s

Biology | Class 12th

CBSE Board Paper 2018

2

CBSE Board Paper 2018 Set - 1

General Instructions:

1. There are a total of 26 questions and five sections in the

question paper. All questions are compulsory.

2. Section A contains questions number 1 to 5, very short-answer

type questions of 1 mark each.

3. Section B contains questions number 6 to 10, short-answer type

I questions of 2 marks each.

4. Section C contains questions number 11 to 22, short-answer

type II questions of 3 marks each.

5. Section D contains question number 23, value based question

of 4 marks.

6. Section E contains questions number 24 to 26, long-answer type

questions of 5 marks each.

7. There is no overall choice in the question paper, however, an

internal choice is provided in one question of 2 marks, one

question of 3 marks and all the three questions of 5 marks. In

these questions, an examinee is to attempt any one of the two

given alternatives.

Time allowed: 3 Hours Max Marks: 70

3

1. Write the dual purpose served by Deoxyribonucleoside triphosphates in the polymerisation.

Section A

1

2. Name two diseases whose spread can be controlled by the eradication of Aedes mosquitoes.

1

3. How do cytokine barriers provide innate immunity in humans?

1

4. Write the names of the following:

(a) A 15-mya primate that was ape-like.

(b) A 2-mya primate that lived in East African grasslands.

1

5. Mention the chemical change that proinsulin undergoes, to be able to act as mature insulin.

1

6. Your advice is sought to improve the nitrogen content of the soil to be used for cultivation of a non-leguminous terrestrial crop.

(a) Recommend two microbes that can enrich the soil with nitrogen.

(b) Why do leguminous crops not require such enrichment of the soil?

2

Section B

4

7. With the help of an algebraic equation, how did Hardy-Weinberg explain that in a given population the frequency of occurrence of alleles of a gene is supposed to remain the same through generations?

2

8. How did a citizen group called Friends of Arcata Marsh, Arcata, California, USA help to improve water quality of the marshland using Integrated Waste Water Treatment? Explain in four steps.

2

9. You have obtained a high yielding variety of tomato. Name and explain the procedure that ensures retention of the desired characteristics repeatedly in large populations of future generations of the tomato crop.

2

10. (a) Name the source plant of heroin drug. How is it obtained from the plant?

(b) Write the effects of heroin on the human body.

2

OR

Although a prokaryotic cell has no defined nucleus, yet DNA is not scattered throughout the cell. Explain.

5

11. Draw a diagram of a mature human sperm. Label any three parts and write their functions.

12. (a) Expand VNTR and describe its role in DNA. fingerprinting

(b) List any two applications of DNA fingerprinting technique.

3

13. Differentiate between Parthenocarpy and Parthenogenesis. Give one example of each.

14. Medically it is advised to all young mothers that breastfeeding is best for their new-born babies. Do you agree? Give reasons in support of your answer.

15. Explain the mechanism of ‘sex determination’ in birds. How does it differ from that of human beings?

3

3

3

3

16. (a) How has the development of bioreactor helped in biotechnology?

(b) Name the most commonly used bioreactor and describe its working.

3

Section C

6

17. Explain the roles of the following with the help of an example each in recombinant DNA technology:

(a) Restriction Enzymes

(b) Plasmids

3

18. Explain out-breeding, out-crossing and crossbreeding practices in animal husbandry.

3

19. (a) Organic farmers prefer biological control of diseases and pests to the use of chemicals for the same purpose. Justify.

(b) Give an example of a bacterium, a fungus and an insect that are used as bio control agents.

3

20. (a) Differentiate between analogous and homologous structures.

(b) Select and write analogous structures from the list given below:

(i) wings of butterfly and birds

(ii) vertebrate hearts

(iii) tendrils of bougainvillea and cucurbita

(iv) tubers of sweet potato and potato

3

7

21. (a) ‘‘India has greater ecosystem diversity than Norway.’’ Do you agree with the statement? Give reasons in support of your answer.

(b) Write the difference between genetic biodiversity and species biodiversity that exists at all the levels of biological organization.

3

OR

Explain the effect on the characteristics of a river when urban sewage is discharged into it.

22. How has the use of Agrobacterium as vectors helped in controlling Meloidegyne incognitia infestation in tobacco plants? Explain in correct sequence.

3

Section D

23. Looking at the deteriorating air quality because of air pollution in many cities of the country, the citizens are very much worried and concerned about their health. The doctors have declared health emergency in the cities where the air quality is very severely poor.

(a) Mention any two major causes of air pollution.

(b) Write any two harmful effects of air pollution to plants and humans.

(c) As a captain of your school Eco-club, suggest any two programmes you would plan to organise in the school to bring awareness among the students on how to check air pollution in and around the school.

4

8

24. (a) Describe any two devices in a flowering plant, which prevent both autogamy and geitonogamy.

(b) Explain the events up to double fertilisation after the pollen tube enters one of the synergids in an ovule of an angiosperm.

OR

(a) Explain menstrual cycle in human females.

(b) How can the scientific understanding of the menstrual cycle of human females help as a contraceptive measure?

25. (a) Write the scientific name of the organism Thomas Hunt Morgan and his colleagues worked with for their experiments. Explain the correlation between linkage and recombination with respect to genes as studied by them.

(b) How did Sturtevant explain gene mapping while working with Morgan?

5

5

Section E

9

OR

(a) State the ‘Central dogma’ as proposed by Francis Crick. Are there any exceptions to it? Support your answer with a reason and an example.

(b) Explain how the biochemical characterisation (nature) of ‘Transforming Principle’ was determined, which was not defined from Griffith’s experiments.

26. (a) Following are the responses of different animals to

various abiotic factors. Describe each one with the help of an example.

(i) Regulate (ii) Conform (iii) Migrate (iv) Suspend

(b) If 8 individuals in a population of 80 butterflies die in a week, calculate the death rate of population of butterflies during that period.

OR

(a) What is a trophic level in an ecosystem? What is ‘standing crop’ with reference to it?

(b) Explain the role of the ‘first trophic level’ in an ecosystem.

(c) How is the detritus food chain connected with the grazing food chain in a natural ecosystem?

5

10

1. Deoxyribonucleoside triphosphates (DNTPs) consist of a deoxyribose sugar, a nitrogenous base (adenine, guanine, thymine or cytosine) and three phosphate groups. There are four different types of DNTPs (dATP, dTTP, dGTP, dCTP).

PCR (polymerase chain reaction) is a technique used in molecular biology to amplify a particular DNA segment (a piece of DNA sequence is repeatedly copied). Multiple copies are produced from a single DNA segment through PCR using DNTPs. Basically, PCR is DNA replication on a very large scale.

In polymerisation, DNTPs serves two important purposes:

• The DNTPs act as building blocks/substrate for the new DNA strands being synthesized during polymerisation.

• They also serve as the energy source for polymerisation as the DNTPs contain high-energy phosphate bonds.

Solutions (Set-1)

Section A (Solutions)

11

2. Aedes mosquito (also known as the yellow fever mosquito) is commonly found in tropical and subtropical regions. It can be identified by white markings present on its legs. Only the female Aedes mosquito bites and sucks blood. The most common species is Aedes aegypti.

It acts as the principal vector for the transmission of various diseases such as dengue, zika fever, chikungunya, and yellow fever. By taking preventive measures (such as covering water containers, tanks, changing the water of cooler regularly) to eradicate Aedes mosquitoes, the spread of dengue and chikungunya can be controlled.

Aedes mosquito

3. Innate immunity (also known as inborn immunity) in humans is present from birth. It is non-specific type of defence. It provides different barriers to the entry of pathogens into the body.

Cytokine barriers:

Cytokines are a category of proteins synthesized by the immune system and which act as chemical messengers. They favour cell-to-cell communications. Cytokines serve to initiate and regulate immune responses.

It includes interferons that are produced by virus-infected cells, which aim to protect non-infected/normal cells from viral

12

infection by inhibiting viral replication. Interferons play a vital role in controlling viral diseases but they do not kill the virus.

4. (a) Dryopithecus

Dryopithecus lived 15 mya (million years ago) in Africa and Europe. They were ape-like, hairy and they walked like gorillas. They had semi-erect posture and large brain size. They were arboreal (lived on tress). Dryopithecus is considered as the direct ancestor of modern day apes.

(b) Australopithecus

They lived in East African grasslands about 2 mya (million years ago). They had erect posture, walked on two legs, and were omnivores. Most importantly, they used weapons carved out of stone to hunt. They were cave dwellers and possessed characters of both humans and apes.

5. In humans, the pancreas (β cells of Islets of Langerhans) produces insulin, a relatively smaller protein in an inactive form like a pro-hormone. This pro-hormone is known as pro-insulin. Mature insulin consists of two polypeptide chains, namely the A chain (having 21 amino acids) and the B chain (having 30 amino acids) which are joined together by disulphide bonds. However, proinsulin contains an extra polypeptide chain, the C peptide that is removed during maturation of proinsulin into insulin. The removal of this extra stretch of the polypeptide is important for proinsulin to be able to function as mature insulin.

Through recombinant DNA technology, it is possible to artificially synthesize mature insulin.

13

MATURATION OF PROINSULIN INTO INSULIN

6. Our atmosphere contains enough nitrogen, but the plants cannot take up this atmospheric nitrogen directly. It needs to be converted into absorbable forms (nitrates and nitrites) by certain microbes such as bacteria and cyanobacteria.

(a) Nitrogen-fixing bacteria like Azospirillum and Azotobacter fix atmospheric nitrogen while living freely in the soil and helps in enriching the nitrogen content of the soil.

(b) Leguminous crops such as pea, beans and pulses do not require such enrichment of the soil. These crops contain Rhizobium bacteria in a symbiotic association in their root

Section B (Solutions)

14

nodules. Rhizobium is capable of fixing atmospheric N2 only in the symbiotic form. They cannot fix N2 independently. This bacterium makes the soil fertile.

7. G.H. Hardy and W. Weinberg together proposed the Hardy-Weinberg principle, which states that if a population is not undergoing any kind of evolutionary change, it is said to be in genetic equilibrium. The term “genetic equilibrium” means that the frequency of occurrence of alleles of a gene remains fixed and same throughout generation to generation. It implies that allele frequencies in a population are stable and the gene pool (total genes and their alleles) of a population remains constant. This principle also states that the sum total of all the allelic frequencies of a gene is 1.

Hardy and Weinberg used an algebraic equation to explain this:

P2 + 2pq + q2 = 1

Where, p = frequency of the dominant allele

q = frequency of the recessive allele

pq = frequency of heterozygotes

This equation is a binomial expansion of (p + q)2.

Five factors affect the Hardy-Weinberg equilibrium:

• Gene flow

• Genetic drift

15

• Mutation

• Natural selection

• Genetic recombination

Any change in frequency of alleles in a population results in evolution.

In prokaryotes, DNA is not scattered throughout the cell although they do not have a defined nucleus. DNA is found lying centrally in the cytoplasm in a condensed form. The coiled DNA is maintained in place by non-histone basic proteins having positive charge. DNA being negatively charged is held by these basic polyamines and this region is termed as nucleoid or genophore. The DNA is packed in large loops in the nucleoid.

A PROKARYOTIC CELL SHOWING NUCLEOID

8. A citizen group called FOAM (friends of Arcata marsh) developed an integrated wastewater treatment process along with the biologists of Humboldt State University.

OR

16

The sewage water is treated in an integrated manner by utilizing both artificial and natural processes.

The process occurs in four steps:

• Conventional sedimentation, filtration and treatments with chlorine are carried out.

• Next, an innovative approach is taken to remove dangerous pollutants like dissolved metals.

• A series of six connected marshes is developed across 60 hectares of marshland.

• Lastly, certain plants, algae, fungi and bacteria are seeded into the marshland to neutralize and absorb the pollutants.

Thus, as the water flows through the marshes, it gets purified naturally.

9. Plant breeding is the procedure that ensures retention of the desired characteristics repeatedly in large populations of future generations of a crop. It refers to the genetic improvement of crop plants. It aims to create and maintain desired plant characteristics such as better yield, disease and pest resistance and increased tolerance to environmental stress. To retain the desired traits in a plant, hybridisation (crossing) of pure lines is carried out, followed by artificial selection.

The main steps followed during plant breeding are:

• Collection of variability (germplasm)

• Evaluation and selection of parents

• Crossing among selected parents

• Selection and testing of superior recombinants

• Testing, release and commercialisation of new crops

17

Organisms whose genes are manipulated artificially are referred to as genetically modified organisms. Such genetically modified crops have been useful in many ways.

10. Heroin commonly called as smack is an example of opiate narcotics, made from morphine. It is a white, odourless and bitter crystalline compound. Clinically it is used as an analgesic (painkiller).

(a) Chemically, heroin is diacetyl morphine. it is obtained by acetylation of morphine. Morphine is extracted from the latex of poppy plant Papaver somniferum.

(b) Heroin is generally taken by snorting or through injections. It acts as a depressant by affecting the central nervous system (CNS). It binds to the specific opiod receptors present in our CNS and slows down body functions. It acts as an effective sedative and is useful for patients who have undergone surgery.

18

11. STRUCTURE OF A MATURE SPERM

A mature sperm cell is made up of a head, a neck, a middle piece and a tail. It is covered by a plasma membrane. The head region consists of an elongated nucleus and little cytoplasm.

Acrosome – a cap-like structure present over the nucleus contains hydrolytic enzymes (called sperm lysins) which are capable of dissolving the membranes covering the ovum. These enzymes help the sperm cell to enter and fertilize the ovum. The acrosome is derived from Golgi apparatus. Sperm lysins include:

• Hyaluronidase – dissolve hyaluronic acid

• zona lysin – dissolves zona pellucida

• corona penetrating enzyme – dissolves corona radiata

Neck – the short neck contains 2 distinct granules having a significant role. The proximal centriole is required for the first cleavage division of zygote formed after fertilisation. The distal centriole gives rise to the axial filament of the tail and helps to maintain it.

Mitochondria – the middle piece of the sperm contains numerous mitochondria, which are spirally arranged around the axial filament. As it is known as the powerhouse of the cell, it produces energy and facilitates the sperm motility. The energy produced helps the sperm to swim and reach the site of fertilization.

Section C (Solutions)

19

12. DNA fingerprinting is a technique developed by Alec Jeffreys. It is used for determining the nucleotide sequence of a segment of DNA, which is unique to every individual. As 99.9% of the base sequence is the same among humans, this technique helps to distinguish humans from one another by identifying differences in repetitive DNA.

(a) VNTRs refer to Variable number tandem repeats. It belongs to a category of satellite DNA called as minisatellites. The size of the VNTR varies from 0.1 to 20 kb. A small DNA sequence is arranged tandemly in many copy numbers. The number of repeats shows a high degree of polymorphism (variation at the genetic level). This sequence normally do not code for any proteins, but they form the basis of DNA fingerprinting.

(b) DNA fingerprinting technique has a number of forensic applications:

• In case of disputes, this technique forms the basis of paternity testing. As the polymorphs are inheritable, they serve as very useful identification tools.

20

• Another application involves criminal identification and forensics. DNA collected from a crime scene can help to identify the criminal and the victim.

13.

14. Yes, breastfeeding is best for new-born babies. During pregnancy, the maternal pituitary gland secretes prolactin that stimulates the growth and differentiation of mammary glands. A process called lactation occurs during pregnancy, which facilitates secretion of milk. The fluid secreted during the initial days after parturition is called colostrum. It is thick, sticky and yellowish fluid rich in proteins and antibodies but contains almost no fat. These antibodies are required for the new-born baby to develop immunity and resistance against diseases. Most importantly, colostrum contains IgA (secretory immunoglobulin) that protects the mucous membranes of the upper respiratory tract. Breastfeeding the new-born is recommended by doctors for bringing up a healthy child.

15. Mechanism of sex determination in birds:

ZW-ZZ type of sex determination is observed in birds. They exhibit female heterogamety, which means that the female individual produces two different types of gametes, while the male individual produces two similar gametes. Thus, the egg determines the sex of the offspring. Both the male and female individual possesses two sex chromosomes, but the female

21

contains heteromorphic sex chromosomes. The male is produced ZZ gametes, and the female produces ZW gametes.

Mechanism of sex determination in humans:

XY-XX type of sex determination is observed in humans. Both male and female have the same number of chromosomes, 44 autosomes and 2 sex chromosomes. Y-chromosome is shorter than the X-chromosome. Humans exhibit male heterogamety, which means that the male individual produces two different types of gametes (XY), while the female individual produces two similar gametes (XX). Thus, the sperm determines the sex of the child. There is always a 50% probability of having either a male or a female child because, during spermatogenesis, 50% of the total sperms produced contain X-chromosome and the other 50% have Y-chromosome. If the sperm is having X-chromosome fuses with the ovum, the zygote develops into a female child (XX). And if the sperm is having Y-chromosome fuses with the ovum, the zygote develops into a male child.

22

NOTE: The mechanism of sex determination shown by birds and humans differ from each other because birds show female heterogamety and humans show male heterogamety.

16. The term biotechnology refers to the use of living organisms and their products to modify and improve human health and the environment. Genetically modified organisms are used to achieve the aim.

(a) Bioreactors, also known as fermenters are chambers in which microorganisms are cultured in a liquid medium. In these vessels, the raw materials are biologically converted into specific products. The development of bioreactors greatly helped in producing the desired products on a large scale. Now, large volumes (100-1000 L) f culture can be processed in a single bioreactor. These fermenters provide and regulate optimum growth conditions such as temperature, pH, substrate and salt concentration and maintain a continuous supply of oxygen.

(b) The most commonly used bioreactors are of stirring type. The structure of a stirred bioreactor is cylindrical which facilitates even mixing of raw materials and the liquid culture throughout the tank. It also ensures a continuous supply of oxygen. The reactor is provided with a baffle and a rotating stirrer that offers excellent mixing. The tank also possesses inlets/sampling ports that allows the person to alter the volumes of the culture. The tank has multiple controlling systems such as pH, temperature, foam, oxygen delivery and agitator system. A figure of a stirred-tank reactor is shown below.

23

17. Recombinant DNA technology involves the creation of recombinant DNA, which possesses only the desired genes using multiple tools such as DNA manipulative enzymes, vectors and host organisms.

(a) Restriction Enzymes (RE):

RE was discovered by Werner Arber in 1963. These enzymes were isolated from bacteria (E.coli) where they were involved in the defence mechanism. RE serve as chemical knives/molecular scissors that cut DNA into distinct fragments. There are several RE known till date and each RE recognize a specific nucleotide sequence in DNA and cut at particular sites in DNA. After cutting the DNA into fragments, the desirable gene is incorporated into it.

For example, the restriction enzyme Eco RI isolated from Escherichia coli recognizes the following nucleotide sequence,

And cuts between G and A (guanine and thymine). The action of RE produces overhanging stretches of DNA called sticky

24

ends. The desirable gene is incorporated into the DNA and later joined by the enzyme DNA ligase.

NOTE: RE are isolated only from prokaryotes because it is their natural defence mechanism against bacteriophage infection.

(b) Plasmids:

Plasmids are extra chromosomal, autonomous, circular, double-stranded, and self-replicating fragments of DNA present in bacteria and yeast. They are used as vectors in r-DNA technology. They are capable of replicating independently and do not contain any vital genes necessary for bacteria/yeast. They may be responsible for providing pathogenicity to bacteria and yeast. Plasmids may contain the genes for antibiotic resistance. They possess multiple restriction sites. The most commonly used plasmid is pBR322.

25

For example, EcoRI is used to cut pBR322. As a result, the plasmid becomes linear possessing sticky ends. Then the desirable gene is inserted into the plasmid and the joints are sealed using DNA ligase. This modified plasmid is then taken by the bacteria where it replicates and produces the desired product.

18. Animal husbandry refers to the agricultural practice of breeding and raising domestic animals. Animal breeding aims at increasing the yield of animals and improving the desirable qualities of the produce.

Out-breeding: it refers to the practice of breeding of unrelated animals. These animals must not have a common ancestor.

Out-crossing: it refers to the practice of breeding of animals of the same breed, but they have no common ancestor on either side for 4-6 generations. The offspring of this mating is called an outcross. This practice helps to overcome inbreeding

26

depression (continuous breeding of closely related individuals of the same breed, which decreases productivity).

Cross breeding: it refers to the practice of mating of animals from two different breeds. It helps to combine the desirable traits of two different breeds. In this method, the superior male of one breed is mated with a superior female of another breed. Thus giving rise to superior progeny. Hisardale is the new breed of sheep obtained by crossing Bikaneri ewes and Marino rams.

19. Organic farming is an integrated farming method of crop and livestock production that aims at reducing the use of pesticides, fertilizers, genetically modified organisms, antibiotics and growth hormones. It strives for sustainability and enhancement of soil fertility and biological diversity.

(a) Organic farmers develop a smart system where pests are not completely eradicated, rather kept at manageable levels by relying on biological control method. These farmers believe that complete eradication of the pests is undesirable because a number of beneficial predatory organisms depend on them for their survival. They seek to understand the interaction between different organisms. Conventional farming practices use chemical methods to kill both useful and harmful life forms indiscriminately. Thus, the use of biocontrol measures greatly reduces the use of toxic chemicals and pesticides. Moreover, humans are consuming these toxic chemicals along with their food, which can be fatal.

(b) Biocontrol refers to the use of biological methods for controlling plant diseases and pests. This method relies on natural predation.

Bacterium – Bacillus thuringiensis (Bt) is a bacterial Biocontrol agents that is used to control caterpillars of butterflies. The

27

dried spores of Bt are mixed with water and sprayed onto the plants. This toxin attacks the larval gut and kills them.

Fungus – Trichoderma, a free-living fungus is used as a Biocontrol agent against numerous plant pathogens.

Insect – Dragonflies are used to get rid of mosquitoes. A beetle, ladybird is used to control aphids.

20. (a)

(b) Analogous structures leads to convergent evolution. These structures differ anatomically and have different origin but they perform similar functions.

(i) Wings of butterfly and birds: butterflies are invertebrates whereas birds belong to vertebrates. The wings in both perform a common function, flying.

28

(iv) Tubers of sweet potato and potato: sweet potato is a modified underground root whereas potato is a modified underground stem. Both have different origin but perform a common function, storage of food.

21. (a) Sociobiologist Edward Wilson used the term ‘biodiversity’ to describe the combined diversity (heterogeneity) at all levels of biological organisation. Our biosphere exhibit immense diversity not only at the species level but at all levels of biological organisation ranging from macromolecules within cells to biomes. If we consider India at the ecosystem level, we observe that India has greater ecosystem diversity than a Scandinavian country Norway. India has deserts, dense rain forests, mangroves, coral reefs, wetlands, alpine meadows and estuaries.

(b) Differences between genetic and species biodiversity are given below:

29

When it comes to treating sewage water, it is observed that solid wastes are relatively easy to remove, but dissolved salts (nitrates, phosphates) and metal ions are difficult to remove.

Effects on the characteristics of a river when urban sewage is discharged into it

• Domestic sewage consists of biodegradable organic matter, which readily decomposes by the action of microorganisms, which can multiply using these organic substances as substrates. It is possible to estimate the amount of biodegradable organic matter in sewage water by measuring Biochemical Oxygen Demand (BOD).

• Microorganisms involved in biodegradation of organic matter of the river consume a lot of oxygen, and as a result, there is a sharp decline in dissolved. This causes mortality of fish and other aquatic creatures.

• Presence of large amounts of nutrients in water bodies also causes excessive growth of planktonic (free-floating) algae, called an algal bloom that imparts a distinct colour to the water bodies. Algal blooms cause deterioration of the water quality and death of aquatic animals. Some bloom-forming algae are extremely toxic.

22. A nematode, Meloidegyne incognitia infects the roots of tobacco plants. It feeds on root cells and causes them to grow into large knots. Its infection reduces the yield largely. The strategy adopted to protect tobacco plants is RNA interference, which is a naturally occurring mechanism in all eukaryotes as a method of cellular defence that leads to silencing of specific genes. This method involves silencing of specific mRNA due to formation of dsRNA molecule. This dsRNA molecule prevented the translation of the original mRNA.

OR

30

The use of agrobacterium vectors has greatly helped in controlling Meloidegyne incognitia infestation in tobacco plants. The nematode specific genes, which were responsible for parasitism, were introduced in tobacco plants through agrobacterium vectors. The genes introduced in the host plant, produced both sense RNA and anti-sense RNA, which were complementary to each other. These two RNAs formed a dsRNA molecule, which initiated silencing of the specific mRNA in the nematode, when taken up by the parasite. As a result, the translation of mRNA in the parasite was prevented and it was unable to survive in the transgenic tobacco plant. Through RNA interference, the plant got itself protected from infestation.

23. (a) There are a number of causes of air pollution.

• Smokes from thermal power plants, smelters and other manufacturing industries release particulate matter and air pollutants, which remain, suspend in the air. These industries also emit toxic harmful gases.

Section D (Solutions)

31

• The increasing use of automobiles is a major concern for causing air pollution. Automobile exhaust increases the concentration of CO2 and other greenhouse gases in the environment.

(b) Intense air pollution greatly affects health of all organisms.

• Air pollutants when inhaled by humans and animals get stuck in their respiratory tract and causes serious diseases such as asthma, heart diseases and eye irritation.

• Air pollution also affects the growth of plants. It leads to their premature death and reduces their yield.

(c) As a captain of my school eco-club, I will suggest following 2 programmes for awareness among students:

• An assessment of the school’s atmosphere should be carried out to determine the exact level of air pollutants. Next, a prevention plan should be developed that aims at lowering the concentration of air pollutants. Strict guidelines must be followed, which include restricting smoking around the school area and installing efficient ventilation systems in school.

• Periodic regular orientations should be held in school for creating awareness about air pollution and its harmful effects. Students must be encouraged to keep their environment clean and healthy.

32

24. (a) In several flowering plants, male and female flowers are borne on different plants that mean a flower is either male or female. This condition is known as Dioecy. It prevents both autogamy and geitonogamy. Example of dioecious plant – papaya.

In a few flowering plants, maturity of the stamens and carpels in a bisexual flower is not synchronised. That is the male and female flowers mature at different times. This condition is known as Dichogamy. Example of dichogamous plants – Cotton and Ficus.

(b) Double fertilization

After entering one of the synergids,

• The pollen tube releases the two male gametes into the cytoplasm of the synergids.

• One of the gametes moves towards the egg cell and fuses with its nucleus. This process completes the syngamy.

• This results in the formation of a diploid cell, the zygote.

• The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN).

• As this involves the fusion of three haploid nuclei, it is termed triple fusion.

• Since two types of fusions, syngamy and triple fusion take place in an embryo sac; the phenomenon is termed double fertilisation.

• The central cell after triple fusion becomes the primary endosperm cell (PEC) and develops into the endosperm while the zygote develops into an embryo.

Section E (Solutions)

33

• Later the synergids also disintegrate.

(a) The reproductive cycle in the female primates is called menstrual cycle. The first menstruation that begins at puberty is called menarche. In human females, menstruation is repeated at an average interval of about 28/29 days. Ovulation occurs during the middle of each menstrual cycle. There are 4 phases of menstrual cycle:

• Menstrual phase – menstrual flow (bleeding) occurs during this phase, which lasts for 3-5 days. It occurs because of breakdown of endometrial lining of the uterus. Menstruation occurs only if the released ovum is not fertilised. It usually occurs about 14 days after ovulation.

• Follicular phase – in this phase, primary follicles grow into mature Graafian follicles. The endometrium also proliferates. The levels of LH and FSH increase gradually. The growing follicles start to secrete estrogen. The combined effect is the thickening of endometrium. Both LH and FSH attain a peak during the middle of each cycle. Rapid secretion of LH leads to its maximum level during mid-cycle, called as LH surge. LH surge induces rupture of Graafian follicle.

• Ovulation – LH surge induces ovulation, which usually occurs on 14th day of a cycle.

OR

34

• Luteal/secretory phase – after ovulation, the egg is moved through the fallopian tubes where it awaits fertilization. The remaining cells of Graafian follicle forms Corpus luteum. It is responsible for thickening and maintaining the endometrium as it secrets large amounts of progesterone and small amounts of estrogen. LH hormone also stimulates the corpus luteum to secrete estrogen and progesterone, which inhibit the release of LH and FSH. As a result, development of new follicles is prevented. During this phase levels of estrogen and progesterone will rise, while levels of LH and FSH will drop. Low level of LH causes degeneration of corpus luteum leading to decline in progesterone level that causes menstruation.

If fertilization occurs, corpus luteum is maintained and is not degenerated.

35

(b) The scientific understanding of the menstrual cycle of human females can help as a contraceptive measure. Periodic abstinence is one method in which the couples avoid sexual intercourse from day 10 to 17 of the menstrual cycle because ovulation occurs at this time. As chances of fertilisation are very high during this period, it is called the fertile period. Therefore, by abstaining from coitus during this period, conception could be prevented.

25. (a) Drosophila melanogaster

Thomas Hunt Morgan carried out several dihybrid crosses in Drosophila to study genes that were sex-linked. Morgan hybridised yellow-bodied, white-eyed females to brown-bodied, red-eyed males and intercrossed their F1 progeny. He observed that the two genes did not segregate independently of each other and the F2 ratio deviated very significantly from the 9:3:3:1 ratio. In the resulting progeny, the proportions of parental gene combinations were much higher than the non-parental type. Morgan attributed this due to the physical association or linkage of the two genes and coined the term linkage to describe this physical association of genes on a chromosome and the term recombination to describe the generation of non-parental gene combinations.

(b) Alfred Sturtevant (student of T.H. Morgan) used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and ‘mapped’ their position on the chromosome. If the distance between two genes is large, they show higher frequency of

36

crossing over and vice-versa. Today genetic maps are extensively used as a starting point in the sequencing of entire genomes.

(a) Central dogma was proposed by Francis Crick. It explains the flow of information from DNA to RNA and finally to trait expressing polypeptides (proteins).

Exceptions to central dogma:

An exception to the central dogma - The biggest revolution in the central dogma was the discovery of retroviruses, which transcribe RNA into DNA with the use of a special enzyme called reverse transcriptase produced by some viruses. The resulting flow of information was RNA → DNA → RNA → proteins. This was discovered by H. Temin and D. Baltimore.

Another exception is the discovery of Prions. The resulting flow of information is Protein → Protein. That is, proteins directly replicating themselves by making conformational changes in other proteins.

(b) Biochemical Characterisation of Transforming Principle:

Oswald Avery, Colin MacLeod and Maclyn McCarty (1933-44), worked to determine the biochemical nature of ‘transforming principle’ in Griffith's experiment. They purified proteins, DNA and RNA from the heat-killed S cells to see which ones could transform live R cells into S cells. They discovered that DNA alone from S bacteria caused the transformation of R bacteria. They also discovered that protein-digesting enzymes

OR

37

(proteases) and RNA-digesting enzymes (RNases) did not affect transformation, so the transforming substance was not a protein or RNA. Digestion with DNase did inhibit transformation, suggesting that the DNA caused the transformation. They concluded that DNA is the hereditary material. In summary,

1. Protein + Protease -> transformation occurred (R cell to S type)

2. RNA + RNAase -> transformation occurred (R cell to S type)

DNA + DNAase → transformation inhibited

Hence DNA alone is the transforming material

26. (a) Responses of different animals to various abiotic factors:

• Regulate - Some organisms are able to maintain homeostasis by physiological or behavioural means, which ensures constant body temperature. All birds and mammals are capable of such regulation (thermoregulation and osmoregulation).

• Conform - Most animals and plants cannot maintain a constant internal temperature. Their body temperature changes with the surrounding temperature. In aquatic animals, the osmotic concentration of the body fluids changes according to the osmotic concentration of the water body. Example: Frogs and Polar bear.

• Migrate - Some organisms temporarily move away from the stressful habitat to a more hospitable area and return when the stressful period is over. For example, birds, during winter undertake long-distance migrations to more hospitable areas. Every winter, we see thousands of migratory birds coming from Siberia to Keolado National Park in Rajasthan.

38

• Suspend - In bacteria, fungi and lower plants, thick-walled spores are formed which help them to survive unfavourable conditions. In higher plants, seeds and some vegetative reproductive structures serve as means to cope up periods of stress as well as help them in dispersal. They do so by reducing their metabolic activity and going into a state of ‘dormancy’. Example: Fish, Snails.

(b) Death rate is defined as the ratio of the number of deaths occurring in a population of a particular area during a particular period of time.

If 8 individuals in a population of 80 butterflies die in a week,

Death rate in the population is given by 8/80 = 0.1 individuals per week.

(a) All organisms occupy a particular place in their natural surroundings or in a community according to their interaction with other organisms. Based on their feeding habits, they occupy a specific place in the food chain that is known as their trophic level.

The standing crop is measured as the mass of living organisms (biomass) or the number of organisms living in a unit area at a particular time. The biomass of a species is expressed in terms of fresh or dry weight.

(b) The most important trophic level within the ecosystem is the first level: primary producers. These organisms live without feeding off another level. They only need sunlight and water to survive. They convert solar energy into food. All other organisms depend on these primary producers to obtain their nutrition.

OR

39

(c) The detritus food chain (DFC) is made up of decomposers (fungi and bacteria) which are heterotrophic/saprotrophic organisms. They obtain their energy and nutrition by degrading dead organic matter or detritus. They secrete digestive are easily absorbed by them.enzymes that breakdown dead organic matter into simple inorganic materials that

Contrary to a terrestrial ecosystem, where larger fraction of energy flows through the detritus food chain, in an aquatic ecosystem, grazing food chain (GFC) is the major conduit for energy flow. Detritus food chain is connected with the grazing food chain at some levels: some of the organisms of detritus food chain (DFC) are prey to the grazing food chain (GFC) animals. We observe that in a natural ecosystem few animals like cockroaches and crows are omnivores.

40