Half Yearly Examination 2024-25

CLASS XII
SUBJECT – BIOLOGY

FULL MARKS : 70 TIME: 3 HRS

Instructions:

● The Question paper has been divided into 5 sections – A,B,C,D & E. And 33
Questions.
● Section A consist of 16 questions of 1 mark each, Section B consists of 5
questions of 2 marks each , Section C consists of 7 questions of 3 marks each, Section D
consist of 2 questions of 4 mark each & Section E consists of 3 questions of 5 marks
each.
● There is no overall choice. However, internal choices have been provided in some
questions. A student has to attempt only one of the alternatives in such questions.
● All questions are compulsory.
● Draw diagram wherever necessary.

1. Mature Graffian follicle is generally present in the ovary of a healthy female around
A. 5 - 8 day of menstrual cycle
B. 11 - 17 day of Menstrual cycle
C. 18 - 23 day of Menstrual cycle
D. 24 - 28 day of Menstrual cycle

2. In Yeast , DNA which is double stranded, 18% of the bases were shown to be Guanine. The
% of the other three bases expected to be present in this DNA are :
A. C 36% , A 18%, T 18%
B. C. 18% , A 36%, T 36%
C. C 18%, A 32% , T 32%
D. C 18% , A 18%, T 18%

3. A polygenic trait is controlled by 3 genes A, B and C. In a cross AaBbCc. X AaBbCc , the

phenotypic ratio of the offspring was observed as: 1: 6: X: 20 : X: 6: 1. What is the possible
value of X?
A. 3
B. 9
C. 15
D. 25

4. In a dihybrid cross, if you get 9:3:3:1 ratio it denotes that

A. the alleles of two genes are interacting with each other
B. two traits are linked.
C. the number of alleles of a gene.
D. the alleles of two genes are segregating independently.

5. Which one of the following statements is correct?

A. Cleistogamous flowers always exhibit autogamy
B. Chasmogamous flowers always exhibit geitonogamy
C. Cleistogamous flowers exhibit both autogamy and geitonogamy
D. Chasmogamous flowers never exhibit autogamy.

6. Escherichia coli. fully labeled with 15N is allowed to grow in 14N medium. The two strands
of DNA molecule of the first generation of bacteria have.
A. Different density and do not resemble parent DNA.
B. Different density but resemble parent DNA.
C. Same density and resemble parent DNA.
D. Same density but do not resemble parent DNA

7. Choose the correct option regarding retrovirus.

A. An RNA virus that synthesizes DNA during infection
B. A DNAvirus that synthesizes RNA during infection .
C. A ssDNA virus
D. A dsRNA virus

8. Which of the following is not correctly matched for the organism and its cell wall degrading
enzyme?
A. Algae - Methylase
B. Fungi - Chitinase
C. Bacteria - Lysozyme
D. Plant cells - Cellulase

9. The mode of action of the copper ions in an IUD is to

A. Increase the movement of sperms
B. Decrease the movement of sperms
C. Make the uterus unsuitable for implantation
D. Make the cervix hostile to sperms

10. Name the microbe that help in the production of Statin commercially:
A. Propionibacterium sharmanii
B. Trichoderma polysporum
C. Aspergillus niger
D. Monascus purpureus

11. The sporozoites that cause infection when a female Anopheles mosquito bites a person,
are formed in
A. Liver of the person
 B. RBCs of mosquitoe
C. Salivary glands of mosquito
D. Gut of mosquito

12. In the embryos of a typical dicot and a grass, true homologous structures are
A. Coleorhiza and coleoptile
B. Coleoptile and scutellum
C. Cotyledons and scutellum
D. Hypocotyl and radicle

Assertion (A) and Reason ( R ). Answer these questions by selecting the appropriate option
given below : A. Both assertion and reason are true and reason is the correct explanation of
assertion. B. Both assertion and reason are true but reason is not the correct explanation of
assertion. C. Assertion is true but reason is false. D. Both assertion and reason are false.
13. Assertion(A ) : The person heterozygous for sickle cell trait produces both ( HbA) and
abnormal haemoglobin( HbS).
Reason( R) : The normal allele and the sickle cell allele are codominant.
14. Assertion (A) : During pregnancy the levels of hormones like estrogen &
progesterone are increased.
Reason(R) : The increased production of these hormones is essential for fetal
growth.
15. Assertion(A): In mycorrhiza the fungus symbiont absorb phosphorus for the plant.
Reason (R ): Mycorrhiza is a symbiotic association of fungus with roots of higher
plants.

16. Assertion (A): Tobacco contains nicotine which stimulates the adrenal gland.
Reason (R ): Nicotine increases the blood pressure and the heart rate.

SECTION B

17. Which chromosome carries the mutated gene causing beta - thalassemia?What are the
problems caused by the mutation ?

18. Why do you see two different types of replicating strands in the DNA replication fork ?
Explain.

19. Name two sexually transmitted diseases.What complications can it lead to if untreated?

20. What is Apomixis ? Comment on its significance.

21. CTTAAG
GAATTC
a. What are such sequences called? Name the enzyme used that recognizes such
nucleotide sequences.
 b. What is their significance in biotechnology? OR.
What is GEAC ? Write its objectives .

SECTION C

22 . At what stage implantation of embryo takes place?

What stimulates pituitary to release the hormone responsible for parturition? Name the
hormone.
OR
Meiotic division during Oogenesis is different from that in Spermatogenesis. Explain how &
why?

23. a. Calculate the length of the DNA of bacteriophage lambda that has 48502 base
pairs?
b . Why did Hershey and chase use radioactive sulphur and Phosphorus in their
experiment? Explain.

24. Given below is the karyotype obtained after analysis if foetal cells for a probable genetic
disorder:

Based on the above karyotype what would be the sex of an unborn foetus?Which genetic
disorder it would be suffering from?What could be the reason for such a condition?write the
symptoms of such a condition.

25. Describe the role of heat, primers and bacterium Thermus aquaticus in the process of PCR.
26. A. How does a normal cell change into a neoplastic tumor?
B. Why are cancer patients often given alpha interferon as a part of the treatment?

27.Large quantities of sewage is generated everyday in cities and towns, which is treated in
sewage treatment plants to make it less polluted. Given below is the flow diagram of one of the
stages of STP.
Observe the given flow diagram and answer the questions accordingly.
Primary effluent is passed into large aeration tank —---------------> effluent passed into settling
tank to form the sediment
a. Why is the primary effluent passed into large aeration tanks?
b. Write the technical term used for the sediment formed.Mention its significance.
c. Explain the final step that results in the formation of biogas in the large tank before the
treated effluent is released into water bodies.

28. a . Explain the contribution of S. L Miller’s experiment on origin of life.

b . State Hardy Weinberg’s principle & equation.

SECTION D

29. Women of 35 years age with a married life of eight years and having normal reproductive
cycles visit a doctor along with her husband for consultation for infertility. They were not using
any contraceptive methods. They have no child.The doctor advises them for following
investigations:
- Seminal analysis of the husband
- Follicular study of the wife
- Blood test for FSH for both
With your knowledge ,answer the following questions:
a. Why do you think the seminal analysis of the husband is done?
b. The blood report of the wife showed low FSH value.What is it indicative of?
c. In which phase of the menstrual cycle is the blood sample of a woman taken if, on analysis, it
shows high levels of LH & estrogen?

OR

c. The high level of which gonadotropin/ ovarian hormone in the blood sample of the wife taken
on day 20 of her reproductive(menstrual)cycle would indicate the luteal phase of the ovarian
cycle?
d. What methods of ART will be recommended by the doctor to the couple ,if seminal analysis is
found to be good.

30. Observe the schematic representation of genes involved in lac operon

p i p o z y a
i. Identify the region where the repressor protein will attach normally?
ii. The active site of the enzyme coded by gene “y” in the bacterium has been blocked by an
inhibitor, how will it affect the lac operon?
iii. The protein produced by the i gene has become abnormal due to unknown reasons.Explain its
impact on lactose metabolism stating the reason.

SECTION E

31 . The base sequence in mRNA is

5’ - UCAUUACCACGAUUCUUUAAAAGA - 3’

I. Give the strand of DNA from which I had been transcribed .

II. How many amino acids will be translated from it.
III. Do you find any stop codon in the above sequence . Name it.
IV . How many amino acids would change if substitution of ‘U’ by ‘C’ takes place at the 5th
codon.
V. Write the number of amino acids that would be in the polypeptide synthesised by a similar
mRNA , where in the fourth codon instead of ‘C’ there is ‘U’. Justify your answer.

OR

A. Explain the mechanism of sex determination in birds. How does it differ from that of
human beings?
B. A colourblind man marries a woman with normal vision whose father was colourblind.
Work out a cross to show the genotype of the couple and their respective sons.

32 . A . Explain any four devices that flowering plants have developed to encourage cross
pollination.
B. Why do plants discourage self pollination? State any one reason.
C.

OR

Cryptorchidism is a condition in which the testes fails to descend into the scrotum.It can
also lead to compromised sertoli cell function and has an impact on Leydig cell function.
i. Identify at least 3 parameters of male fertility which get affected due to cryptorchidism.
ii.Which process will get affected if mature spermatids are not released from sertoli cells?
iii. What is the function of sertoli cells & leydig cells
iv. Name the accessory glands of male reproductive system.

33.a. Why is Agrobacterium tumifaciens a good cloning vector? Explain.

b. Name the most commonly used bioreactor and describe its working.

OR
a. Name the selectable markers in the cloning vector pBR322? Mention the role they play.
b. Explain the methods (at least three)by which a host can be made competent in rDNA
technology.

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