PSEB CLASS 12 BIOLOGY GUESS PAPER MARCH 2026 SOLVED

Section - A

1.     Each question carries one mark.
        Multiple Choice Questions.

(i)

Identify the parts labelled A, B, C and D in the diagram of an antibody molecule and select the correct options : [ ] A - Light chains, B - Heavy chains, C - Antigen-binding sites, D - Disulphide bonds A - Disulphide bonds, B - Antigen-binding sites, C - Heavy chains, D - Light chains A - Antigen-binding sites, B - Light chains, C - Heavy chains, D - Disulphide bonds A - Antigen-binding sites, B - Disulphide bonds, C - Light chains, D - Heavy chains

1

(ii)

The enzyme Taq polymerase used in PCR has been isolated from the bacterium : Agrobacterium tumefaciens Thermus aquaticus Streptomyces albus Escherichia coli

1

(iii)

What causes the inactive form of Bt toxin (Protoxin) to get converted into it's active form in the body of an insect ? Temperature of the gut Enzymes present in saliva Alkaline pH of the gut There is no specific reason

1

(iv)

Two species competing for the same resource can avoid competition by choosing different habits. This phenomenon is called ____________ and was supported by ____________. Competitive exclusion, Gause Competitive exclusion, MacArthur Resource partitioning, Gause Resource partitioning, MacArthur

1

(v)

The flow of energy through lower to higher trophic levels in an ecosystem is : always unidirectional. sometimes unidirectional and sometimes bidirectional. always bidirectional. cannot be determined with certainty.

1

(vi)

Meenu visited Mahendra Chaudhary Zoolgical Park (Chattbir Zoo) as part of her environmental studies school tour. She observed several animals such as tigers, rhinoceroses and lions being maintained and bred in enclosures. This is an example of which type of conservation ? In situ conservation Ex situ conservation On-site conservation Natural habitat conservation

1

(vii)

Geitonogamy involves transfer of pollen grains : from anther to stigma of the same flower. from anther to stigma of another flower on the same plant. from anther to stigma of a flower on a different plant of the same species. between flowers of different species.

1

(viii)

The testes in humans are located outside the abdominal cavity within a pouch called the scrotum. This positioning helps in : avoiding compression by internal organs. maintaining a temperature slightly lower than the body temperature for sperm formation. protecting the testes from physical injury during movement. helping in the storage of urine before excretion.

1

(ix)

Diaphragms are contraceptive devices used by females. Which of the following statement is correct about diaphragm ? They are introduced into the uterus to prevent implantation. They are placed to cover the cervical region and act as a physical barrier for sperm entry. They act as spermicidal agents. They prevent ovulation by hormonal regulation.

1

(x)

A pure tall (TT) plant is crossed with a pure dwarf (tt) plant to produce F1 offspring. The F1 offspring are then self-crossed. What is the ratio of true-breeding tall (TT) to true-breeding dwarf (tt) plants in the F2 generation ? (A) 1 : 1 (B) 1 : 2 : 1 (C) 1 : 3 (D) 3 : 1

1

(xi)

In E.coli, the lac operon gets switched on when : lactose is present and it binds to the repressor. repressor binds to operator. RNA polymerase binds to the operator. lactose is absent.

1

(xii)

Darwin observed different types of beaks in finches adapted to different feeding habits on the Galapagos Islands. This diversity provides evidence for : intraspecific competition. interspecific competition. origin of species by natural selection. origin of species by mutation.

1

Fill in the blanks :

(xiii)

The sequence of base pairs in a double-stranded DNA or RNA molecule that reads the same in forwards and backward on each strand is called ____________ nucleotide sequence.

1

(xiv)

The process of release of ovum from the mature follicle is called ____________.

1

(xv)

____________ test is performed to confirm typhoid.

1

(xvi)

Alexander Fleming, Ernst Chain and Howard Florey were awarded the Nobel Prize in 1945 for the discovery of ____________.

1

Write True or False.

(xvii)

The Amazon rainforest is known as the "lungs of the planet" because it produces a large amount of carbon dioxide.

1

(xviii)

Surgical methods of contraception prevent gamete formation.

1

(xix)

Cocaine is obtained from the leaves of the Erythroxylum coca and acts as a stimulant on the central nervous system.

1

(xx)

Saccharomyces cerevisiae is used in the production of alcohol and bread.

1

Section - B

Each question carries 2 marks.

2.

Bt toxin is produced by bacteria Bacillus thuringiensis but it does not kill the bacteria itself. Why is it so ?

2

3.

Define food chain. Construct a grazing food chain using any 5 of the following : Grass, Earthworm, Snake, Vulture, Frog, Grasshopper, Decaying plant matter. OR Explain why the introduction of an alien species reduces the biodiversity of an area ? Support your answer with an example.

2

4.

Draw a well-labelled diagram of a longitudinal section of a flower showing all its parts.

2

5.

State the role of oxytocin in parturition.

2

6.

In case of an infertile couple, the male has very low sperm count but the female has normal functioning reproductive system. Suggest and explain the Assisted Reproductive Technology (ART) useful to this couple. OR In our society, women are often blamed for giving birth to daughters. Explain scientifically why this belief is incorrect ?

2

7.

A man with blood group A marries a woman with blood group B. They have two children, one with blood group AB and the other with blood group O. Work out a genetic cross to show how this is possible ? OR A normal male marries a female carrier for colour blindness. Show the type of progeny formed with help of Punnett square.

2

8.

Classify the following pairs as analogous or homologous organs : Wings of butterflies and birds Thorns of bougainvillea and tendrils of cucurbita Forelimbs of whale, bat, cheetah and human Sweet potato and Potato

2

9.

Write any two points of difference between innate and acquired immunity.

2

10.

Samina saw her mother adding a little curd to warm milk at night. She asked why she was doing this ? She replied that the curd acts as an inoculum to convert milk into curd. Name the bacterium responsible. Explain how adding a small amount of curd (inoculum) helps in making the entire milk turn into curd ?

2

11.

What is biolistic method for introducing alien DNA into a host cell ? OR Why is restriction endonuclease called as 'Molecular Scissor' ? Explain its role in biotechnology.

2

Section - C

Each question carries 3 marks.

12.

Name the type of interaction seen in each of the following examples : Cuscuta growing on a hedge plant Mycorrhizae living on the roots of higher plants An orchid growing on the branch of a mango tree

1 1 1

13.

Given diagram is of transcription unit. Identify the strands A and B in this diagram and write the basis on which you identified them. [ ] OR Why is human genome project called a mega project ?

3 3

14.

After industrialisation in England, the population of melanised moths (dark-coloured) increased while that of white-winged moths decreased. State the reason behind this change. Explain this change in terms of natural selection.

1 2

15.

Name the causing agent of malaria. Also write down its symptoms and modes of transmission.

3

16.

The diagram below shows an agarose gel electrophoresis setup : [ ] Label the positive (+) and negative (-) terminals. What type of charge is carried by DNA molecules and how does this property help in their separation ? Name the method used to isolate the separated DNA fragments from the gel. OR What is gene therapy ? Illustrate using the example of adenosine deaminase deficiency.

1 1 1 3

Section - D

Each question carries 5 marks.

17.

What is meant by monosporic development of the female gametophyte ? Draw a diagram of a mature embryo sac and label any four parts. OR Given below is the diagram of a part of seminiferous tubule of human testis : [ ] Name the cells that undergo spermiogenesis. Name the type of cell division responsible for generation of primary spermatocyte. Differentiate between spermiogenesis and spermiation. How many sperms will be produced from 50 primary spermatocytes ?

2 3 1 1 2 1

18.

Given below is a karyotype obtained after analysis of foetal cells : [ ] Based on the karyotype, what chromosomal disorder is detected ? Write its cause. Write the symptoms of the disorder. OR Frederick Griffith performed an experiment on Streptococcus pneumoniae to study inheritance. Describe the experimental setup. What were his major observations when he injected mice with different strains ? What conclusion did Griffith draw from this experiment ?

1 2 2

19.

Read the paragraph carefully and answer the following questions : A constant input of solar energy is the basic requirement for any ecosystem to function and sustain. Primary production is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight (gm-2) or energy (kcal m-2). The rate of biomass production is called productivity. It is expressed in terms of gm-2 yr-1 or (kcal m-2) yr-1 to compare the productivity of different ecosystems. It can be divided into gross primary productivity (GPP) and net primary productivity (NPP). Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP). Net primary productivity is the available biomass for the consumption to heterotrophs (herbivores and decomposers). Secondary productivity is defined as the rate of formation of new organic matter by consumers. Primary productivity depends on the plant species inhabiting a particular area. It also depends on a variety of environmental factors, availability of nutrients and photosynthetic capacity of plants. Therefore, it varies in different types of ecosystems. Name the factors that influence the primary productivity of an ecosystem. Suppose the GPP of a grassland ecosystem is 2000 kcal m-2 yr-1 and NPP is 1200 kcal m-2 yr-1. Calculate the respiratory loss (R). Why is all the primary productivity not available to herbivores for consumption ? Differentiate between GPP and NPP. OR Define biodiversity. Describe the three important components of biodiversity with suitable examples.

1 1 1 2 2+3

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BIOLOGY QUESTION PAPER

ANSWER KEY

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SECTION – A

Multiple Choice Questions

(i) C – Antigen-binding sites, Light chains, Heavy chains, Disulphide bonds

(ii) B – Thermus aquaticus

(iii) C – Alkaline pH of the gut

(iv) D – Resource partitioning, MacArthur

(v) A – Always unidirectional

(vi) B – Ex situ conservation

(vii) B – From anther to stigma of another flower on the same plant

(viii) B – Maintaining temperature slightly lower than body temperature

(ix) B – Covers cervical region and acts as barrier

(x) A – 1 : 1

(xi) A – Lactose binds to repressor

(xii) C – Origin of species by natural selection

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Fill in the Blanks

(xiii) Palindromic

(xiv) Ovulation

(xv) Widal

(xvi) Penicillin

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True / False

(xvii) False

(xviii) False

(xix) True

(xx) True

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SECTION – B

2. Bt toxin is produced in an inactive protoxin form. It becomes active only in the alkaline gut of insects. Hence, it does not kill the bacteria itself.

3.
Food Chain: A linear sequence of organisms through which energy passes.
Example: Grass → Grasshopper → Frog → Snake → Vulture

OR

Alien species reduce biodiversity by competing with native species and may cause extinction. Example: Nile perch in Lake Victoria.

4. Diagram of L.S. of flower showing: Sepals, Petals, Stamen (Anther, Filament), Pistil (Stigma, Style, Ovary).

5. Oxytocin stimulates strong uterine contractions during childbirth (parturition).

6. Suitable ART: ICSI (Intracytoplasmic Sperm Injection) or Artificial Insemination.

OR

Sex of child is determined by father (XX in female, XY in male).

7. Parents genotype: IAi × IBi
Offspring: AB, A, B, O

OR

Carrier female × Normal male cross gives:
Carrier female, Normal female, Colour blind male, Normal male

8.
Butterfly & Bird wings – Analogous
Bougainvillea & Cucurbita – Homologous
Forelimbs – Homologous
Sweet potato & Potato – Analogous

9.
Innate immunity – Present from birth, non-specific
Acquired immunity – Develops after exposure, specific

10.
(a) Lactobacillus
(b) Bacteria multiply and produce lactic acid, converting milk to curd

11.
Biolistic method: DNA-coated particles are shot into cells using gene gun

OR

Restriction enzymes cut DNA at specific sequences

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SECTION – C

12.
(a) Parasitism
(b) Mutualism
(c) Commensalism

13.
Strand A – Template strand (3' → 5')
Strand B – Coding strand (5' → 3')

OR

Human Genome Project is called mega project due to huge cost, data size and global collaboration.

14.
Industrial smoke darkened tree trunks. Dark moths survived better (natural selection).

15.
Agent – Plasmodium
Symptoms – Fever, chills, sweating
Transmission – Female Anopheles mosquito

16.
Negative terminal – Near wells
Positive terminal – Opposite end
DNA is negatively charged and moves toward positive electrode
Method – Elution

OR

Gene therapy: Introducing functional gene. Example – ADA deficiency treated by inserting ADA gene into lymphocytes.

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SECTION – D

17.
Monosporic development – Embryo sac develops from one functional megaspore

OR

(a) Spermatids
(b) Mitosis
(c) Spermiogenesis – Transformation
Spermiation – Release
(d) 200 sperms

18.
Disorder – Down’s Syndrome
Cause – Trisomy 21
Symptoms – Intellectual disability, flat face, short stature

OR

Griffith experiment showed transformation principle.

19.
(a) Plant species, nutrients, environmental factors
(b) R = 2000 – 1200 = 800 kcal m⁻² yr⁻¹
(c) Some energy lost in respiration
(d) GPP – Total production
NPP – GPP minus respiration

OR

Biodiversity – Diversity at genetic, species and ecosystem level.

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