advantages:-

  1. Inbreeding is necessary if we want to evolve a pure line in any animal. 
  2.  It helps in the accumulation of superior genes and the elimination of less desirable genes 

Disadvantages:- 

  1. reduces fertility 
  2. decreases productivity. 

Question 34- “Specific Bt Toxin gene is incorporated into the cotton plant so as to control the infestation of Bollworm”. Mention the organism from which the gene was isolated and explain its mode of action.

Answer: Specific Bt toxin genes isolated from Bacillus thuringiensis is incorporated into cotton is coded by the genes cryIAc and cryIIAb that control the cotton bollworms.  

  1. Bacillus forms protein crystals that contain a toxic insecticidal protein. 
  2. Once an insect ingests the inactive toxin, it is converted into an active form.
  3. The toxin in the form of crystals gets solubilised due to alkaline pH in the gut.
  4. The activated toxin binds to the surface of gut epithelial cells and perforates the walls causing the death of insect larva. 

Question 35- Answer the following: 

a. State any two criteria for determining biodiversity hotspots. Name any two hotspots designated in India.

b. Differentiate between in-situ and ex-situ approaches for conserving biodiversity. Give examples. 

Answer:  a. Criteria for determining biodiversity hot spots are: 

i)  High levels of species richness.  

ii) The high degree of endemism.

Hotspots In India – the Western Ghats, Himalaya (Indo-Burma/Sunderland to be accepted). 

b. In-situ Conservation– Threatened /endangered plants and animals are provided with urgent measures to save from extinction within their natural habitat and they are protected and allowed to grow naturally. Examples-  wildlife sanctuaries, national parks, biosphere reserves, sacred groves. Ex-situ Conservation –Threatened animals and plants are taken out from their natural habitat and placed in a setting where they can be protected and given care Examples – in botanical gardens, zoological gardens, seed, pollen, gene banks. 

Question 36- When the gene product is required in large amounts, so transformed bacteria with the plasmid inside the bacteria are cultured on a large scale in an industrial fermenter which then synthesizes its protein. This product is extracted from the fermenter for commercial use. 

a) Why is the used medium drained out from one side while the fresh medium is added from the other? 

b) List some optimum conditions for achieving the desired product in a bioreactor.

Answer: (a) To maintain the cells in their physiologically most active log/exponential phase. 

(b) Temperature, pH, substrate, salts, vitamins, oxygen 

Question 37- Observe the diagram and answer: 

a. Name the metals used as the catalytic converter. 

b. Which gases are released after the exhaust hydrocarbon is passed through the catalytic converter? 

c. Name the poisonous gas missing in exhaust pollutant of an automobile in the above diagram?

Answer: a. Platinum-palladium Rhodium 

b. CO2, H20 and CO 

c. Nitric oxide 

Question 38- Answer the following: 

a) How do DNA fragments migrate and resolve in Gel electrophoresis? 

b) How lane one is different from lane 2, 3 and 4 in the Gel electrophoresis set up? 

c) How pure DNA fragments are made observable in the visible light? 

Answer: a) The DNA fragments resolve according to their size through sieving effect provided by the agarose gel. Hence, the smaller the fragment size, the farther it moves.

b) The given agarose gel electrophoresis shows the migration of undigested DNA fragments in lane 1 and digested set of DNA fragments in lane 2 to 4.

c) The separated DNA fragments can be visualized only after staining the DNA with a compound known as ethidium bromide followed by exposure to UV radiation. 

Question 39- Answer the following: 

a) Construct a complete transcription unit with promoter and terminator on the basis of the hypothetical template strand given below: 

b) Write the RNA strand transcribed from the above transcription unit along with its polarity. 

c) Mention two events in which DNA is unzipped. 

d) Predict the consequences when both the template and the coding strands of a DNA segment participate in the transcription process? 

Answer:

c) Replication and Transcription.

d) If both the stands take part in transcription.  one segment of DNA would be coding for two different proteins which will complicate the genetic information machinery  Two RNA molecules will be produced, complementary to each other, hence form a double-stranded RNA. 

Question 40- Answer the following: 

a. Explain the different steps involved in the secondary treatment of sewage.

b. Microbes can be used to decrease the use of chemical fertilizers. Explain. 

c. List the changes that occur when an ovule matures into a seed. 

d. Alien species are highly invasive and are a threat to indigenous species. Give examples. 

Answer: a. Primary effluent is passed into large aeration tanks with constant mechanical agitation and air supply. Useful aerobic microbes grow rapidly and form flocs. Flocs while growing to consume organic matter and thus reduce the biochemical oxygen demand (BOD), the effluent is passed into settling tank. The bacterial flocs settle at the bottom of the tank and it forms activated sludge, a small part this is used as an inoculum in the aeration tank and the remaining part is passed into large tanks called anaerobic sludge digesters. 

b. Rhizobium bacteria present in the root nodules of leguminous plants form a symbiotic association and fixes atmospheric nitrogen into organic forms, which is used by the plant as nutrient. Free-living bacteria in the soil Azospirillum and Azotobacter can fix atmospheric nitrogen thus enriching the nitrogen content of the soil. Many members of the genus Glomus (Fungi) form mycorrhizal symbiotic associations with higher plants in these, the fungal symbiont absorbs phosphorus from soil and passes it to the plant. 

c. The changes that occur when an ovule matures into a seed are: 

i) Integuments of ovules harden as tough protective seed coats.

ii) The micropyle remains as a small pore in the seed coat.

iii) As the seed matures, its water content is reduced.  

iv) Seeds become relatively dry (10-15 per cent moisture by mass).  

v) The general metabolic activity of the embryo slows down.  

vi) The embryo may enter a state of inactivity called dormancy. 

d. (i) Nile perch introduced into Lake Victoria in East Africa led to the extinction of Cichlids fish. (ii) Parthenium/Lantana/Eichhornia are invasive plants and pose a threat to indigenous species. (iii) Introduction of African catfish (Clarias gariepinus) to aquaculture is a threat to Indian catfishes. 

Question 41- Suggest and explain the assisted reproductive techniques which will help a couple to have children, where the female had a blockage in the fallopian tube and the male partner had a low sperm count. 

Answer: As the male partner is suffering from low sperm count, Intracytoplasmic sperm injection technique should be used to directly inject sperm into the ovum. As the female partner is having a blockage in the fallopian tube, In vitro fertilization, followed by embryo transfer (ET) will help her conceive. In this case, sperms from the male partner will be collected and injected into the ovum of the female partner to form zygote under simulated conditions in the laboratory and embryo with more than 8 blastomeres will be transferred into the uterus IUT – intrauterine transfer, to complete its further development. 

Question 42-

With reference to the above schematic diagram of Spermatogenesis (a) and Oogenesis(b), answer the following:  

a) About 300 million spermatozoa may be present in a human male ejaculation at one time. Calculate how many spermatocytes will be involved to produce this number of spermatozoa. 

b) How many spermatids will be formed? 

c) How many chromatids are found during Oogenesis in Primary oocyte and First polar body in a human female?

Answer: a) Each primary spermatocyte will undergo meiosis-I and meiosis-2 which will result in 4 spermatozoa 300 million/4=75 million 

b) Since replication has occurred by this stage 46×2 = 92 chromatids 

c) Meiosis –I is completed by this time 92/2 =46 chromatids

CBSE Class 12th Biology Examination 5 Marks Important Questions: 

Question 43- Answer the following: 

a. Certain phenotypes in the human population are spread over a gradient and reflect the contribution of more than two genes. What is the term used for the types of inheritance? Describe it with the help of an example in the human population.  

b. Summarize the process by which the sequence of DNA bases in Human GenomeProject was determined using the method developed by Frederick Sanger. Name a free-living non-pathogenic nematode whose DNA has been sequenced.

Answer: a. Polygenic inheritance is used for this type of inheritance. If we assume skin colour is controlled by three genes A, B, C. Dominant forms (A, B, C) are responsible for dark skin colour and recessive form (a, b, c) for light skin colour. The genotype with all dominant alleles (AABBCC) will be darkest skin colour and with recessive alleles will be light test skin colour (aabbcc). The genotypes (AaBbCc) will be of intermediate skin colour i.e. with three dominant alleles and three recessive alleles. 

b. The sequences were arranged based on some overlapping regions present in them (Alignment of these sequences was not humanly possible). Therefore, the specialized computer-based programme was developed. These sequences were subsequently annotated and were assigned to each chromosome. Chromosome 1. Caenorhabditis Elegans.  

Question 44- Answer the following: 

a. People living in the coastal areas are forced to evict their dwelling units as the sea has inundated into the land areas. State the possible reasons and suggest measures that could be taken to reduce the deleterious changes in the environment.

b. A young sperm whale, 33-foot long was found dead off the coast. It had a large amount of human trash like trash bags, polypropylene sacks, ropes, net segments etc. amounting to 29 kilograms in its digestive system. The whale died because of inflammation of the abdominal lining. Analyze the possible reasons for such mishaps and suggest measures that can be taken to reduce such incidents.

Answer: a. Increase in the level of greenhouse gases has led to considerable heating of Earth leading to global warming, the temperature of Earth has increased by 0.6 oC most of it during the last three decades. El Nino effect is leading to increased melting of polar ice caps as well as of other snow caps. This has resulted in a rise in sea level that can submerge many coastal areas. 

The measures are: Cutting down the use of fossil fuel, improving the efficiency of energy usage, reducing deforestation and planting trees, slowing down the growth of the human population,  reduce the emission of greenhouse gases into the atmosphere. 

b. We are increasing the use of non-biodegradable products. At least one layer of plastic is there in most of the products we buy. We have started packaging even our daily use products like milk and water in polybags. In cities, fruits and vegetables are packed in polystyrene and plastic packaging and we contribute heavily to environmental pollution. 

Measures that could be taken are: We can support the government’s initiative across the country by reducing the use of plastics and the use of eco-friendly packaging. We can do our bit by carrying a cloth or other natural fibre carry-bags when we go for shopping and by refusing to take the polythene bags from shopkeepers. It is important that all garbage generated is sorted. The biodegradable materials can be put into deep pits in the ground and be left for the natural breakdown. It leaves only the nonbiodegradable to be disposed of, the quantity of which should be minimized. The need to reduce our garbage generation should be a prime goal. 

Question 45- a. What is mutation breeding? Give an example of a crop and the disease to which resistance was induced by mutation. 

b. Differentiate between the pisciculture and aquaculture. 

Answer: a. Inducing mutation artificially using chemicals /radiations /and selecting plants with desirable characters. Mung Bean; Yellow mosaic virus. 

b. Production and culturing of fishes is known as pisciculture whereas production and culturing of all types of aquatic organisms in water bodies is known as aquaculture. 

Question 46- a. Name the infection in which the patient is advised anti-retroviral drug. Also, name the causative organism. 

b. How do vaccines prevent subsequent microbial infections? 

c. Many microbial pathogens enter the guts of humans along with the food. Name the psychological barrier that protects the human body from such pathogens. 

Answer: a. AIDS caused by the Human Immunodeficiency Virus.

b. Vaccines prevent microbial infections by initiating production of antibodies against these antigens to neutralise the pathogenic agents during later actual infection. The vaccines also generate memory – B and T-cells that recognize the pathogen quickly on subsequent exposure.

c. Normal cells show a property called contact inhibition by virtue of which contact with other cells inhibits their uncontrolled growth. Cancer cells appear to have lost this property. These cells grow very rapidly, invading and damaging the surrounding normal tissues. Cells sloughed from such tumours reach distant sites through blood, and wherever they get lodged in the body, they start a new tumour there. This property called metastasis. psychological barriers are the acid in the stomach and saliva in the mouth. 

Question 47- “ Indiscriminate human activities have strengthened the greenhouse effect resulting in Global Warming.” Give the relative contribution of various Green House Gases and explain the fate of the energy of sunlight reaching the earth’s surface contributing to Global Warming.  

Answer: Clouds and gases reflect one-fourth of incoming solar radiation/absorb some of it/but almost half of incoming solar radiation falls on Earth’s surface heating it/while a small is reflected backs/Earth’s surface re-emits heat in the form of infrared radiation/but part of this does not escape into space as atmospheric gases absorb a major fraction of it.

Question 48- Given below is a table depicting population interactions between species A and species B. 

Type of interactionSpecies ASpecies B
a(-)(+)
b(+)(-)

i. Name the two types of population interactions a and b in the above table.

ii. Justify giving three reasons, how the type of interaction (b) is important in an ecological context.

Answer: i. (a) – Amensalism (b) – Predation. 

ii. Justifications- 

a. Nature’s way of transferring energy fixed by plants to higher trophic levels/conduits for energy transfer. 

b. Keep the prey population under control. 

c. Predators help in maintaining species diversity in a community, by reducing the intensity of competition among competing for prey species.

Question 49- Aneuploidy of chromosomes in human beings results in certain disorders. Draw out the possibilities of the karyotype in common disorders of this kind in human beings and its consequences in individuals.

Answer: Down’s syndrome, Turner’s syndrome, Klinefelter’s syndrome are common examples of Aneuploidy of chromosomes in human beings.  Down’s syndrome results in the gain of an extra copy of chromosome 21- trisomy. Turner’s syndrome results due to the loss of an X-chromosome in human females- X0 monosomy. Klinefelter’s Syndrome is caused due to the presence of an additional copy of X-chromosome resulting in XXY condition.

Down’s Syndrome: The affected individual is- short-statured with small round head furrowed tongue and partially open mouth; the palm is broad with characteristic palm crease; physical, psychomotor and mental development is retarded.

Klinefelter’s Syndrome: The affected individual is a male with the development of breast, i.e., Gynecomastia. Such individuals are sterile.

Turner’s Syndrome: The affected individual shows the following characters-  Females are sterile as ovaries are rudimentary and the lack of other sexual characters. 

Question 50-  a. Differentiate between spermatogenesis and oogenesis.

b. ‘Parturition is induced by a complex Neuroendocrine mechanism.’ Justify. 

Answer: a. Oogenesis is different from that of spermatogenesis in the following ways: 

i. In spermatogenesis, continuous production of sperms takes place from puberty to old age. Whereas, in oogenesis, the oocytes are generated before birth/ in the foetus. 

ii. A large number of these oocytes degenerate during the phase from birth to puberty. The oocytes continuously decrease in number, and it gets completely exhausted at menopause.

iii. A primary spermatocyte completes the first meiotic division leading to the formation of two equal-sized secondary spermatocytes whereas, Primary oocyte divides unequally by the first meiotic division resulting in the formation of a large haploid secondary oocyte and a tiny first polar body. 

iv. Meiotic division of secondary oocyte gets temporarily arrested at Prophase-I stage. It is completed only when sperm comes in contact with the zona pellucida layer of the ovum. There is no such event in spermatogenesis. 

v. Spermatogonium produces 4 functional spermatozoa whereas an oogonium produces one functional ovum and 3 non-functional polar bodies. 

b. Parturition is a process where vigorous contraction of the uterus at the end of pregnancy causes expulsion/delivery of the foetus. The signals for parturition originate from the fully developed fetus and the placenta which induce mild uterine contractions called foetal ejection reflex. This triggers the release of oxytocin from the maternal pituitary. Oxytocin acts on the uterine muscle and causes stronger uterine contractions, which in turn stimulates the further secretion of oxytocin. The stimulatory reflex between the uterine contraction and oxytocin secretion continues resulting in stronger and stronger contractions. This leads to the expulsion of the baby out of the uterus through the birth canal – parturition, after the infant is delivered; the placenta is also expelled out of the uterus. 

These were some of the important questions from previous year question paper, sample papers and the NCERT textbooks. The students are advised to go through  these questions in order to get an idea about the questions that can be asked in the CBSE Class 12th Biology Board Examination 2020. 

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