ISC Class 12 Biology Exam 2025 : Most Important Questions Answers for Last-Minute Revision

ISC Class 12 exams are going on and you have very little time left for the Biology exam. Therefore, we are providing important questions in this article. You can study these thoroughly and score good marks in your exams.

To help you make your preparation effective, we have compiled a list of important questions with detailed explanations.

These questions for the ISC Class 12 Biology exam cover various topics from the syllabus.

ISC Class 12th Biology Exam 2025 Most Important Questions

Q1. Sapna has been diagnosed with an infection of the reproductive tract caused by bacteria. She experiences burning sensation during urination, pain around her genitalia and observes pus containing discharge. Her doctor tells her that this infection has an incubation period of two to five days but can be cured.

What is the biological name of the causative agent of the disease Sapna is suffering from?

Answer :- Neisseria gonorrhoeae

Q2. The number of chromosomes in the leaf cells of a male plant is 40 and in the stem cells of a female plant is 60. If they are artificially hybridised, what will be the number of chromosomes in the endosperm? 

Answer :- 80

Q3. How many cycles of PCR are required to produce 250 molecules of DNA, starting with a single parental strand?

Answer :- 8 cycles

Q4. Which is the most commonly prescribed non-steroidal oral contraceptive pill in India?

Answer :- ‘Saheli’

Q5. Tall pea plants having green pods were crossed with dwarf pea plants having yellow pods. Out of 80 plants, how many are likely to be tall plants in the F2 generation?

Answer :- 60 plants

Q6. Write a 6-nucleotide long palindromic sequence on a double stranded DNA, which was reverse transcribed by the following nucleotide sequence on a retroviral RNA.

3’-GUA- - - 5’

Answer :- 

5’- CATATG -3’

3’- GTATAC -5’

Q7. Observe the relation between the first two words and then fill in the fourth word.

Histamine: Mast cells :: Antibody: ____________ 

Answer :- Plasma cells / B-Lymphocytes

Q8. Which of the following sequences of mRNA will NOT translate completely?

(a) 5' - AUG UUC AGC UCG UGA - 3'

(b) 5'- AUG AAC UAA CCA CUC - 3'

(c) 5' - AUG UUA CUC GCG UAA - 3'

(d) 5' - AUG CCA UAC GAC UAG - 3'

Answer :- (b) 5'- AUG AAC UAA CCA CUC - 3'

Q9. Given below are two statements marked Assertion and Reason. Read both the statements carefully and choose the correct option.

Assertion: Earthworm is called a detritivore.

Reason: It breaks down the water-soluble inorganic nutrients, which percolate down into the soil.

(a) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.

(b) Both Assertion and Reason are true but Reason is not the correct explanation for Assertion.

(c) Assertion is true and Reason is false.

(d) Both Assertion and Reason are false.

Answer :- (c) Assertion is true and Reason is false.

Q10. Given below are two statements marked Assertion and Reason. Read both the statements carefully and choose the correct option.

Assertion: E. coli was transformed by inserting the foreign DNA by using PvuI in pBR322.

Reason: It was done to make the transformed cells survive in the culture medium containing ampicillin.

(a) Both Assertion and Reason are true, and Reason is the correct explanation for Assertion.

(b) Both Assertion and Reason are true, but Reason is not the correct explanation for Assertion.

(c) Assertion is true and Reason is false.

(d) Both Assertion and Reason are false.

Answer :- (c) Assertion is true and Reason is false.

Q11. What is the advantage of growing apomictic seeds in crop improvement?

Answer :- Helps in retaining parental characters. 

Q12. A farmer has two fields in which he wants to grow a cereal crop and a legume. He cannot afford to spend a lot of money on chemical fertilisers to increase the fertility of the soil. Suggest a way to help him resolve the issue.

Answer :- By recommending him to use cyanobacteria and / or Nitrogen-fixing bacteria

Q13. Answer the following questions. 

(a) Expand the abbreviation MALT. 

(b) David is a molecular biologist. He uses a technique to know the amino acids occupy the first and the last positions in the Chain – A and Chain – B of insulin. Name the scientist whose contribution made it possible for David to use this technique. 

Answer :- (a) Mucosal Associated Lymphoid Tissues

(b) Sanger

Q14. In a flower, the megaspore mother cell formed four megaspores without undergoing meiosis. One of the four megaspores developed into the embryo sac. What would be the ploidy level of the antipodal cells in this embryo sac?

Answer :- Diploid (2n)

Q15. Name the pollinating agent of a flowering plant with large and coloured flowers with sticky stigma. State one characteristic feature of the pollen grains produced in such flowers. 

Answer :- Insect pollinated.

One characteristic feature of pollen grain - pollen kit / sticky pollen grains

Q16. What happens to an inferior competitor if:

(i) a superior competitor is present in the same environment? 

(ii) the superior competitor is removed from the environment?

Answer :-

(i) The inferior competitor will be eliminated as per Gause’ ‘Competitive exclusion principle’.

(ii) The inferior competitor expands its territory as per ‘Competitive release’.

Q17. Name the technique used to detect the presence of HIV in the body of an individual, Justify the principle associated with this technique.

Answer :- Technique: ELISA

Principle: Presence of specific anti-viral antibodies

Q18. Name any two factors responsible for the loss of biodiversity in a geographical region. 

Answer :- Habitat loss and fragmentation, Alien species invasion

Q19. What are ZIFT and GIFT? 

Answer :- Zygote Intra Fallopian Transfer, Gamete Intra Fallopian Transfer

Q20. All STDs do not affect the genital organs. Justify the statement. 

Answer :- STDs like AIDS and Hepatitis – B do not directly affect the genital organs because AIDS affects only the immune system and Hepatitis affects the liver.

Q21. Why are the biocontrol agents preferred over the chemical pesticides?

Answer :- Biocontrol agents do not pollute the environment and are highly specific in nature

Q22. Explain the role of any two biocontrol agents by mentioning their target pests.

Answer :- Baculovirus – Specifically attack insect pests.

Bacillus thuringiensis – Specifically destroys cotton bollworm/ corn borer

Q23. Why are some molecules called bioactive molecules?

Answer :- Bioactive molecules modulate the metabolic pathways of the living organism.

Q24. Mention the respective source and state the specific use of any two bioactive molecules. 

Answer :- Streptokinase: Source – Streptococcus: Use – Intravascular clot buster

Cyclosporin: Source – Trichoderma polysporum: Use– Immunosuppressant

Q25. Enumerate the cause and the characteristic symptom of PKU. 

Answer :- Cause: Deficiency of the enzyme phenylalanine hydroxylase

Symptom: Retarded mental growth/ light skin pigmentation

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