CBSE 12 Biology 2025-26: Most Important Case Based Questions with Answers – Free PDF

The CBSE Class 12 Biology board exam is scheduled for Friday, March 27, 2026. In this article, we've provided the most important case-based questions along with detailed answers.

Case-based questions carry good marks in the CBSE board exam. If you practice these questions well, you can easily score full marks in the exam.

You can easily download the PDF from the table below and complete your exam preparation.

CBSE 12 Biology 2025-26 Important Case-Based Questions

Case No.1: Read the following and answer any four questions from (i) to (v) given below.

Packaging of DNA Helix

Each diploid human cell contains a genome of approximately 6 billion base pairs of DNA packaged into 23 pairs of chromosomes. Because each base pair is around 0.34 nanometers long, each diploid cell therefore contains about 2 meters of DNA. How is this chromosomal DNA compacted into the microscopic space of the eukaryotic nucleus? In eukaryotes, DNA is wrapped around an octamer of proteins known as histones to form structures called nucleosomes. H1, H2A, H2B, H3 and H4 are five major types of histones involved in eukaryotic DNA packaging. Each nucleosome is linked to the next one with the help of a linker DNA. This is also known as the “beads on a string” structure.

The following questions are multiple-choice questions. Choose the most appropriate answer:

(i) The histone octamer of a nucleosome is formed of

(a) two molecules each of H1, H2A, H2B and H3.

(b) two molecules each of H2A, H2B, H3 and H4.

(c) two molecules each of H2A, H2B and H3 and one molecule each of H1 and H3.

(d) one molecule each of H2A, H2B and two molecules each of H1, H3 and H4.

Answer: (b) two molecules each of H2A, H2B, H3 and H4.

(ii) Histones are rich in

(a) acidic amino acids.

(b) polar amino acids.

(c) basic amino acids.

(d) non-polar amino acids.

Answer: (c) basic amino acids.

(iii) DNA is wrapped tightly around the histone octamer due to the interaction of

(a) negative charges on DNA with positive charges on histone proteins.

(b) negative charges of histones with negative charges on DNA.

(c) positive charges on DNA with negative charges on histone proteins.

(d) positive charges on DNA with positive charges on histone proteins.

Answer: (a) negative charges on DNA with positive charges on histone proteins.

(iv) What does “bead” represent in “beads on string structure”?

(a) DNA

(b) Histone octamer

(c) Linker DNA

(d) Nucleosome

Answer: (d) Nucleosome

(v) Assertion: Chromatin appears as euchromatin and heterochromatin in a nucleus.

Reason: Euchromatin stains lightly and heterochromatin stains darkly.

(a) Both assertion and reason are true, and the reason is the correct explanation of the assertion.

(b) Both assertion and reason are true, but the reason is not the correct explanation of the assertion.

(c) Assertion is true, but reason is false.

(d) Both assertion and reason are false.

Answer: (b) Both assertion and reason are true, but the reason is not the correct explanation of the assertion.

Case No.2: A disease is an abnormal condition affecting a healthy living organism. It is broadly divided into infectious and non-infectious. Infectious diseases are caused by pathogens, such as bacteria, viruses, fungi, parasites and can be easily transmitted from one person to another. Hence it is also known as a contagious or communicable disease. Common Cold, Tuberculosis, flu, ringworm, and malaria are some examples of infectious diseases. Non-infectious diseases cannot be transmitted from one person to another and are called non-infectious diseases, it is also known as non-communicable diseases. These diseases can be caused by genetic disorders, unhealthy diets, lack of physical activity, excessive use of tobacco, drugs or alcohol and a few environmental factors.

Based on the above information, answer the following questions:

(i) Name the vector that spreads dengue and chikungunya disease.

(ii) What are the symptoms of pneumonia? Also, mention the names of pathogens for this disease in humans.

(iii) What are cancer-causing genes known as?

(iv) What is the main cause of Rheumatoid arthritis?

(v) Name the toxin that is released by Plasmodium-infected cells responsible for the chills and fever in the patient.

Answer: 

(i) Dengue and chikungunya are vector-borne diseases that are spread by a mosquito vector called Aedes mosquitoes.

(ii) The common symptoms of pneumonia are difficulty in respiration, fever, chills, cough, and headache. In humans, pneumonia is caused due to bacteria Streptococcus pneumoniae and Haemophilus influenzae that infect the alveoli (air-filled sacs) of the lungs.

(iii) Cancer-causing genes are known as oncogenes. There are certain proto- oncogenes or cellular oncogenes that, under specific conditions, convert into tumour cells.

(iv) Rheumatoid arthritis is an autoimmune disorder in which body cells lose their ability to differentiate between self and foreign cells. In this disorder, the body cells start attacking self-cells due to some genetic or other reasons.

(v) The toxin that is released by the Plasmodium-infected cells, which is responsible for the chills and fever in the patient, is known as Haemozoin.

Q.No.3: Read the passage given below and answer the following questions:

Biodiversity is all the different kinds of life forms found in one area - the variety of animals, plants, fungi, and even microorganisms like bacteria that make up our natural world. Each of these species and organisms works together in ecosystems, like an intricate web, to maintain balance and support life. It supports everything in nature that organisms need to survive. It provides food, clean water, medicine, and shelter. But as humans put increasing pressure on the planet, using and consuming more resources than ever before, it risks upsetting the balance of ecosystems and losing biodiversity.

(i) Give one example of a species showing genetic diversity.

(ii) Why did the biologists not give any count on the number of prokaryotes?

(iii) What are the special characteristics that account for the higher biological diversity of the tropics?

(iv) Give the equation that represents the species-area relationship on a log scale.

(v) What are the identifying features that are shown by the stable community?

Answer: 

(i) One species is the mango, having 1,000 different genetic varieties growing in India.

(ii) The biologists did not give any count on the numbers of prokaryotes because the conventional taxonomic methods are not suitable for identifying microbial species, and many species are simply not culturable under laboratory conditions.

(iii) Tropics are not subjected to frequent glaciations and have remained relatively undisturbed in past years; also, tropics are less seasonal, and there is more solar energy available in the tropics.

(iv) The equation is:

log S = log C + Z log A

where,

S = Species richness, A = Area

Z = slope of the line (regression coefficient)

C = Y-intercept

(v) A stable community should not show too much variation in productivity from year to year; it must be either resistant or resilient to occasional disturbances (natural or man-made), and it must also be resistant to invasions by alien species.

Download PDF CBSE 12th Biology Case-Based Questions 2025-26

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👉 CBSE Class 12 Study Materials

CBSE Class 12 study material includes latest syllabus, sample papers, NCERT books and solutions, previous year question papers, chapter-wise important questions, case study questions, assertion-reason questions, and competency-based questions — all updated as per the latest CBSE board exam pattern.