CBSE Class 12 Biology Guess Paper 2027

(i) Name the protozoan species that is responsible for causing the most serious and even fatal malarial disease. (ii) Name the host in which the parasite completes its sexual stages and explain the changes taking place. (iii) How does the parasite damage the human body after entering the blood stream? (iv) Suggest two effective preventive measures to control the spread of this disease in endemic regions.

Assertion (A): Flocs are masses of bacteria associated with fungal filaments in secondary treatment of sewage. Reason (R): Flocs help in digestion of solid waste by anaerobic respiration. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation for Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true.

Study the sequence of bases in a DNA molecule that is shown below and answer the questions. 5' – GAATTC – 3' / 3' – CTTAAG – 5'. (a) What are such sequences called? Name the enzyme used that recognises such nucleotide sequences. (b) How do these enzymes function?

Albinism is known to be due to an autosomal recessive mutation. The first child of a couple with normal skin pigmentation was an albino. What is the probability that their second child will also be an albino? A. 100% B. 25% C. 50% D. 75%

How many pollen grains and ovules are likely to be formed in the anther and the ovary of an angiosperm bearing 50 microspore mother cells and 50 megaspore mother cells respectively ? (A) 100, 25 (B) 200, 50 (C) 50, 50 (D) 200, 100

Identify the plant in which emasculation is not required for artificial hybridization. (A) Rice (B) Wheat (C) Pea (D) Papaya

The experimental proof on the thermal stability of genetic material was first provided by experiments of A. Hershey and Chase B. Meselson and Stahl C. Frederick Griffith D. Jacob an Monod

RNA interference (RNAi) helps in making tobacco-plant resistant to a nematode (Meloidegyne incognitia). Choose the correct option that shows how RNAi is achieved : (A) Preventing the process of translation of mRNA of the nematode. (B) Preventing the process of replication of DNA of the nematode. (C) Preventing the process of transcription of DNA of the plant. (D) Preventing the process of replication of DNA of the plant.

Amplification of gene of interest by using DNA polymerase may go upto (A) 0.1 million times (B) 1 million times (C) 1 billion times (D) 1 trillion times

The sequence of nitrogenous bases in a segment of a coding strand of DNA is 5' - AATGCTAGGCAC - 3'. Choose the option that shows the correct sequence of nitrogenous bases in the mRNA transcribed by the DNA. (A) 5' - UUACGAACCGAG - 3' (B) 5' - AAUGCUAGGCAC - 3' (C) 5' - UUACGUACCGUG - 3' (D) 5' - AACGUAGGCAGC - 3'

The population pyramids of Japan for the years 1950, 2007 and on the basis of the estimated value for 2050 have been shown below to answer the questions that follow: A. Comment upon the growth status of the population on the basis of shape of the age pyramids. B. What insights can you gain about their population dynamics? [For visually impaired students: The population pyramids of Japan for the years 1950, 2007 and on the basis of the estimated value for 2050 have broad-based, inverted bell and urn shape, respectively. Answer the following questions: A. Comment upon the growth status of the population on the basis of shape of the age pyramids. B. What insights can you gain about their population dynamics?]

Suggest a suitable contraceptive device for the following cases with justification. (i) Mohini does not want to take the risk of conception and sexually transmitted infections (STIs). (ii) Lalita has two children and does not want any more children. (iii) Geeta wants a contraceptive that she can take till she wants to avoid conception and can resume back to her fertile life without the intervention of the doctor. Also, it should have a lower failure rate.

Assertion (A): The milk produced by transgenic cow 'Rosie' was nutritionally more balanced product for human babies than natural cow milk. Reason (R): It was human protein enriched milk containing human alpha lactalbumin. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation for Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true.

Assertion (A): A floating cover placed over the slurry in a biogas plant keeps on rising. Reason (R): This cover keeps on rising due to the gas produced in the tank by the microbial activity. A. Both A and R are true and R is the correct explanation of A. B. Both A and R are true and R is not the correct explanation of A. C. A is true but R is false. D. A is False but R is true.

Analyse the following two cases of sex determination in different organisms: Case I: Males have (XO) sex chromosomes and females have two copies of same sex chromosome (XX). Case II: Females have two different sex chromosomes (ZW) and males have two copies of same sex chromosome (ZZ). Identify the type of heterogamety in each case, giving one example of each.

Two statements are given - one labelled as Assertion (A) and the other labelled as Reason (R). Select the appropriate option: (A) Both (A) and (R) are true and (R) is the correct explanation of (A). (B) Both (A) and (R) are true, but (R) is not the correct explanation of (A). (C) (A) is true, but (R) is false. (D) (A) is false, but (R) is true. Assertion (A) : Saheli is the World's first non-steroidal oral contraceptive pill. Reason (R) : It has been developed by National Institute of Immunology, New Delhi.

Given below is a pyramid found in an ecosystem, where each bar represents the standing crop available in the trophic level. (i) Identify the kind of pyramid and with the help of an example explain the conditions where this kind of pyramid is possible in nature. (ii) Write any two limitations of ecological pyramids.

In an ecosystem there was loss of biodiversity due to some project in that area. (i) How will biodiversity be affected? (2 points) (ii) List two major causes of loss of biodiversity.

[Case context] The graph below shows the Antibody concentration in young calves. Study the graph and answer the questions that follow: (Graph: Passive immunity (Colostrum) peaks early then falls; Active immunity (Immune system) rises after a high risk period between 1 d and 14 d.) A. What do you think is the difference between passive and active immunity in this case? (1) B. What kind of immunity will be observed when a vaccine is administered to the calf and why? (2) Attempt either subpart C or D. C. What kind of trend does passive immunity show and why? (1) OR D. Why does active immunity start after 14 days as per the above graph? [For visually impaired students: The antibody concentration in a young calf was studied. It was found that the antibodies derived from colostrum (passive immunity) decreased from day 1 to 14, while the antibodies derived from immune cells (active immunity) increased between day 1 to day 14 and remained steady thereafter. A. What do you think is the difference between passive and active immunity in this case? (1) B. What kind of immunity will be observed when a vaccine is administered to the calf and why? (2) Attempt either subpart C or D. C. What kind of trend does passive immunity show and why? (1) OR D. Why does active immunity start after 14 days as per the above graph?]

Identify the stage of follicle where primary oocyte undergoes first meiotic division. Also mention the products of this division.

The primary endosperm nucleus is formed by fusion of which of the following? A. A male gamete and a female gamete B. A male gamete and two polar nuclei C. A female gamete and two synergids D. Two male gametes and an egg cell

[Case: The diagram below shows the distribution of two barnacle species, Chthamalus and Balanus on a rocky sea shore. When Balanus is experimentally removed, Chthamalus expands its range in lower intertidal zone.] (a) Identify and define the ecological phenomenon demonstrated by this observation. OR (a) State the principle that explains the elimination of one species. (b) Two different species can compete for the same resource. Give another example of it. (c) How do species avoid competition in nature? Explain with an example.

(i) What are bio-fertilizers? (ii) Name the different types of microorganisms used as bio-fertilizers in organic farming and explain how each contributes to soil fertility. (iii) Write two advantages of using bio-fertilizers over chemical fertilizers.

[Case context] Given below is a set of information about some fruits and seeds. (P: Nucellar cells surrounding the embryo sac develop into embryos. Q: Ovary develops into the fruit by the application of growth hormones. R: Thalamus contributes to fruit formation. S: Ovary matures into a fruit after fertilisation.) On the basis of the information provided above, answer the following questions with justification for each answer. A. How many embryo sacs will be present in each ovule of S before maturation and how many egg(s) will be present in each embryo sac when the embryo sac is developed from a single megaspore? (1) B. (i) Which of these fruits exhibits polyembryony? Will their embryos exhibit genetic variation? Justify. (ii) What will be ploidy of the embryonic cells in the above case? (2) Attempt either subpart C or D. C. Which of these fruits can be considered as parthenocarpic? Give a reason. (1) OR D. Which of the fruits P, Q, R or S is a true fruit with seeds? Give a reason. (1)

Some plant and animal pathogens serve as one of the tools of recombinant DNA (rDNA) technology. A. Name one animal and one plant pathogen and discuss the pathogenic nature of both. State how they serve as a tool in rDNA technology. B. What are the enzymes needed for rDNA technology? C. A farmer owns a cotton farm land which is getting infested with coleopteran pests. He is not willing to use the microbes to protect his farm. i. Name an alternate method to introduce the gene of interest the pathogen would have otherwise delivered and discuss how the alternate method would deliver the gene. ii. State how this gene would control the pest. OR BamH1 is a restriction enzyme which recognizes the sequence 5' - GGATCC - 3'. The restriction activity of this enzyme is between G and G. A. Construct the palindrome for the above sequence. B. Draw a labeled diagram to show the formation of recombinant DNA (rDNA) using BamH1. C. PBR322 is a plasmid that has a restriction site for this enzyme at the tetracycline resistant gene. If BamH1 were to be used, how will it impact the response of the transformant with rDNA to antibiotics- ampicillin and tetracycline. Justify.

Trace the events from fertilization till implantation of blastocyst in human female. Also mention the site where fertilization takes place.

Shown below are certain aquatic vertebrates (salmon, shark, dolphin, ichthyosaur, blue whale, seal), where natural selection has favoured them to develop certain characteristics which enable them to live in water. A. Name and explain the phenomenon exhibited by the above animals. B. Which one of the above is the most primitive one? What is its significance? [For Visually impaired students: A. What are the basic conceptual similarities and differences between Lamarckian and Darwinian theory of evolution? B. Give an example that supports any one of the above two theories.]

Observe the given picture carefully. A mixture of DNA with fragments ranging from 100 base pairs to 1800 base pairs were separated by electrophoresis on agarose gel with the following arrangement: (a) What result will be obtained in staining with ethidium bromide? Explain with reasons. (b) The above setup was modified as shown below and a band with 100 base pairs was obtained at X. What changes were made to the previous design to get a band at 'X'. Why did the band appear at 'X'?

In order to isolate genetic material of a bacterium, the cell must be treated with A. Lysozyme, ribonuclease, protease, chilled ethanol B. Cellulase, ribonuclease, protease, chilled ethanol C. Chitinase, ribonuclease, chilled ethanol, water D. Ribonuclease, protease, chilled ethanol, water

A culture plate of Lactobacillus shows blue-coloured colonies and colourless colonies. Explain the principle involved in the formation of such variance in the colour of colonies.

Given below is a stretch of DNA showing the coding strand of structural gene of transcription unit. 5' - ATG ACC GTA TTT TCT GTA GTG CCC GTA CTT CAG GCA TTA 3'. (i) Write the corresponding template strand and m-RNA strand that will be transcribed along with its polarity. (ii) If GUA of transcribed mRNA is an intron, then depict the sequence involved in formation of mRNA / mature / processed hnRNA strand: (1) In a bacterium (2) In humans. (iii) How many amino acids the resulting polypeptide will have after the process of translation in humans?

(i) Case studies of some couples are given below. They were not able to have kids even though the parents were not taking any precautions. They also do not wish to adopt a child or take the help of donors. Study these cases and suggest an appropriate Assisted Reproductive Technology that could help them. Case I – Female – Normal Reports. Male – Normal sperms but no connection between epididymis and vas deferens. Case II – Female – Normal eggs but Fallopian tube blocked. Male – Normal sperms. Male reports are normal. Case III – Female – Normal Reports. Male – Low sperm count with less motility. (ii) Copper releasing intrauterine devices are very effective and popular contraceptive devices. Name any two copper releasing IUDs. Write two reasons that make them effective contraceptives.

Student to attempt either option-(A) or (B) : (A) (a) Distinguish between the two cells enclosed in a mature male gametophyte of an angiosperm. (b) Study the diagram given below showing the modes of pollination. Answer the questions that follow. [Diagram: three flowers showing pollen transfer methods labelled 1, 2 and 3.] (i) The given diagram shows three methods of pollen transfer in plants. Examine them carefully and write the technical terms used for pollen transfer methods '1', '2' and '3'. (ii) How do the following plants achieve pollination successfully ? (a) Water lily (b) Vallisneria. (iii) Write advantages of pollen transfer in method '3'. OR (B) Given below is the diagram of human ovum surrounded by a few sperms. Observe the diagram and answer the questions that follows : [Diagram: ovum with zona pellucida, perivitelline space, cells of the corona radiata, and sperms labelled P, Q and R.] (i) Compare the fate of sperms 'P', 'Q' and 'R' shown in the diagram. (ii) Write the role of Zona pellucida in this process. (iii) Analyse the changes occurring in the ovum after the entry of sperm. (iv) How acrosome and middle piece of a human sperm are able to play an important role in human fertilization ?