CBSE Class 12 Board Exam Biology Paper 2018 2019 2020 With Solutions

Class 12 Subject Biology Paper Set 1 with Solutions

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Question 1: (Marks 1)

Meselson and Stahl carried out centrifugation in CsCl2 density gradient to separate :

(A) DNA from RNA

(B) DNA from protein

(C) The normal DNA from 15N-DNA

(D) DNA from tRNA

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Answer :

C / The normal DNA from 15N-DNA

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Question 2: (Marks 1)

Self-pollination is fully ensured if

(A) the flower is bisexual.

(B) the style is longer than the filament.

(C) the flower is cleistogamous.

(D) the time of pistil and anther maturity is different.

OR

Zoospores are the reproductive units to carry asexual reproduction in

(A) Chlamydomonas

(B) Spirogyra

(C) Yeast

(D) Rhizopus

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Answer :

C / Flower is Cleistogamous

OR

A / Chlamydomonas

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Question 3: (Marks 1)

Micropropagation can be achieved by

(A) Self-pollination

(B) Asexual reproduction

(C) Tissue culture

(D) Vegetative propagation

OR

The microbes commonly used in kitchens are

(A) Lactobacillus and Yeast 4

(B) Penicillium and Yeast

(C) Microspora and E. coli

(D) Rhizopus and Lactobacillus

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Answer :

C / Tissue Culture

OR

A / Lactobacillus and Yeast

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Question 4: (Mark 1)

The main barrier that prevents the entry of micro-organisms into our body is

(A) Antibodies

(B) Macrophages

(C) Monocytes

(D) Skin

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Answer :

D / Skin

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Question 5: (Marks 1)

Nematode specific genes were introduced into the tobacco host plant using a vector

(A) pBR 322

(B) Plasmid

(C) Bacteriophage

(D) Agrobacterium

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Answer :

D / Agrobacterium

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Question 6: (Marks 2)

Given below is one of the strands of a DNA segment :

(a) Write its complementary strand.

(b) Write a possible RNA strand that can be transcribed from the above DNA molecule formed.

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Answer :

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Question 7: (Marks 2)

Wings of birds and wings of butterflies contribute to locomotion. Explain the type of evolution such organs are a result of.

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Answer :

They are not anatomically similar in structure but perform same function , hence these are analogous structure

type of evolution is convergent evolution

similar habitat of birds and butterflies has resulted in selection of similar adaptive features (wings) in different groups of organisms , but towards the same function

convergent evolution

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Question 8: (Marks 2)

It is often observed that the chances of a person suffering from measles in his or her lifetime are low if he or she has suffered from the disease in their early childhood. Justify the statement.

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Answer :

Memory cells develop during measles in early childhood , subsequent encounters with the same pathogen elicits a highly intensified secondary / anamnestic response

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Question 9: (Marks 2)

List the three hormones produced in women only during pregnancy. What happens to the levels of estrogen and progesterone during pregnancy ?

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Answer :

- Human chorionic gonadotropin / hCG ,

- Human placental lactogen / hPL ,

- Relaxin ,

(The level of progesterone & estrogen) increases

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Question 10: (Marks 2)

A student on a field trip suddenly felt breathlessness and started to sneeze very badly. Name this response and explain what it is due to.

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Answer :

Allergy = 1 Allergens ( dust / pollen / mites etc.), due to release of chemicals like histamine/serotonin ( from mast cells)

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Question 11: (Marks 2)

With the help of a suitable example, explain how cross-breeding is carried out in developing a new breed in animals.

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Answer :

Mating of superior male of one breed with superior female of another breed. =1 Hisardale , is a new breed of sheep ( developed in punjab) by crossing Bikaneri Ewes and Marino Rams.

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Question 12: (Marks 2)

Name the genus of baculovirus that acts as a biological control agent in spite of being a pathogen. Justify by giving three reasons that make it an excellent candidate for the job.

OR

‘‘Micro-organisms play an important role for the biological treatment of sewage.’’ Justify

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Answer :

Genus -Nucleopolyhedrovirus, species-specific , Narrow spectrum , No negative impact on (plants /mammals/ birds /fish ) non-target organisms

OR

Primary effluent passed into aeration tanks, constantly agitated and air is pumped in , allows growth of useful aerobic microbes into flocs / (masses of bacteria and fungal filaments), these microbes consume organic matter and reduce Biochemical oxygen demand (BOD) of effluent.

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Question 13: (Marks 3)

Draw a schematic transverse section of a mature anther of an angiosperm. Label its epidermis, middle layers, tapetum, endothecium, sporogenous tissue and the connective.

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Answer :

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Question 14: (Marks 3)

Differentiate between wind pollinated and insect pollinated flowers.

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Answer :

Wind pollinated

Pollen grains are light non sticky /well exposed stamens/ Large feathery stigma / single ovule in each flower / Small flowers packed in inflorescence Any three

Insect pollinated

Pollen grains sticky/ Floral rewards / Fragrant / Nectar rich / Large Flower

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Question 15: (Marks 3)

Generally it is observed that human males suffer from hemophilia more than human females, who rarely suffer from it. Explain giving reasons.

OR

F₁ progeny of pea plant bearing violet flowers and snapdragon plant bearing red flowers were selfed to produce their respective F₂ progeny. Compare the phenotypes, the genotypes and the pattern of inheritance of their respective F₂ progeny.

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Answer :

This is a sex linked / X chromosomes linked recessive disease ,the heterozygous female / carrier for hemophilia may transmit the disease to sons (male progeny), The possibility of hemophilic female/daughter is rare because mother of such a female has to be at least carrier and the father should be hemophilic.

OR

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Question 16: (Marks 3)

Explain the changes that milk undergoes when suitable starter/inoculum is added to it. How does the end product formed prove to be beneficial for human health ?

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Answer :

Lactobacillus / lactic acid bacteria (LAB) present in inoculum , grows in milk at a suitable / optimum temperature, multiplies converting milk to curd, produce acids that coagulate and partially digest the milk proteins.

This improves its nutritional quality by increasing Vitamin B12 , LAB check diseases causing microbes in our stomach.

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Question 17: (Marks 3)

Alien species invasion has been a threat to biodiversity. Justify with the help of a suitable example. List any other three causes responsible for such a loss.

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Answer :

Example of Alien species invasion

Nile Perch , introduced into lake Victoria ( in East Africa) , led to extinction of Cichlid fish (more than 200 species) in the lake

Introduction of African cat fish (Clarias gariepinus) , for aquaculture, posing threat to indigenous catfish

Introduction of carrot grass (Parthenium) / Lantana / Water hyacinth (Eicchornia) ,which are invasive weed , that pose threat to native species or any other appropriate example.

Causes of biodiversity loss

(i) Habitat loss and fragmentation

(ii) Over exploitation

(iii) Co-extinction

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Question 18: (Mark 3)

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Answer :

OR

Plant breeding is the purposeful manipulation of plants species in order to create desired plant types, that are better suited for cultivation / better yield / disease resistant.

Classical Plant breeding involves crossing or hybridisation of pure lines,followed by artificial selection to produce plants with desirable traits of higher yield / nutrition / resistance to diseases.

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Question 19: (Marks 3)

Explain the three steps carried out in the formation of recombinant DNA using the enzyme EcoRI.

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Answer :

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Question 20: (Marks 3)

Name any two natural cloning vectors. Give reasons that make them act as cloning vectors. Write the two characteristics the engineered vectors are made to possess.

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Answer :

Plasmids , bacteriophages

ability to replicate within bacterial cells , high copy number within the bacterial cells

Characteristics of engineered Vectors: easy linking of foreign DNA, Selection of recombinants from non- recombinants /selectable marker

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Question 21: (Marks 3)

Explain the difference between commensalism and mutualism types of interactions, with the help of a suitable example of each.

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Answer :

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Question 22: (Marks 3)

Insulin in the human body is secreted by pancreas as prohormone/proinsulin. The schematic polypetide structure of proinsulin is given below. This proinsulin needs to undergo processing before it becomes functional in the body. Answer the questions that follow :

(a) State the change the proinsulin undegoes at the time of its processing to become functional.

(b) Name the technique the American company Eli Lilly used for the commercial production of human insulin.

(c) How are the two polypeptides of a functional insulin chemically held together ?

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Answer :

(a) ‘C’ Peptide is removed

(b) r-DNA technology / Recombinant DNA Technology

(c) Disulphide bonds

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Question 23: (Marks 3)

The cytological observations made in a number of insects led to the development of the concept of genetic/chromosomal basis of sex-determination mechanism. Honey bee is an interesting example to study the mechanism of sex-determination. Study the schematic cross between the male and the female honey bees given below and answer the questions that follow :

(a) Identify the cell divisions ‘A’ and ‘B’ that lead to gamete formation in female and male honey bees respectively.

(b) Name the process ‘C’ that leads to the development of male honey bee (drone).

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Answer :

(a) ‘A’ - meiosis

‘B’-Mitosis

(b) ‘C’-Parthenogenesis

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Question 24: (Marks 3)

Study the age pyramids ‘A’, ‘B’ and ‘C’ of the human population given below and answer the questions that follow :

(a) Identify pyramids ‘B’ and ‘C’.

(b) Write the basis on which the above pyramids are plotted.

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Answer :

(a) B- Stable population

C- Declining population

(b) Age Distribution of male and female of human population

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Question 25: (Marks 5)

(a) IVF is a very popular method these days that is helping childless couples to bear a child. Describe the different steps that are carried out in this technique.

(b) Would you consider Gamete Intrafallopian Transfer (GIFT) as an IVF ? Give a reason in support of your answer.

OR

(a) Draw a sectional view of a human ovary and label primary follicle, tertiary follicle, Graafian follicle and corpus luteum in it.

(b) Name the gonadotropins and explain their role in oogenesis and the release of ova.

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Answer :

(a) Ova from the wife /donor (female) and sperms from the husband / donor (male ) , are collected and induced to form zygote , in simulated conditions in laboratory , the zygote/ early embryos(upto 8 blastomeres) then transferred into the fallopian tube(ZIFT), and embroyos with more than 8 blastomeres, into the uterus(IUT) to complete its further development.

(b) No, GIFT cannot be considered as IVF technique because fertilisation takes place in the female body / in vivo.

OR

a.

b. LH , FSH

Both LH and FSH increase during follicular phase and stimulate follicular development , secretion of estrogen by the growing follicles, LH surge ( rapid secretion of LH) during mid cycle / 14th day induces rupture of graffian follicle, and release of ova / secondary oocyte

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Question 26: (Marks 5)

Describe the experiment carried out by Hershey and Chase. Write the conclusion they arrived at.

OR

(a) Describe the observations made on collection of white winged moths and dark winged moths in England between the years 1850 and 1920. What did these observations lead to ?

(b) How is the use of herbicides, pesticides and antibiotics by humans for various purposes, comparable with the observations made on moths in the above question ? What is this type of phenomenon called ?

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Answer :

They grew viruses on a medium containing radioactive phosphorus /P³², and some on radioactive sulphur / S³⁵, radioactive DNA contained in viruses grown on radioactive phosphorus , radioactive protein contained in virus grown in radioactive sulphur , Radioactive phages were allowed to attach to E.coli bacteria , the virus coats were removed from bacteria by agitating in a blender, the virus particles were seperated from the bacteria on centrifugation , Bacteria infected with viruses containing radioactive DNA were radioactive, where as bacterial cells infected with virus containing radioactive protein did not show radioactivity ,

Conclusion- DNA is therefore the genetic material that is passed from virus to bacteria

OR

a. Before industrialisation tree bark were covered with light coloured lichens , In this background white winged moth survived but dark coloured moth was picked out by predators,post industrialisation tree trunks become dark due to industrial smoke and soot,under this condition the white winged moth did not survive due to predators,while the dark winged/melanised moth survived, this showed that organism that are better adapted to survive are selected by Nature/Natural selection.

b. Excessive use of these chemicals has resulted in the selection of resistant varieties , in a much lesser time ( scale) .

These are examples of evolution by anthropogenic action.

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Question 26: (Marks 5)

Describe the model of phosphorus cycle in the terrestrial ecosystem.

OR

Describe the DDT biomagnification occurring in an aquatic food chain. State the negative effects the process has on the organisms at the last trophic level of the food chain.

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Answer :

Phosphorus a major constituent of biological membrane / nucleic acids / cellular energy transfer system , Many animals need it for making shells / bones and teeth , the natural reservoirs of phosphorous is rock , which contains phosphorous in the form of phosphate reserves , on weathering minute amount of phosphate dissolve in soil solution , and absorbed by roots , Herbivores , obtain these elements from plants waste products and dead organism , decomposed by phosphate-solubilising bacteria , releasing phosphorus

Marks can also be awarded for the steps given in the proper sequence, with other details

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