BIOLOGY Code no.

044
MARKING SCHEME
CLASS – XII (2025–26)

Q. Answer Mar
No. ks

Section - A

1 C. Mitotic division of nucleus of generative cell 1

2 B. A male gamete and two polar nuclei 1

3 C. FSH and LH 1

4 C. Frederick Griffith 1

5 A. Probes 1

6 C. Hardy-Weinberg principle says that phenotype frequencies in a population 1

are stable and are constant from generation to generation.

7 B. 25% 1

8 D. The leg mutation might lead to reproductive isolation and speciation due to 1
an effect on the mating call.

9 D. Sporozoites, Liver infection, Erythrocyte infection, Gametocytes 1

10 B AUGAGACGGACUGCAUUCCCAACCUGA 1

11 A. Lysozyme, ribonuclease, protease, chilled ethanol 1

12 C. Both I and II 1

13 C. A is true but R is false 1

14 A. Both A and R are true and R is the correct explanation of A 1

15 A. Both A and R are true and R is the correct explanation of A 1

16 C. A is true but R is false 1

Section - B

17 It is achieved by emasculation and bagging techniques. 2

If the female parent bears bisexual flowers, removal of anthers from the flower
bud before the anther dehisces using a pair of forceps is necessary. This step
is referred to as emasculation.
Emasculated flowers have to be covered with a bag of suitable size, generally
made up of butter paper, to prevent contamination of its stigma with unwanted
pollen. This process is called bagging. (1 x 2 = 2)

18 In a transcription unit, the activity of RNA polymerase at a given promoter is 2

regulated by accessory proteins that have an ability to recognise start sites.
(1)
These regulatory proteins can act both positively (activators) and negatively
 (repressors) with the operator which is adjacent to the promoter in an operon
unit. (1)

19 A. Innate immunity/ non-specific type of defense/immunity present at the time 2

of birth is effected. It provides different types of barriers to the entry of the
foreign agents into our body/destroys microbes/ prevents microbial growth.
(1)
B. Cellular barrier-monocytes is with least count and it can phagocytose and
destroy microbes. (1)

20 Anyone who can use/modify any living organism or their products using 2
technology is considered to be a biotechnologist. (1)
Thus, cheese maker can be a biotechnologist as he uses microbes like
bacteria, fungi to make cheese for commercial purpose. (1)

21 A. 2
(i) The first (upright) pyramid of biomass corresponds to a terrestrial
ecosystem. Producers have maximum biomass, decreasing with
herbivores (primary consumer), secondary consumer and tertiary
consumer. Second (inverted) pyramid refers to a small standing crop of
phytoplankton supporting a large standing crop of zooplankton/aquatic
ecosystem. (1)
(ii)

(1)
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For Visually impaired students
(Answer same as (i) and (ii) above)
OR
B.
(i) 3000+1300+427+427+378= 5532 (1)
(ii)
(a) Animals are mobile and can migrate to escape harsh conditions or
explore new areas.
(b) Animals have adapted to changing environments, developing complex
nervous systems and receptors. Their responses are adaptive and
ensure survival. Iii. Plants, being fixed, have fewer evolutionary
adaptations for water, minerals, and sunlight. (Anyone, 1)

Section – C

22 (i) Condoms; these act by blocking the entry of microbes and sperms in the 3
cervix.
 (ii) Surgical methods such as Tubectomy by her or vasectomy by her
husband as these are highly effective but irreversible methods.
(iii) Oral pills containing progestogens or progestogen – estrogen combination
are very effective and can be repeated as long as the female derives to
prevent conception. (1 x 3 =3)

23 (i) (a) ovum is haploid (n) and (c) – blastomeres are diploid(2n). 3
(ii) If the trophoblast (L) does not attach to the endometrium properly, it can
lead to implantation failure, pregnancy loss, and other pregnancy
complications.
(iii) In case X, the cells of these embryos will have identical genome as they
have developed from the same zygote. (1 x 3 =3)
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For Visually impaired students
 The mitotic division called cleavage starts as the zygote moves through
the isthmus of the oviduct towards the uterus and forms 2,4,8,16 daughter
cells called blastomeres. The embryo with 8 to 16 cell blastomeres is
called a morula. (1)
 The morula transforms into blastocyst and the blastomeres in the
blastocyst are arranged into an outer layer called trophoblast and an inner
group of cells attached to trophoblast called the inner cell mass. (1)
 The trophoblast layer then gets attached to the endometrium of the uterus
and the inner cell mass gets differentiated as the embryo. After
attachment, the uterine cells divide rapidly and cover the blastocyst which
gets embedded in the endometrium leading to implantation. (1)

24 BbGg Male is crossed with BbGg female which are both heterozygous of 3
both the characters for eye and coat colour. (1)

Punnett square (1)

1/16 or 6.25 % - blue eyes and white coat (1)

25 A. These animals exhibit convergent evolution, structures that are not 3

anatomically similar are evolved to perform similar functions adapted to
the same habitat, the fins of Salmon and Shark, flippers of Dolphins, Seals
and whales. One can say that it is the similar habitat that has resulted in
selection of similar adaptive features in different groups of organisms but
toward the same function: hence, analogous structures are a result of
convergent evolution. (2)
 B. Ichthyosaurs. Some of the land reptiles went back into water to evolve into
fish like reptiles probably 200 mya. (1)
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For Visually impaired students
A. Both Lamarck and Darwin believed that living things had hereditary traits,
traits they could pass on to their offspring. They believed that some traits
were more useful than others, and that over time the more useful trait
would become more common. (1)
The difference is that Lamarck believed that the changes in an organism
experienced during its life could be passed on to succeeding
generations/Inheritance of acquired characters. So, for instance Giraffe
stretched its neck its entire life to eat leaves on tall plants, it would have
longer-necked offspring (or any other example of Lamarckian theory). (1)
Darwin believed that small, random and gradual genetic variations
followed by natural selection lead to evolution- that some giraffes just
naturally had longer necks, and these were the ones who had more
children. So over time natural selection led to evolution of Giraffes with
long neck were fitter than short-necked. (1)

26 'Flocs' and 'activated sludge' in sewage treatment help to reduce the BOD. 3
(i) Flocs: These are masses of bacteria held together by slime and fungal
filaments to form mesh-like structures. These are used during the
secondary sewage treatment in the aeration tank to increase the rate of
decomposition. The microbes digest a lot of organic matter, converting it
into microbial biomass and releasing a lot of minerals. As a result, the
BOD of sewage is reduced. As the BOD of waste is reduced to 1% of
raw sewage. it is passed into the settling tank. In these tanks, flocs are
allowed to undergo sedimentation.
(ii) Activated sludge: The sediment of the settling tank is called activated
sludge. A part of it is used as an inoculum in aeration tanks. The
remaining part is passed into a large tank called an anaerobic sludge
digester. In these tanks, anaerobic microbes are present that digest the
organic mass as well as aerobic microbes of activated sludge. The
remaining sludge is used as manure or compost.

27 ELISA-Enzyme Linked Immunosorbent Assay. (1) 3

ELISA is based on antigen-antibody interaction. (1)
The ways to detect the presence of infection or disease by ELISA are as
follows:
The presence of antigens (proteins, glycoproteins, etc.) is detected. /
Antibodies produced against the pathogens are detected. (1)
OR
The technology is called RNA Interference(RNAi)
The principle is to block certain genes through a process called gene silencing
(1)
It involves silencing of a specific mRNA due to its complementary Double-
stranded RNA /dsRNA molecule that binds to and prevents translation of the
 mRNA(silencing). (1)
Example: Resistance to nematode (Meloidogyne incognita) in tobacco is
achieved by this (or ANY OTHER RELEVANT EXAMPLE.) (1)

28 1950 – Expanding - The population structure in 1950 exhibits a broad-based 3

pyramid with a wider base, indicating a higher percentage of young individuals.
This suggests a population with a higher birth rate. (1)
2007 – Stable - The narrowing of the pyramid towards the top signifies a lower
proportion of elderly individuals. By 2007, the population was more stable with
the number of pre reproductive and reproductive age nearly being the same.
Bell shape indicates static population. (1)
2050– Declining - By 2050 urn shaped pyramid shows a declining population
where birth rates are very less and the populations of elderly people will
increase. (1)
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For Visually impaired students
Answer same as above

Section – D

29 A. One embryo sac is present in each ovule and one egg is present in each 4
embryo sac when the embryo sac is developed from a single megaspore.
(1)
B.
(i) P exhibits polyembryony due to occurrence of more than one embryo in
a seed. Embryos developed from nucellar cells by apomixis, a form of
asexual reproduction, don't show genetic variation. (1)
(ii) Cells of embryos developed from diploid nucellar cells are diploid (2n)
as these are formed by apomixis, a form of asexual reproduction. (1)
Student to attempt either subpart C or D.
C. Q, because this fruit is developed without fertilisation and will thus be
seedless. (1)
OR
D. Fruit S is a true fruit with seeds. True fruits develop from the ripened ovary
after fertilisation and fertilised ovules mature into seeds. (1)

30 A. Passive immunity – Ready made antibodies from colostrum / less effective 4

/ transient / no memory cells involved
Active Immunity – Made by host’s immune system/ Lag phase/ memory
based/ largely effective. (1)
B. In the first weeks of life, the calf's immunity is strong because it absorbs
antibodies from the cow’s colostrum and milk. This passive immunity
peaks at day 1 and declines as the calf ages as the antibodies are used
up to develop immunity against diseases. (1)
Attempt either subpart C or D.
C. As the calf grows, its immune system starts to produce antibodies in
response to bacteria or viruses in the environment and by the time it is
nearly 14 days old after which active immunity takes charge.
OR
 D. Active immunity. As the vaccine contains a weakened or inactivated form
of the pathogen (microbe), which is harmless but still recognized by the
immune system as foreign. The immune system responds by activating
lymphocytes, which produce specific antibodies to fight the pathogen.
This process also leads to the creation of memory cells, which provide
long-term protection by "remembering" to fight this pathogen if the person
is exposed again. (2)
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For Visually impaired students
Same answers as given in parts a-d.

Section – E

31 A. 5

B. (2)
 Transcription in eukaryotes involves one of three types of polymerases,
depending on the gene being transcribed. RNA polymerase II transcribes
all of the protein-coding genes, whereas RNA polymerase I transcribes
rRNA genes, and RNA polymerase III transcribes rRNA, tRNA, and small
nuclear RNA genes. (1)
 The primary transcripts contains the coding region, exon, and non-coding
region, intron, hnRNA undergoes a process where the introns are
removed and exons are joined to form mRNA by the process called
splicing. (1)
 The hnRNA undergoes two additional processes called capping and
tailing. In capping, an unusual nucleotide, methyl guanosine triphosphate,
is added to the 5′-end of hnRNA. In tailing, adenylate residues (about 200–
300) are added at 3′-end in a template independent manner. (1)
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For visually impaired students
A. If both strands act as a template, they will code for RNA molecules with
different sequences as complementarity does not mean the strands are
identical and hence, the sequence of amino acids in the proteins would be
different. Hence, one segment of the DNA would be coding for two different
proteins, and this would complicate the genetic information transfer
machinery. (1)
The two RNA molecules if produced simultaneously would be
complementary to each other, hence would form a double stranded RNA.
This would prevent RNA from being translated into protein and the exercise
of transcription would become a futile one. (1)
B. Transcription in eukaryotes involves one of three types of polymerases,
depending on the gene being transcribed. RNA polymerase II transcribes
 all of the protein-coding genes, whereas RNA polymerase I transcribes
rRNA genes, and RNA polymerase III transcribes rRNA, tRNA, and small
nuclear RNA genes. (1)
The primary transcripts contain the coding region, exon, and non-coding
region, intron, hnRNA undergoes a process where the introns are removed
and exons are joined to form mRNA by the process called splicing. (1)
The hnRNA undergoes two additional processes called capping and tailing.
In capping, an unusual nucleotide, methyl guanosine triphosphate, is added
to the 5′-end of hnRNA. In tailing, adenylate residues (about 200–300) are
added at 3′-end in a template independent manner. (1)
OR
A. Aminoacylation is the process by which amino acids become activated by
binding with its aminoacyl tRNA synthetase in the presence of ATP. If two
charged tRNAs come close during translation process the formation of
peptide bond between them is energetically favourable. (1)
B. The cellular factory responsible for synthesising proteins is the ribosome.
In its inactive state it exists as two subunits: a large subunit and a small
subunit. When the small subunit encounters an mRNA the process of
translation of the mRNA to protein begins. There are two sites in the large
subunit for subsequent amino acids to bind to and thus be close enough
to each other for the formation of a peptide bond. The ribosome also acts
as a catalyst 23S rRNA in bacteria is the enzyme-ribozyme for the
formation of peptide bonds. (2)
C. ARGININE, THREONINE, HISTIDINE, LEUCINE, PROLINE, GLYCINE,
PROLINE PROLINE, PROLINE, LEUCINE SERINE (1)
Stop codon UGA/UAG/UAA (1)
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For visually impaired students
A and B as above
C. Insertion or deletion of one or two bases changes the reading frame from
the point of insertion or deletion. Such mutations are referred to as frame-
shift mutations. Insertion or deletion of three or multiple of three bases
does not alter the frame. However, the mutation takes place. This proves
that codon is a triplet and it is read in a contiguous manner. (2)

32 A. Retrovirus in animals and Agrobacterium tumefaciens can transform 5

normal cells into cancerous cells/Agrobacterium tumefaciens is
responsible for causing crown gall disease/ it can transfer its T-DNA to
transform normal plant cell into tumorous cells (1). They are used as
cloning vectors to deliver desirable genes into animal/plant cells. (1)
B. restriction enzyme and ligase. (1).
C.
(i) Biolistic/ gene guns can be used. The plant cells are bombarded with
high velocity micro particles of gold or tungsten coated with DNA. (1)
(ii) the cry gene will create Bt toxin inside the pest which will get activated
in the alkaline gut of the pest and cause the gut epithelial lining to
disintegrate. (1)
 OR
A. 5' - GGATCC - 3'.
3’- CCTAGG-5’ (1)
B. drawing and labelling (2)

C. If ampicillin is added, the bacteria will show resistance as the gene is

intact and will survive;
if tetracycline is added, the bacteria will die as it will show insertional
inactivation/ gene is not functional owing to insertion of the gene of
interest in the tetracycline region of selectable marker. (2)

33 A. Totally unrelated species could also compete for the same resource. For 5
instance, in some shallow South American lakes, visiting flamingoes and
resident fishes compete for their common food, the zooplankton in the
lake. (1)
B. Resources need not be limiting for competition to occur; in interference
competition, the feeding efficiency of one species might be reduced due
to the interfering and inhibitory presence of the other species, even if
resources (food and space) are abundant. (1)
C. Gause and other experimental ecologists believed when resources are
limited the competitively superior species will eventually eliminate the
other species. The Abingdon tortoise in Galapagos Islands became
extinct within a decade after goats were introduced on the island,
apparently due to the greater browsing efficiency of the goats. (1)
D. One such mechanism is ‘resource partitioning’. If two species compete for
the same resource, they could avoid competition by choosing, for
instance, different times for feeding or different foraging patterns.
MacArthur showed that five closely related species of warblers living on
the same tree were able to avoid competition and co-exist due to
behavioural differences in their foraging activities. (1)
E. A species whose distribution is limited to a smaller area because of the
presence of a competitively superior organism, is found to increase its
range when the competing species is experimentally removed. For
example, in the coasts of Scotland the superior barnacle (Balanus)
 dominates the intertidal region and excludes the smaller barnacle from
that zone. (1)
OR
A. Food chain and the first law of thermodynamics. The solar energy trapped
by the plants is transformed to chemical energy through the process of
photosynthesis. When the plants are consumed by the animals, the
organic matter (chemical energy) is transferred to the animal, some
amount of this organic matter is lost as heat through the process of
respiration. Death of the producers or consumers will result in the organic
matter getting transferred to detritivores. As demonstrated, energy is not
created nor destroyed but transformed from one form to another. (2)
B. Greater diversity is seen in regions closer to equator/ tropics (latitudinal
range of 23.5° N to 23.5° S) harbour more species. (1)
Reasons for this greater diversity are: (Any two reason)
● Speciation is generally a function of time, unlike temperate regions
subjected to frequent glaciations in the past, tropical latitudes have
remained relatively undisturbed for millions of years and thus, had a
long evolutionary time for species diversification
● Tropical environments, unlike temperate ones, are less seasonal,
relatively more constant and predictable. Such constant
environments promote niche specialisation and lead to a greater
species diversity
● There is more solar energy available in the tropics, which contributes
to higher productivity; this in turn might contribute indirectly to
greater diversity. (2, any two)

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