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501. The common name mad cow disease refers to:
ⓐ. potato spindle-tuber viroid disease
ⓑ. bovine spongiform encephalopathy
ⓒ. human Creutzfeldt-Jacob disease
ⓓ. tobacco mosaic virus disease
Correct Answer: bovine spongiform encephalopathy
Explanation: Mad cow disease is the common name for bovine spongiform encephalopathy, a prion-associated neurological disease of cattle. Creutzfeldt-Jakob disease is the named human prion disorder, so it should not be substituted for the cattle disease. Potato spindle tuber disease is associated with a viroid, whereas tobacco mosaic disease is caused by a virus. The identification depends on keeping four relations aligned: the common name, the formal disease name, the affected host, and the type of infectious agent. This comparison separates prion diseases from viroid and viral plant diseases. The host term bovine is the additional clue that fixes the cattle disease rather than the human prion disorder.
502. The human disease associated with a prion in the stated classification is:
ⓐ. acquired immunodeficiency syndrome
ⓑ. human smallpox disease
ⓒ. Creutzfeldt-Jacob prion disease
ⓓ. human influenza disease
Correct Answer: Creutzfeldt-Jacob prion disease
Explanation: Creutzfeldt-Jacob disease is the named human neurological disorder associated with prions. AIDS, smallpox, and influenza are viral diseases and involve nucleic-acid-containing agents with protein coats. The prion differs compositionally as the infectious material is abnormal protein without RNA or DNA. The human example parallels bovine spongiform encephalopathy in cattle, but the two disease names should not be interchanged. Correct identification requires combining host category with agent type rather than grouping all serious human infections under viruses. It is a human neurological prion disease, distinct from the cattle disease bovine spongiform encephalopathy.
503. Match each disease with the correct host-agent combination. A Column II entry is used once.
| Column I | Column II |
|---|---|
| P. Potato spindle tuber disease | 2. Cattle; prion |
| Q. Bovine spongiform encephalopathy | 3. Human; prion |
| R. Creutzfeldt-Jacob disease | 1. Potato plant; viroid |
ⓐ. P-1, Q-2, R-3
ⓑ. P-2, Q-3, R-1
ⓒ. P-3, Q-1, R-2
ⓓ. P-1, Q-3, R-2
Correct Answer: P-1, Q-2, R-3
Explanation: Potato spindle tuber disease affects potato plants and is caused by a viroid, so P maps to 1. Bovine spongiform encephalopathy is the prion-associated disease of cattle, giving Q-2. Creutzfeldt-Jacob disease is the corresponding human prion disease and fixes R-3. The two prion diseases share an agent category but differ in host and disease identity, so the combined host-agent entries remain distinct. The mapping also separates the naked-RNA viroid from the protein-only prion.
504. An infectious sample contains no detectable RNA or DNA, but abnormal protein is present. The most defensible identification is:
ⓐ. an infectious viroid
ⓑ. a protein-only prion
ⓒ. an RNA-containing virus
ⓓ. a DNA bacteriophage
Correct Answer: a protein-only prion
Explanation: Absence of both RNA and DNA rules out viruses and viroids, since each requires nucleic acid. The presence of abnormal infectious protein matches the defining composition of a prion. An RNA virus would contain RNA enclosed by a protein coat, while a bacteriophage commonly contains double-stranded DNA within a capsid. The conclusion is based on the infectious component rather than on host or particle shape. This compositional reasoning provides a direct way to distinguish a prion from all nucleic-acid-containing acellular agents. Absence of nucleic acid rules out viruses and viroids, while infectious abnormal protein identifies a prion.
505. A prion differs from an ordinary virus because a prion:
ⓐ. is a complete wall-less prokaryotic cell
ⓑ. is infectious RNA without a protein coat
ⓒ. is DNA enclosed within a polyhedral capsid
ⓓ. is infectious protein with no nucleic acid
Correct Answer: is infectious protein with no nucleic acid
Explanation: An ordinary virus contains either RNA or DNA associated with a protein capsid. A prion has no nucleic acid and is composed of infectious abnormal protein. RNA without a coat describes a viroid, while DNA inside a capsid could describe a DNA virus. A prion is not cellular and cannot be considered a reduced prokaryote. The distinction identifies the material carrying infectivity rather than relying only on the fact that both agents lie outside normal five-kingdom classification. Both agents are infectious, but only the ordinary virus contains a nucleic-acid genome enclosed by protein.
506. Assertion: Prions are not classified as viruses.
Reason: Prions consist of infectious abnormal protein and contain no viral nucleic-acid genome.
ⓐ. Both statements are true, and the Reason correctly explains the Assertion
ⓑ. Both statements are true, but the Reason does not explain the Assertion
ⓒ. The Assertion statement is true, whereas the Reason statement is false
ⓓ. The Assertion statement is false, whereas the Reason statement is true
Correct Answer: Both statements are true, and the Reason correctly explains the Assertion
Explanation: Viruses are nucleoprotein particles containing either RNA or DNA within a protein capsid, whereas prions are infectious abnormal proteins with no nucleic-acid genome. This compositional difference makes the assertion true and the reason accurate. The absence of nucleic acid means that a prion does not satisfy the defining organisation of a virus. The reason therefore explains why prions are classified separately despite their ability to cause transmissible disease. Shared infectivity alone cannot make two agents structurally equivalent. The decisive distinction is the nature of the infectious material, not merely the disease produced or dependence on a host.
507. Three infectious samples are analysed. In P, an infective nucleic acid is normally enclosed by a protein coat. Q remains infectious as free low-molecular-weight RNA without a protein coat. R contains no nucleic acid, and its infectivity is associated with an abnormal protein. Which identification is correct?
ⓐ. P-viroid, Q-virus, R-prion
ⓑ. P-virus, Q-viroid, R-prion
ⓒ. P-prion, Q-virus, R-viroid
ⓓ. P-virus, Q-prion, R-viroid
Correct Answer: P-virus, Q-viroid, R-prion
Explanation: Sample P has the defining nucleoprotein organisation of a virus: its genetic material is nucleic acid and is ordinarily enclosed within a protein capsid. Sample Q is a viroid because the infectious agent is a small, free RNA molecule without a protein coat. Sample R is a prion because no nucleic acid is present and the abnormal protein itself is infectious. The identification depends on the nature and packaging of the infectious material, not merely on the fact that all three agents are acellular. The evidence separates a nucleic-acid–protein viral particle, naked infectious RNA, and protein-only infectivity.
508. A lichen is best described as:
ⓐ. a fungal organism whose own cells have permanently acquired photosynthetic pigments
ⓑ. a colony composed entirely of prokaryotic cells joined in a mutual association
ⓒ. a stable symbiotic association between fungal and photosynthetic partners
ⓓ. an acellular infectious particle consisting of nucleic acid surrounded by protein
Correct Answer: a stable symbiotic association between fungal and photosynthetic partners
Explanation: A lichen is a stable association involving a fungus and a photosynthetic partner, usually described as an alga in the stated classification. The two partners cooperate so closely that the association appears and functions as one visible unit. The fungal partner does not itself become photosynthetic, and the association is not a single prokaryotic colony or an acellular agent. Recognising the composite nature of a lichen is essential for understanding why it is discussed separately from the five kingdoms: its partners belong to established biological groups, while the lichen represents their mutual association.
509. The photosynthetic partner in a lichen is called the:
ⓐ. phycobiont
ⓑ. mycobiont
ⓒ. capsomere
ⓓ. plasmodium
Correct Answer: phycobiont
Explanation: The phycobiont is the algal or photosynthetic partner of a lichen. It contains chlorophyll and prepares organic food through photosynthesis. The mycobiont is the fungal partner, which provides shelter and helps absorb water and minerals. Capsomeres are viral protein subunits, while a plasmodium is a feeding stage of slime moulds. The term phycobiont identifies both partner type and nutritional function, preventing reversal of the reciprocal roles within the lichen association. The phycobiont contains the photosynthetic machinery and supplies organic food to the composite association. Its name distinguishes it from the fungal mycobiont, which performs the structural and absorptive functions.
510. The fungal partner of a lichen is known as the:
ⓐ. phycobiont
ⓑ. viroid
ⓒ. capsid
ⓓ. mycobiont
Correct Answer: mycobiont
Explanation: The mycobiont is the fungal component of a lichen. Its hyphae form much of the protective framework and help absorb or retain water and mineral substances. The photosynthetic component is the phycobiont, which produces organic food. These names identify both partner type and function, so interchanging them would reverse the biological roles within the association. A viroid is infectious RNA without a protein coat, while a capsid is the protein covering of a virus; neither term names a lichen partner. The prefix myco- refers to the fungal partner, which helps distinguish mycobiont from the photosynthetic phycobiont.
511. The principal contribution of the phycobiont to a lichen association is:
ⓐ. production of infectious protein
ⓑ. preparation of food by photosynthesis
ⓒ. formation of a fungal fruiting body
ⓓ. absorption of all minerals through hyphae
Correct Answer: preparation of food by photosynthesis
Explanation: The phycobiont contains photosynthetic pigments and synthesises organic food using light. This food supports the partnership and supplies the nutritional contribution that the heterotrophic fungal partner cannot produce independently. The mycobiont provides shelter and assists in obtaining water and minerals. Infectious-protein production is associated with prions, and a fungal fruiting body is not the defining food-producing function of the lichen partner. The relation is reciprocal: photosynthetic carbon supply comes primarily from the phycobiont, while physical protection and absorption are linked with the mycobiont. Organic food produced by this partner supports both components, while the fungus contributes protection and resource absorption.
512. The mycobiont benefits the lichen association most directly by providing:
ⓐ. formation of a protein capsid around the alga
ⓑ. photosynthetic synthesis of organic food
ⓒ. shelter plus uptake of water and minerals
ⓓ. ingestion and internal digestion of food
Correct Answer: shelter plus uptake of water and minerals
Explanation: Fungal hyphae form much of the structural framework of a lichen. They surround and protect the photosynthetic cells and help absorb or retain water and mineral substances from the surroundings. The phycobiont performs photosynthesis and supplies organic food, so food production is not the principal contribution of the mycobiont. The fungal partner also does not form a viral coat or use animal-like ingestion. Its combined protective and absorptive role complements the autotrophic role of the phycobiont and helps maintain the stable association. This division of labour allows the partners to occupy conditions in which either component may be less successful alone.
513. Assertion: A lichen represents a mutually useful association.
Reason: The phycobiont prepares food, while the mycobiont supplies shelter, water and mineral support.
ⓐ. Both statements are true, and the Reason correctly explains the Assertion
ⓑ. Both statements are true, but the Reason does not explain the Assertion
ⓒ. The Assertion statement is true, whereas the Reason statement is false
ⓓ. The Assertion statement is false, whereas the Reason statement is true
Correct Answer: Both statements are true, and the Reason correctly explains the Assertion
Explanation: Both partners contribute a different resource or function. The photosynthetic phycobiont produces organic food, while the fungal mycobiont provides a protected environment and assists with water and mineral acquisition. Since each partner benefits from the contribution of the other, the association is mutualistic. The reason gives the reciprocal functional basis of the assertion rather than merely listing two organisms found together. A lichen should not be viewed as one organism exploiting another in the same way as a parasite harming its host. The reason names the complementary contributions that make the relationship beneficial to both partners and explains the assertion.
514. If the phycobiont is removed from an otherwise intact lichen association, the function most directly lost is:
ⓐ. formation of the protective fungal framework
ⓑ. absorption of water by the fungal partner
ⓒ. loss of photosynthetic production of organic food
ⓓ. mineral support supplied by the mycobiont
Correct Answer: loss of photosynthetic production of organic food
Explanation: The phycobiont is the autotrophic partner and provides organic food through photosynthesis. Removing it eliminates the principal food-producing component of the association. The mycobiont may still retain its fungal framework, absorb some water, and contribute mineral support, but the reciprocal partnership can no longer function normally without photosynthetic carbon input. This changed-partner condition separates the roles of the two partners: the phycobiont supplies food, while the mycobiont supplies shelter and assists in water and mineral acquisition. The fungal component cannot replace the removed partner's photosynthetic function. Loss of the food-producing partner disrupts the nutritional balance of the association.
515. If the mycobiont is removed while the photosynthetic partner remains alive, the most immediate loss to the association is:
ⓐ. all chlorophyll from the phycobiont
ⓑ. the RNA genome of the association
ⓒ. the ability to produce any organic molecule
ⓓ. shelter and water-mineral support
Correct Answer: shelter and water-mineral support
Explanation: The mycobiont provides the fungal framework of the lichen and helps obtain or retain water and mineral substances. Removing it immediately removes this shelter and absorptive support from the association. The photosynthetic partner may still retain chlorophyll and some capacity to produce organic food, so loss of photosynthesis is not the first direct consequence. A lichen also has no single shared RNA genome comparable with a virus. The result shows why the partnership is mutualistic: each partner contributes a distinct function that the other does not fully replace. The immediate prediction follows from the removed partner's normal role rather than from a loss of every function of the composite body.
516. The reciprocal functions of lichen partners are summarised below.
The correct identification is:
| Partner | Nutritional type | Major contribution |
|---|---|---|
| P | Autotrophic | Produces organic food |
| Q | Heterotrophic | Provides shelter and assists water-mineral absorption |
ⓐ. P-mycobiont and Q-phycobiont
ⓑ. P-phycobiont and Q-mycobiont
ⓒ. P-prion and Q-viroid
ⓓ. P-virus and Q-fungus
Correct Answer: P-phycobiont and Q-mycobiont
Explanation: Partner P is autotrophic and produces organic food, so it is the photosynthetic phycobiont. Partner Q is heterotrophic and supplies the fungal framework, shelter, and assistance in water-mineral absorption, so it is the mycobiont. The table must be solved by linking nutritional type with contribution rather than by memorising the two names separately. Reversing the labels would assign photosynthesis to the fungus and absorptive support to the alga, contradicting the stated roles. Prions, viroids, and viruses are acellular infectious agents and cannot serve as the two cellular partners of a lichen.
517. Lichens are generally absent from heavily polluted areas. The most defensible use of this observation is that lichens:
ⓐ. can serve as qualitative indicators of air pollution
ⓑ. provide an exact numerical pollution index without measurement
ⓒ. grow best when atmospheric pollution is greatest
ⓓ. prove that every pollutant has the same biological effect
Correct Answer: can serve as qualitative indicators of air pollution
Explanation: Lichens are sensitive to polluted atmospheric conditions and commonly fail to grow where air pollution is substantial. Their presence or absence can provide a qualitative indication of environmental air quality. This relation does not supply an exact pollutant concentration, identify the responsible chemical, or prove that every pollutant acts identically. Other environmental conditions must also be considered before interpreting a site. From this observation, the valid inference is limited but useful: reduced lichen growth may signal polluted air, making the association an ecological indicator rather than a numerical measuring instrument.