HBSE Class 12 Sample Paper 2025 Answers Biology

Board Of School Education Haryana

मॉडल पेपर
उत्तर
2025

MARKING SCHEME (2024-25)
CLASS – XII
BIOLOGY (CODE- 865)
Q. No Expected Answer/ Value Point Marks
1 b) Syncarpous 1
2 c) 60000-80000 1
3 8 1
4 Autosome linked recessive trait 1
5 a) AUG codes for Methionine, and it also act as initiator 1
codon.

6 H.M.S. Beagle 1
7 d) Thymine 1
8 Four 1
9 ELISA (Enzyme Linked Immuno-sorbent Assay) 1
10 b) As a blood cholesterol lowering agent. 1
11 c) Stanley Cohen and Herbert Boyer 1
12 b) Protein 1
13 b) 0.4 1
14 a) Gross primary productivity minus respiration 1
losses.

15 Trophic level 1
16 (a) Both A and R are true an R is right explanation of 1
A

17 (a) Both A and R are true an R is right explanation of 1
A

18 (c) A is true and R is false 1
19 a) Syngamy: Fusion of one male gamete with 1
nucleus of egg cell to form diploid zygote.
b) Triple fusion: Fusion of other male gamete
with two polar nuclei to form triploid 1
primary endosperm nucleus.

20 a) Motivate people for small families through 1
contraceptive methods
b) Statutory raising the marriageable age of
females to 18 and males to 21 years 1

21 Test cross 1
To determine the genotype of an organism. 1
22 Theory of chemical evolution was proposed by
Oparin and Haldane. They proposed that the first
2
form of life could have come from pre-existing
non- living organic molecules and the formation
of life was preceded by chemical evolution.
23 Ascaris 1
Two symptoms of ascariasis are as follows:
(i) Internal bleeding and anemia 1/2
(ii) Fever
1/2

Or
Interferons are the proteins which are secreted 1
by virus infected cells.

Interferons protect non infected cells from 1
further viral infection.

24 Restriction Enzymes are molecular scissors 1
which cut DNA at specific locations.
Role in r-DNA technology:
The cut piece of DNA is linked with plasmid
1
DNA to form recombinant DNA and to further,
transfer in host organism for cloning.
Or
Gel electrophoresis is a technique to separate, the
fragments of DNA, cut by action of restriction
enzymes, under electric field.
1
Separated DNA fragments can be visualized only
after staining the DNA followed by exposure to 1/2
UV radiation.
Ethidium bromide.
1/2
25 Pyramid of energy is always upright because 2
some energy is always lost in form of heat, when
energy flows from one trophic level to next
trophic level in pyramid of energy.
Or
The close association between egrets and grazing 1
cattle is called commensalism.
The reason for this interaction is that when
1
grazing cattle move, they stir up and flush out
insects from vegetation that otherwise will be

difficult for egrets to find and catch.

26

3

Diagrammatic sectional view of seminiferous
tubule in human being.
27 Salient features of the Double-helix structure of
DNA:
½
(i) DNA structure constitutes two polynucleotide
chains, where the backbone is made by sugar-
phosphate, and the nitrogenous bases are
flanked inside.
(ii) The two chains have anti-parallel polarity. It ½
means, if one chain has the polarity 5'→3', the
other has 3'→5'.
(iii) The bases in two strands are paired through
hydrogen bond. ½
(a). Adenine is linked with two hydrogen bonds
with Thymine.
(b) Guanine is linked with Cytosine with three
H-bonds.
(c) Therefore, purine comes opposite to a

pyrimidine.
(iv) The pitch of the helix is 3.4 nm and there are
roughly 10 bp in each turn. The distance ½
between a bp in a helix is approximately 0.34
nm.
(v) The two chains are coiled in right handed ½
fashion.
(vi) The plane of one base pair stacks over the other ½
in double helix.

28 Sr. Name of Reasons Symptoms
No genetic
disorder

1. Klinefelter’s An additional Overall masculine
1
syndrome copy of X development with
chromosome Gynaecomastia
resulting into a
karyotype of 47,
XXY

2. Down’s Trisomy 21 Small rounded head, furrowed
syndrome tongue, partially open mouth,
1
palm broad with characteristic
crease

3 Turner’s Absence of one Sterile female with
syndrome of the X rudimentary ovary 1
chromosomes

Or
RNA Polymerase I: it transcribes rRNAs (28S,
18S, 5.8S).
1½
RNA Polymerase II: It transcribes precursor of
mRNA and heterogeneous nuclear RNA.
RNA Polymerase III: It helps in transcription of
tRNA, 5srRNA, and snRNAs.

Gene Splicing: Primary transcript in eukaryotes
contain both exons and introns. These introns are
non- coding parts in transcript. Therefore, the
1½
removal of introns and joining of exons is called
gene splicing.

29 Secondary treatment of sewage is also called
biological treatment because in this treatment,
sewage is biodegraded with the help of
microorganisms.
Micro-organisms have following roles in sewage
treatment:
(i) Masses of bacteria and fungi (Flocs)
are produced when primary effluent is
passed into large aeration tanks which
consumes major part of organic matter
3
in effluent reducing it’s BOD.
(ii) Now this effluent is passed to settling
tank where bacterial flocs settle as
activated sludge. Small amount of
activated sludge works as inoculum
when passed back into aeration tank.
(iii) Remaining part of sludge is taken into
anaerobic sludge digester tanks where
different anaerobic bacteria perform
digestion of sludge to produce Biogas
which is mixture of gases such as
methane, hydrogen sulphide and
carbon dioxide.
Or

(i) Contact inhibition is a property of
normal cells. When normal cells come
1
in contact with other cells inhibit their
uncontrolled growth or tumorous
growth.
(ii) Malignant tumour is the mass of
proliferating, neoplastic rapidly 1
growing cells which invade and
damage surrounding tissues.
(iii) Carcinogens are the physical,
chemical or biological agents which 1
induce transformation of normal cells
into cancerous neoplastic cells e.g.
Radiations (X-rays, gamma rays and
UV rays) and Chemical carcinogen like
tobacco smoke.

30 GMO or Genetically Modified Organisms are
plants, animals, bacteria and fungi, whose genes
1
have been altered by manipulation.
Usefulness of GM plants:
(i) GM crops are more tolerant to abiotic
stresses (Cold, draught, salt, heat). 1
(ii) GM plants have less reliance on
1
chemical pesticides.

31 (i) Rhino Virus 1
(ii) Plasmodium which is a protozoa
1

(iii) Amoebiasis. 2

Three symptoms:
a) Constipation b) abdominal pain c)
Stools with excess mucous and
blood clots.
Or
1
(i) Salmonella typhi
1
(ii) Widal Test.

32 (i) The approach in which we conserve 1
and protect the whole ecosystem and
it’s biodiversity at all levels is called
in situ conservation. To protect entire
forest to save the tiger.
1
(ii) Johannesburg, South Africa.
(iii) Four major causes of biodiversity
losses are:
(a) Habitat loss and fragmentation.
(b) Over-exploitation 2
(c) Alien species invasions
(d) Co-extinctions
Or
Broadly Utilitarian argument:
Biodiversity plays a major role in
many ecosystem services that nature provides.
For example Amazon forest is estimated to
produce 20 percent of the total oxygen in the
earth’s atmosphere with the help of
photosynthesis.

33 (i) In flow chart the hormone released by 1
hypothalamus is gonadotropin
releasing hormone (GnRh)
Function:
 It begins spermatogenesis at the
1
age of puberty.
 It Stimulates secretion of two
gonadotropins: ½
a) Luteinising hormone
b) Follicle stimulating hormone ½
(ii) The hormone released by anterior
pituitary which acts on Leydig cell is
Luteinising hormone. ½
Function: Luteinizing hormone
stimulates synthesis and secretion of
androgens. ½
(iii) The hormone released by Leydig cells ½
is androgen.
Function: Androgen stimulates the
process of spermatogenesis. ½
Or
Labelled diagram of typical
anatropous ovule in flowering plants.

5

34 Transcription: The process of copying the 1
genetic information from one strand of DNA into

RNA is known as transcription.
Process of transcription in bacteria: The
process of transcription in bacteria consists of 3
steps:
1
(i) Initiation of transcription Process:
RNA polymerase binds to promotor
and associates transiently with
initiating factor sigma ρ(σ) initiates
transcription.
(ii) Elongation of transcription process:
1
After binding to promotor, RNA
polymerase facilitates opening of
DNA helix. It uses nucleoside
triphosphates as substrates and
polymerises into nucleotides following
principle of complementarity (except
base pairing of adenosine with uracil
instead of thymine).
D
i
a
g
r
1
a
m

s
h
o
w
i
ng process of Transcription in Bacteria

(iii) Termination of transcription process:
Once the polymerase reaches to
1
terminator region the nascent RNA
falls off. Polymerase transiently
associated with rho (ρ) termination
factor also falls off.

Or
(i) Aim of the experiment done by
Hershey and Chase: 1
They worked to discover whether it
was protein or DNA from virus that
enters bacteria. 1
(ii) They worked on bacteriophage virus
which infects bacteria.
(iii) Main Steps:
(a). They grew some viruses on a medium
1
that contained radioactive phosphorus to
prepare radioactive DNA and some others
on medium that contained radioactive sulfur
to prepare radioactive protein.
(b). Radioactive phages were allowed to
1
attach to E. coli bacteria. Then, the viral
coats were removed from the bacteria by
agitating them in a blender. The virus
particles were separated from the bacteria
by a centrifuge.

(iv) Conclusion: 1
Bacteria which were infected with viruses
that had radioactive DNA were radioactive,
indicating that DNA was the genetic
material that passed from the virus to the
bacteria.

35 (i) Polymerase Chain Reaction 1
(ii) Three steps as given below:
½
(a) Denaturation ½
(b) Primer annealing
½
(c) Extension of primers
½
(iii) Role played by Thermus
aquaticus in PCR:
Repeated DNA amplification in PCR 2
is achieved by the use of a
thermostable DNA polymerase which
is isolated from Thermus aquaticus
bacteria.

Or
 Origin of replication (ori) is a 1
sequence from where replication
starts and any piece of DNA
when linked to this sequence can
be made to replicate within host
cell.
 Recognition sites, in vector, are 1
the sequences needed, to link the
alien DNA. The presence of
recognition site helps particular
restriction enzyme to cut the
vector DNA at a particular
sequence.
 Selectable Marker is a DNA 1
sequence that aids in detecting
and eliminating non-

transformants and allowing
selective growth of
transformants.

In given vector pBR322, the genes
encoding resistance to following
antibiotics are used as selectable
markers:
 tetR resistant to 1
tetracycline.
 ampR resistant to
ampicillin. 1

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