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Biology · 2026 · 5 marks
CBSE 2026 · Region 2 · Set 1 · Q33
(a) A 30 year old man was admitted to a hospital with recurrent infections, weight loss, prolonged fever. His blood report showed a decline in count…
A $\displaystyle 30$ year old man was admitted to a hospital with recurrent infections, weight loss, prolonged fever. His blood report showed a decline in count of T-Lymphocytes thus affecting immune system.
(i)
Identify the disease and its causative agent.
(ii)
How did the pathogen affect the immune system once it entered the body ? Explain.
(iii)
Mention two major modes of its transmission.
(i)
Why are microbes considered effective biocontrol agents in sustainable agriculture?
(ii)
Explain the role of the following as biocontrol agents naming the pests or diseases they help to control :
(1)
A Bacterium
(2)
A Fungus
(3)
Virus
Marking-scheme solution
(i)
• Disease : Acquired Immuno Deficiency Syndrome /AIDS
• Causative Agent : Human Immuno deficiency Virus / HIV
(ii)
After getting the entry into the body of the person virus enters macrophages
RNA genome of the virus replicates to form viral DNA with the help of enzyme reverse transcriptase
The viral DNA gets incorporated into the host cell’s DNA and directs the infected cells to produce virus particles.
Simultaneously HIV enters into helper T-lymphocytes (TH) and produce progeny viruses which attack other T-lymphocytes leading to weakened immune system.
(iii)
By Sexual contact with infected person.
Transfusion of contaminated blood and blood products.
Sharing of infected needles by intravenous drug abusers.
Infected mother to her child through placenta.
(i)
They reduce our dependence on toxic chemicals and pesticides / They create
a system where insects called pests are not eradicated but kept at manageable level by complex system of checks and balances / No negative impact on plants mammals birds fish or non – target insects
(ii)(1)
A Bacterium: Bacillus thuringenesis, controls butterfly caterpillars, larvae eat the spores and toxin is released in the gut of larvae thus killing it
(2)
A Fungi: Trichoderma are free living fungi common in root ecosystem, effective biocontrol agents against several plant pathogens.
(3)
Virus: Baculoviruses under genus Nucleopolyhedrovirus , shows species specific
, narrow spectrum insecticidal application.
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CBSE Class 12 Biology past-paper question from the 2026 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.