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401. In fungi, the most accurate distinction between asexual spores and sexual spores is that:
ⓐ. Asexual spores are always formed only after meiosis
ⓑ. Sexual spores are formed without any nuclear fusion
ⓒ. Asexual spores are formed only inside asci
ⓓ. Asexual spores are formed by mitotic divisions, while sexual spores are formed after sexual processes involving nuclear fusion and meiosis
Correct Answer: Asexual spores are formed by mitotic divisions, while sexual spores are formed after sexual processes involving nuclear fusion and meiosis
Explanation: Asexual spores arise from mitotic divisions and help fungi multiply rapidly without mating. They are produced under favorable conditions for quick dispersal and colonization. Sexual spores are linked to sexual reproduction, where compatible nuclei come together through sexual processes. After nuclear fusion, meiosis restores the chromosome number and leads to formation of sexual spores. This pathway is fundamental to the life cycle pattern in fungi. Hence, mitosis-based asexual spores differ from sexual spores formed after fusion and meiosis.
402. Conidia in fungi are best described as:
ⓐ. Motile spores with flagella formed in water
ⓑ. Thick-walled resting spores formed after fertilization
ⓒ. Asexual spores formed exogenously on specialized hyphae called conidiophores
ⓓ. Sexual spores formed inside basidia
Correct Answer: Asexual spores formed exogenously on specialized hyphae called conidiophores
Explanation: Conidia are asexual spores produced externally (exogenously) on specialized spore-bearing structures called conidiophores. They are not formed inside a sac-like structure, which makes them distinctive among asexual spores. Conidia are produced in large numbers and disperse easily through air, aiding rapid spread. Their production is a common asexual strategy in many fungi. This feature is routinely used to describe asexual reproduction in fungi. Hence, conidia are exogenous asexual spores formed on conidiophores.
403. Sporangiospores are correctly identified as spores that are:
ⓐ. Produced endogenously inside a sporangium
ⓑ. Produced only after meiosis in basidia
ⓒ. Always motile with flagella
ⓓ. Formed only inside asci
Correct Answer: Produced endogenously inside a sporangium
Explanation: Sporangiospores are asexual spores formed within a sac-like structure called a sporangium. Because they develop inside the sporangium, they are termed endogenous spores. When mature, they are released and dispersed to new sites for growth. This is a common asexual reproductive method in many lower fungi groups described in basic classification. The defining feature is “inside sporangium,” not outside on hyphae. Hence, sporangiospores are produced endogenously inside a sporangium.
404. The sexual spores of Ascomycetes are:
ⓐ. Basidiospores
ⓑ. Ascospores
ⓒ. Zoospores
ⓓ. Conidia
Correct Answer: Ascospores
Explanation: Ascomycetes are characterized by the formation of sexual spores called ascospores. These spores are formed inside sac-like structures called asci. The production of ascospores is a defining reproductive feature for this fungal group. This concept is central to connecting fungal classification with reproductive structures. Therefore, identifying Ascomycetes strongly links to ascospores as their sexual spores. Hence, the sexual spores of Ascomycetes are ascospores.
405. Basidiospores are best described as:
ⓐ. Asexual spores formed inside sporangia
ⓑ. Motile spores formed only in aquatic habitats
ⓒ. Thick-walled spores formed only by fragmentation
ⓓ. Sexual spores formed exogenously on basidia
Correct Answer: Sexual spores formed exogenously on basidia
Explanation: Basidiospores are sexual spores characteristic of Basidiomycetes. They are produced on a club-shaped structure called a basidium. A key point is that basidiospores are formed externally (exogenously) on the basidium. This reproductive feature is used to recognize Basidiomycetes in classification. It directly links spore type to the spore-bearing structure. Hence, basidiospores are sexual spores formed exogenously on basidia.
406. A zygospore in fungi is best defined as:
ⓐ. A motile asexual spore with flagella
ⓑ. An asexual spore produced on conidiophores
ⓒ. A thick-walled sexual resting spore formed after fusion of compatible gametangia
ⓓ. A sexual spore formed only inside an ascus
Correct Answer: A thick-walled sexual resting spore formed after fusion of compatible gametangia
Explanation: A zygospore is a sexual spore formed after fusion of compatible gametangia. It develops a thick wall, allowing it to survive unfavorable environmental conditions. Because of this resistance, it is commonly considered a resting spore. This spore type is a key feature in describing sexual reproduction in certain fungal groups. The “fusion + thick wall + resting” idea is central to its definition. Hence, a zygospore is a thick-walled sexual resting spore formed after gametangial fusion.
407. An oospore is best described as:
ⓐ. An asexual spore formed in a sporangium
ⓑ. A thick-walled sexual spore formed after fertilization involving oogonium and antheridium
ⓒ. A sexual spore formed on basidia
ⓓ. An external asexual spore on conidiophores
Correct Answer: A thick-walled sexual spore formed after fertilization involving oogonium and antheridium
Explanation: An oospore is a sexual spore associated with oogamous reproduction, involving an oogonium and antheridium. After fertilization, a thick-walled oospore forms, helping survival under adverse conditions. This thick wall supports its role as a resistant resting stage. The concept highlights sexual reproduction based on differentiated male and female structures. It is a standard sexual spore type discussed in fungal classification frameworks. Hence, an oospore is a thick-walled sexual spore formed after fertilization via oogonium and antheridium.
408. Zoospores in fungi are best described as:
ⓐ. Motile asexual spores bearing flagella
ⓑ. Thick-walled sexual resting spores
ⓒ. Asexual spores formed only in asci
ⓓ. Sexual spores formed only on basidia
Correct Answer: Motile asexual spores bearing flagella
Explanation: Zoospores are asexual spores capable of movement because they possess flagella. This motility helps them disperse efficiently in aquatic or very moist environments. Their defining character is “motile spore,” which distinguishes them from non-motile conidia or sporangiospores. Zoospores represent an asexual dispersal strategy suited to water-mediated habitats. The key point is movement due to flagella. Hence, zoospores are motile asexual spores bearing flagella.
409. The primary biological significance of asexual spores in fungi is that they:
ⓐ. Always increase chromosome number
ⓑ. Require fusion of two nuclei to form
ⓒ. Are produced only once in the life cycle
ⓓ. Enable rapid multiplication and quick dispersal under favorable conditions
Correct Answer: Enable rapid multiplication and quick dispersal under favorable conditions
Explanation: Asexual spores are produced in large numbers and form quickly, allowing fast population increase. They help fungi colonize new substrates efficiently without waiting for sexual reproduction. Because they are typically formed under favorable conditions, they act as a rapid spread mechanism. Dispersal through air or water helps occupy available niches swiftly. This is a key ecological advantage of asexual reproduction in fungi. Hence, asexual spores enable rapid multiplication and quick dispersal.
410. In fungi, the correct sequence of key events that leads to formation of sexual spores is:
ⓐ. Meiosis → karyogamy → plasmogamy
ⓑ. Karyogamy → plasmogamy → meiosis
ⓒ. Plasmogamy → karyogamy → meiosis
ⓓ. Fragmentation → budding → spore formation
Correct Answer: Plasmogamy → karyogamy → meiosis
Explanation: Sexual reproduction begins with plasmogamy, where cytoplasms of compatible cells fuse. This is followed by karyogamy, the fusion of nuclei to form a diploid nucleus. Meiosis then occurs to restore the haploid condition and produce sexual spores. This ordered sequence is a core framework used to understand fungal sexual reproduction. It connects cellular events directly to spore formation. Hence, the correct sequence is plasmogamy → karyogamy → meiosis.
411. Phycomycetes are most commonly found in:
ⓐ. Dry desert habitats only
ⓑ. Aquatic habitats and on decaying wood in moist places
ⓒ. Only inside animal intestines
ⓓ. Only on high-salt pans
Correct Answer: Aquatic habitats and on decaying wood in moist places
Explanation: Phycomycetes are commonly associated with moist conditions and are often found in water or damp environments. Many grow on decaying organic matter like wood, which provides nutrients for saprophytic growth. Their distribution reflects the need for moisture for growth and reproduction. This habitat association is a standard feature used to describe phycomycetes. It also explains why they are common in humid, rainy, or water-rich regions. Hence, phycomycetes occur in aquatic habitats and on decaying wood in moist places.
412. The hyphae of phycomycetes are generally:
ⓐ. Septate
ⓑ. Aseptate (coenocytic)
ⓒ. Made of bacterial cells
ⓓ. Divided into tissues and organs
Correct Answer: Aseptate (coenocytic)
Explanation: Phycomycetes typically show aseptate hyphae, meaning the hyphae lack cross walls. This results in a continuous cytoplasm containing many nuclei, described as coenocytic. This structural feature is frequently used in basic classification of fungal groups. It distinguishes them from many other fungi that commonly have septate hyphae. The concept directly links hyphal organization to group identification. Hence, phycomycetes generally have aseptate (coenocytic) hyphae.
413. Asexual reproduction in phycomycetes commonly occurs by formation of:
ⓐ. Conidia
ⓑ. Ascospores
ⓒ. Zoospores or aplanospores
ⓓ. Basidiospores
Correct Answer: Zoospores or aplanospores
Explanation: Phycomycetes typically produce asexual spores in forms such as zoospores (motile) or aplanospores (non-motile). These spores support rapid multiplication and dispersal, often in moist or aquatic environments. The spore types reflect adaptation to water availability, especially for zoospores. This is a key identifying reproductive feature for phycomycetes in classification. It distinguishes them from Ascomycetes where conidia are common. Hence, phycomycetes commonly reproduce asexually by zoospores or aplanospores.
414. Sexual reproduction in phycomycetes typically results in the formation of:
ⓐ. Zygospores
ⓑ. Basidiospores
ⓒ. Conidia
ⓓ. Ascospores
Correct Answer: Zygospores
Explanation: In phycomycetes, sexual reproduction often occurs by fusion of compatible gametangia. This fusion leads to formation of a thick-walled resting spore called a zygospore. The thick wall helps the spore survive unfavorable conditions. Zygospore formation is therefore a standard hallmark of sexual reproduction in phycomycetes. This concept connects their life cycle to survival strategy. Hence, sexual reproduction in phycomycetes produces zygospores.
415. Ascomycetes are also commonly called:
ⓐ. Club fungi
ⓑ. Algal fungi
ⓒ. Sac fungi
ⓓ. Blue-green fungi
Correct Answer: Sac fungi
Explanation: Ascomycetes are known as sac fungi because their sexual spores are produced inside sac-like structures called asci. This “ascus” structure is the defining feature for naming the group. The term directly reflects their characteristic reproductive morphology. It is a common, high-yield classification fact used in exam questions. This name distinguishes them from basidiomycetes (club fungi). Hence, ascomycetes are called sac fungi.
416. The hyphae of most ascomycetes are typically:
ⓐ. Aseptate and coenocytic
ⓑ. Septate and branched
ⓒ. Without any cell wall
ⓓ. Prokaryotic filaments
Correct Answer: Septate and branched
Explanation: Ascomycetes generally possess septate hyphae, meaning cross walls divide the hyphae into compartments. Their hyphae are also branched, forming an extensive mycelial network. This structural organization supports efficient absorption and spread in the substrate. Septate hyphae are a standard identifying feature for ascomycetes in basic fungal classification. It helps distinguish them from phycomycetes, which are usually aseptate. Hence, most ascomycetes have septate and branched hyphae.
417. The sexual spores of ascomycetes are called:
ⓐ. Basidiospores
ⓑ. Conidia
ⓒ. Zoospores
ⓓ. Ascospores
Correct Answer: Ascospores
Explanation: Ascomycetes produce sexual spores inside sac-like asci, and these spores are called ascospores. This is the defining reproductive feature for the group. The name “Ascomycetes” is linked to ascus formation and ascospore production. This concept is central for identifying fungal groups based on spore type. It connects classification directly with reproduction. Hence, the sexual spores of ascomycetes are ascospores.
418. Asexual reproduction in ascomycetes most commonly occurs by:
ⓐ. Conidia
ⓑ. Zygospores
ⓒ. Basidiospores
ⓓ. Oospores
Correct Answer: Conidia
Explanation: Ascomycetes commonly reproduce asexually by forming conidia. Conidia are produced externally on specialized hyphae called conidiophores and are dispersed easily by air. This enables rapid multiplication under favorable conditions. Conidial reproduction is a frequently tested feature of ascomycetes and helps distinguish them from phycomycetes, where zoospores/aplanospores are emphasized. Thus, conidia represent the most common asexual mode. Hence, asexual reproduction in ascomycetes commonly occurs by conidia.
419. The best single feature to differentiate phycomycetes from ascomycetes is:
ⓐ. Presence of chlorophyll
ⓑ. Hyphal organisation (aseptate vs septate)
ⓒ. Ability to live only in deserts
ⓓ. Presence of seeds
Correct Answer: Hyphal organisation (aseptate vs septate)
Explanation: Phycomycetes typically have aseptate (coenocytic) hyphae, while ascomycetes generally have septate hyphae. This structural difference is stable and easy to observe, making it a strong basis for differentiation. It reflects differences in internal organization and is widely used in classification descriptions. Other options like chlorophyll or seeds are irrelevant to fungi. Habitat alone is not a defining separation criterion here. Hence, hyphal organisation (aseptate vs septate) best differentiates phycomycetes and ascomycetes.
420. A fungus with septate hyphae and sexual spores formed in sac-like asci is most appropriately placed in:
ⓐ. Phycomycetes
ⓑ. Protista
ⓒ. Monera
ⓓ. Ascomycetes
Correct Answer: Ascomycetes
Explanation: Septate hyphae are a common feature of ascomycetes, supporting their structural identification. The defining reproductive feature is formation of sexual spores (ascospores) inside sac-like asci. This sac-based sexual reproduction is the hallmark that gives the group its name. Combining septate hyphae with asci formation clearly indicates Ascomycetes rather than phycomycetes. This is a standard classification identification pattern in fungi. Therefore, the fungus is placed in Ascomycetes.
421. Basidiomycetes are commonly called “club fungi” because they produce:
ⓐ. Basidia that bear basidiospores externally
ⓑ. Asci that bear ascospores internally
ⓒ. Sporangia that bear sporangiospores internally
ⓓ. Oogonia that form oospores after fertilization
Correct Answer: Basidia that bear basidiospores externally
Explanation: Basidiomycetes are defined by the presence of basidia, which are club-shaped reproductive structures. Basidiospores are formed exogenously on the surface of basidia, which is the key diagnostic feature. This external spore formation is central to the identity of the group and explains the term “club fungi.” The basidium is the site where the sexual phase culminates and spores are produced for dispersal. This trait is consistently used to classify and recognize basidiomycetes. Hence, club fungi are those producing basidia with external basidiospores.
422. The sexual spores of basidiomycetes are:
ⓐ. Conidia
ⓑ. Zygospores
ⓒ. Basidiospores
ⓓ. Ascospores
Correct Answer: Basidiospores
Explanation: Basidiomycetes produce sexual spores known as basidiospores. These spores are formed on basidia, which are characteristic reproductive structures of this group. The type of sexual spore is a primary criterion in fungal classification at this level. Basidiospores represent the end product of the sexual process and are important for dispersal and propagation. The name “Basidiomycetes” itself reflects the basidium–basidiospore association. Hence, the sexual spores are basidiospores.
423. A key reproductive feature of basidiomycetes is that basidiospores are formed:
ⓐ. Inside sporangia
ⓑ. Inside asci
ⓒ. Only after budding
ⓓ. Exogenously on basidia
Correct Answer: Exogenously on basidia
Explanation: In basidiomycetes, the basidium is the spore-bearing structure associated with sexual reproduction. Basidiospores develop externally on the surface of the basidium, which is a distinctive trait. This contrasts with ascomycetes, where ascospores are formed inside asci. The external placement facilitates easy dispersal of spores into the environment. This feature is consistently emphasized as the defining reproductive hallmark of basidiomycetes. Hence, basidiospores are formed exogenously on basidia.
424. The mycelium of basidiomycetes is typically:
ⓐ. Septate and branched
ⓑ. Aseptate (coenocytic)
ⓒ. Unicellular only
ⓓ. Prokaryotic filaments
Correct Answer: Septate and branched
Explanation: Basidiomycetes generally possess septate hyphae that form a branched mycelial network. Septation indicates the presence of cross walls, a common structural feature in this group. The extensive branching helps in effective absorption and spread through the substrate. This body organisation supports their saprophytic role in many species and enables formation of complex fruiting bodies in some. Hyphal structure is a stable criterion used in describing fungal groups. Hence, basidiomycetes typically have septate, branched mycelium.
425. Rusts and smuts are important examples of:
ⓐ. Deuteromycetes
ⓑ. Basidiomycetes
ⓒ. Phycomycetes
ⓓ. Protists
Correct Answer: Basidiomycetes
Explanation: Rusts and smuts are well-known plant pathogens classified under basidiomycetes. Their placement reflects key basidiomycete reproductive and structural traits even though their life cycles can be complex. This association is frequently used to connect basidiomycetes with agricultural disease significance. The examples reinforce that basidiomycetes include both saprophytes (like mushrooms) and parasites (like rusts/smuts). Recognizing these examples is a standard classification skill. Hence, rusts and smuts belong to basidiomycetes.
426. Deuteromycetes are commonly called “imperfect fungi” because:
ⓐ. They lack a cell wall
ⓑ. Their sexual reproduction is not known/observed
ⓒ. They are always unicellular
ⓓ. They produce basidiospores only
Correct Answer: Their sexual reproduction is not known/observed
Explanation: Deuteromycetes are grouped based on the absence of a known sexual stage. They reproduce asexually, and the sexual form has not been observed or identified in this classification context. Because sexual characters are major criteria in fungal taxonomy, the lack of sexual information makes them “imperfect” in classification terms. This does not imply they are biologically incomplete, but that their sexual phase is unknown. Many are later reassigned when sexual stages are discovered. Hence, they are called imperfect fungi because sexual reproduction is not known.
427. The asexual spores typical of deuteromycetes are:
ⓐ. Zoospores
ⓑ. Conidia
ⓒ. Basidiospores
ⓓ. Ascospores
Correct Answer: Conidia
Explanation: Deuteromycetes reproduce asexually mainly by forming conidia. Conidia are produced in large numbers and help rapid multiplication and dispersal. Since the sexual stage is unknown, asexual characters like conidia become crucial for identifying and grouping these fungi. This feature is a key textbook point for deuteromycetes and is repeatedly tested. Conidial reproduction also supports their wide distribution in diverse habitats. Hence, deuteromycetes typically form conidia as asexual spores.
428. A defining classification feature of deuteromycetes is that they are known primarily by:
ⓐ. Their fruiting bodies with basidia
ⓑ. Their asci containing ascospores
ⓒ. Their asexual (conidial) stage only
ⓓ. Their motile flagellated spores
Correct Answer: Their asexual (conidial) stage only
Explanation: Deuteromycetes are grouped based on what is observed—mainly their asexual reproduction through conidia. The sexual stage is unknown, so classification relies on the imperfect (asexual) form. This makes their conidial stage the key identifying feature in this group. It also explains why they are treated separately from groups defined by sexual spores (ascospores/basidiospores). The emphasis is on observable reproductive traits. Hence, they are known primarily by their asexual (conidial) stage.
429. The mycelium of deuteromycetes is generally:
ⓐ. Prokaryotic and non-septate
ⓑ. Aseptate (coenocytic) without cross walls
ⓒ. Unicellular with no hyphae
ⓓ. Septate and branched
Correct Answer: Septate and branched
Explanation: Deuteromycetes typically show septate and branched mycelium. Septate hyphae indicate compartmentalization by cross walls, consistent with many higher fungi. This structural feature supports extensive colonization and absorptive nutrition. Although sexual reproduction is unknown, vegetative and asexual features like hyphal structure are well described. The septate, branched mycelium is therefore a standard identifying character for this group. Hence, deuteromycetes generally have septate and branched mycelium.
430. Alternaria, Colletotrichum and Trichoderma are commonly cited examples of:
ⓐ. Basidiomycetes
ⓑ. Phycomycetes
ⓒ. Deuteromycetes
ⓓ. Protista
Correct Answer: Deuteromycetes
Explanation: Alternaria, Colletotrichum and Trichoderma are classic examples placed under deuteromycetes in basic fungal grouping. They are identified largely by their asexual reproduction, especially conidia, since sexual stages are not known in this context. These examples help students connect deuteromycetes with practical relevance, including decomposers and plant-associated forms. Using examples is a key way to anchor group-level identification. This linkage is a standard classification fact. Hence, these organisms are examples of deuteromycetes.
431. Viruses are described as “acellular” because they:
ⓐ. Lack any cell organization and do not have cytoplasm or organelles
ⓑ. Have a well-defined nucleus and cytoplasm
ⓒ. Are made of many tissues and organs
ⓓ. Are unicellular eukaryotes with mitochondria
Correct Answer: Lack any cell organization and do not have cytoplasm or organelles
Explanation: Viruses are not made of cells and therefore do not have cytoplasm, cell membrane, or cell organelles. They exist as nucleic acid enclosed in a protein coat, not as a complete cell unit. Because they lack cellular machinery, they cannot perform independent metabolism. This is why they are termed acellular and are placed at the border of living and non-living. Their reproduction requires a host cell’s machinery. Hence, viruses are acellular because they lack cell organization, cytoplasm, and organelles.
432. The most direct consequence of viruses being acellular is that they:
ⓐ. Can reproduce independently in nutrient media
ⓑ. Must use host cell machinery to replicate
ⓒ. Carry out photosynthesis in sunlight
ⓓ. Have their own ribosomes for protein synthesis
Correct Answer: Must use host cell machinery to replicate
Explanation: Because viruses lack cellular components like ribosomes and metabolic enzymes, they cannot synthesize proteins or replicate their genome on their own. They must enter a living host cell and hijack the host’s machinery for replication and assembly. This absolute dependence is a direct outcome of being acellular. It also explains why viruses are called obligate intracellular parasites. Without a host cell, they remain inert. Hence, acellularity makes viruses dependent on host cell machinery to replicate.
433. The basic structural organization of a virus particle is:
ⓐ. Cell wall + cytoplasm + nucleus
ⓑ. Nucleic acid enclosed in a protein coat
ⓒ. Peptidoglycan wall with 70S ribosomes
ⓓ. Cell membrane with mitochondria
Correct Answer: Nucleic acid enclosed in a protein coat
Explanation: A virus is essentially genetic material (either DNA or RNA) protected by a protein coat called capsid. This simple organization is not equivalent to a cell because it lacks cytoplasm and organelles. The capsid protects the nucleic acid and helps the virus attach to host cells. This basic structure is common across viruses, with some having additional outer envelopes. The core definition, however, remains nucleic acid plus protein coat. Hence, a virus particle is nucleic acid enclosed in a protein coat.
434. Which feature most strongly supports viruses being placed at the border of living and non-living?
ⓐ. They have independent cellular respiration
ⓑ. They show metabolism outside host cells
ⓒ. They are acellular and inert outside a host, but replicate only inside living cells
ⓓ. They possess a complete cellular structure but do not reproduce
Correct Answer: They are acellular and inert outside a host, but replicate only inside living cells
Explanation: Outside a host cell, viruses are metabolically inactive and do not show life processes like respiration or growth. Inside a living host cell, they can replicate and produce new virus particles using host machinery. This dual nature—non-living-like outside and living-like inside—places them at the boundary of living and non-living. Their acellular structure is central to this behavior. The dependence on host cell for replication is a key point. Hence, viruses are at the border because they are inert outside hosts but replicate inside living cells.
435. A correct statement about viral “acellular” nature is that viruses:
ⓐ. Possess cytoplasm but no nucleus
ⓑ. Possess a nucleus but no cytoplasm
ⓒ. Have both cytoplasm and organelles but no membrane
ⓓ. Lack cytoplasm and organelles entirely
Correct Answer: Lack cytoplasm and organelles entirely
Explanation: Viruses are not cells and therefore do not have cytoplasm, nucleus, or organelles. Their structure is minimal: genetic material plus a protein coat, sometimes with an envelope. Because they lack ribosomes and metabolic machinery, they cannot perform independent life activities. This is why they are inert outside host cells. The absence of cytoplasm and organelles is the simplest and most direct meaning of “acellular.” Hence, viruses lack cytoplasm and organelles entirely.
436. The term “obligate intracellular parasite” for viruses is most directly linked to:
ⓐ. Their ability to live freely in soil
ⓑ. Their acellular nature and dependence on host cell machinery
ⓒ. Their capacity to perform photosynthesis
ⓓ. Their presence of 70S ribosomes
Correct Answer: Their acellular nature and dependence on host cell machinery
Explanation: Viruses cannot replicate on their own because they lack cellular machinery like ribosomes and many enzymes. This acellular organization forces them to enter a host cell and use its resources for replication. Therefore, they are “obligate” (must) and “intracellular” (inside cells) parasites. The dependence is a direct outcome of their minimal structure. This concept is central in viral biology and classification. Hence, viruses are obligate intracellular parasites due to acellularity and host dependence.
437. Which option best defines “acellular” in the context of viruses?
ⓐ. Viruses are cells without a cell wall
ⓑ. Viruses are cells without a nucleus
ⓒ. Viruses are not composed of cells and lack cellular organization
ⓓ. Viruses are multicellular organisms without tissues
Correct Answer: Viruses are not composed of cells and lack cellular organization
Explanation: “Acellular” literally means “not made of cells.” Viruses do not show cellular organization, meaning they have no cytoplasm, organelles, or cell membrane as a functional unit. Instead, they exist as nucleic acid packaged in a protein coat. This is fundamentally different from unicellular organisms that still have cell structure. The term therefore emphasizes absence of cell as the basic unit. Hence, viruses are acellular because they are not composed of cells and lack cellular organization.
438. A student claims viruses are living because they reproduce. The most accurate correction is:
ⓐ. Viruses reproduce independently in nutrient broth, so they are living
ⓑ. Viruses are multicellular, so reproduction is expected
ⓒ. Viruses show metabolism outside host cells like bacteria
ⓓ. Viruses reproduce only inside host cells, reflecting dependence due to acellularity
Correct Answer: Viruses reproduce only inside host cells, reflecting dependence due to acellularity
Explanation: Viruses can produce new virus particles only after entering a living host cell. They cannot reproduce independently because they lack cellular machinery and metabolism. Their acellular nature makes them dependent on host ribosomes and enzymes for protein synthesis and genome replication. Outside the host, they remain inert, which is unlike true living cells. This conditional reproduction is why they are considered borderline. Hence, viruses reproduce only inside host cells due to acellularity.
439. The absence of ribosomes in viruses is best explained by the fact that viruses are:
ⓐ. Fully cellular organisms
ⓑ. Acellular entities lacking cytoplasm and organelles
ⓒ. Multicellular organisms with tissues
ⓓ. Autotrophs with chloroplasts
Correct Answer: Acellular entities lacking cytoplasm and organelles
Explanation: Ribosomes are cellular organelles found in cytoplasm and are essential for protein synthesis. Viruses lack cytoplasm and organelles because they are not cells. Therefore, they do not possess ribosomes and cannot synthesize proteins independently. This is why they must rely on host cell ribosomes for translation. The absence of ribosomes is a direct result of being acellular. Hence, viruses lack ribosomes because they are acellular entities without cytoplasm and organelles.
440. The most accurate summary statement about viruses as acellular agents is:
ⓐ. Viruses are simplest cells with only cytoplasm
ⓑ. Viruses are multicellular organisms with specialized tissues
ⓒ. Viruses are independent organisms that grow and divide by binary fission
ⓓ. Viruses are non-cellular particles that replicate only in living cells
Correct Answer: Viruses are non-cellular particles that replicate only in living cells
Explanation: Viruses are non-cellular, meaning they do not have a cell structure with cytoplasm and organelles. They exist as particles composed mainly of nucleic acid and protein. Because they lack metabolic and synthetic machinery, they can replicate only inside living host cells. Outside hosts, they are inactive and do not show life processes. This captures the core concept behind “acellular” and viral dependence. Hence, viruses are non-cellular particles that replicate only in living cells.
441. A fundamental genetic feature of viruses is that they:
ⓐ. Always contain both DNA and RNA in the same particle
ⓑ. Lack nucleic acid completely and contain only protein
ⓒ. Contain DNA only, never RNA
ⓓ. Contain either DNA or RNA as the genetic material, but not both
Correct Answer: Contain either DNA or RNA as the genetic material, but not both
Explanation: A virus carries its hereditary information as nucleic acid packaged inside the capsid. The nucleic acid present is of only one type—either DNA or RNA. This “either-or” nature is a defining property of viruses and is used in basic virus description and classification. It also explains why viral replication strategies differ widely among different viruses. The host cell machinery is used differently depending on whether the genome is DNA or RNA. Hence, viruses have either DNA or RNA, not both.
442. The genetic material of a virus is best described as:
ⓐ. DNA or RNA enclosed within a protein coat
ⓑ. DNA enclosed within a cell wall of chitin
ⓒ. RNA enclosed within a peptidoglycan wall
ⓓ. DNA enclosed within a cellulose wall
Correct Answer: DNA or RNA enclosed within a protein coat
Explanation: A virus particle is essentially nucleic acid protected by a capsid (protein coat). The nucleic acid can be DNA or RNA, depending on the virus. This simple organization differs from cells because there is no cytoplasm or organelles. The capsid protects the genome and helps in attachment/entry into host cells. The nucleic acid is the core information molecule that directs formation of new virus particles inside the host. Hence, viral genetic material is DNA or RNA enclosed in a protein coat.
443. Which statement correctly explains why “DNA virus” and “RNA virus” are separate categories?
ⓐ. DNA viruses cannot infect animals, while RNA viruses infect only plants
ⓑ. DNA viruses always have single-stranded genomes, while RNA viruses always have double-stranded genomes
ⓒ. The viral genome is of a single nucleic acid type (either DNA or RNA), which determines replication strategy
ⓓ. DNA viruses do not have capsids, while RNA viruses always have capsids
Correct Answer: The viral genome is of a single nucleic acid type (either DNA or RNA), which determines replication strategy
Explanation: Viruses are grouped based on the type of nucleic acid they carry as the genome. Because the genome is either DNA or RNA, the pathway used to produce new viral nucleic acid and viral proteins differs. Host enzymes and viral enzymes are utilized differently depending on genome type. This leads to distinct replication mechanisms and is therefore a meaningful classification basis. The category is not about host type or capsid presence, but genome chemistry. Hence, genome type (DNA vs RNA) is a major classification basis.
444. A correct generalization about plant viruses is that many of them have:
ⓐ. Only DNA genomes
ⓑ. RNA as genetic material
ⓒ. Both DNA and RNA simultaneously
ⓓ. No nucleic acid at all
Correct Answer: RNA as genetic material
Explanation: Many viruses infecting plants carry RNA as their genetic material. This is a standard broad trend emphasized while comparing virus types across hosts. The RNA genome directs synthesis of viral components inside infected plant cells using host resources. This feature helps explain why “RNA viruses” are common in plant pathology discussions. It also supports genome-based classification of viruses. Hence, many plant viruses have RNA genomes.
445. Bacteriophages are most commonly described as viruses that usually contain:
ⓐ. Only protein without nucleic acid
ⓑ. RNA genomes in most cases
ⓒ. Both DNA and RNA in the same capsid
ⓓ. DNA as genetic material in most cases
Correct Answer: DNA as genetic material in most cases
Explanation: Bacteriophages are viruses that infect bacteria and are widely known for having DNA genomes in most cases. Their DNA enters the bacterial cell and uses bacterial machinery (and sometimes viral enzymes) to replicate and produce new phage particles. This is a classic example used to highlight that many phages are DNA viruses. Genome type also influences how the phage takes over the host cell. Therefore, bacteriophages are commonly associated with DNA genomes. Hence, bacteriophages usually contain DNA.
446. The statement “viral genome may be single-stranded or double-stranded” refers to:
ⓐ. The nucleic acid (DNA or RNA) present in the virus
ⓑ. The protein coat (capsid) layers
ⓒ. The number of host cells infected
ⓓ. The thickness of the cell wall
Correct Answer: The nucleic acid (DNA or RNA) present in the virus
Explanation: In viruses, the hereditary material is nucleic acid, and that nucleic acid can exist in single-stranded or double-stranded form. This applies to both DNA viruses and RNA viruses, depending on the specific virus. The “strandedness” affects how the genome is copied and expressed inside the host. It is a genome property, not a capsid or host property. This is why it is discussed while describing viral genetic material. Hence, single- or double-stranded refers to the viral nucleic acid.
447. Which option best completes the statement: “A virus cannot have ____ as its genetic material.”
ⓐ. RNA
ⓑ. Both DNA and RNA together
ⓒ. DNA
ⓓ. Either DNA or RNA alone
Correct Answer: Both DNA and RNA together
Explanation: A virus contains only one kind of nucleic acid as its genome. This means a virus particle carries either DNA or RNA, but not both at the same time. This is a core defining property used to distinguish viruses from cellular organisms, which contain both DNA and RNA. The “either-or” genome nature directly impacts how the virus replicates and expresses genes in the host cell. Therefore, having both DNA and RNA together as genome is not a viral feature. Hence, a virus cannot have both DNA and RNA together as genetic material.
448. If a virus particle contains RNA as its only nucleic acid, it is correctly classified as:
ⓐ. A DNA virus
ⓑ. A viroid
ⓒ. An RNA virus
ⓓ. A bacterium
Correct Answer: An RNA virus
Explanation: Viral classification at a basic level uses the type of nucleic acid present in the virion. If the genome is RNA, the virus is termed an RNA virus. The RNA serves as the genetic template that directs synthesis of viral components after entering the host. This genome type determines the replication pathway and the enzymes required. The classification is based on genome chemistry, not on size or host. Hence, a virus with RNA as its only nucleic acid is an RNA virus.
449. The most accurate statement about viral nucleic acid is:
ⓐ. Every virus has DNA and uses it to make RNA
ⓑ. Every virus has RNA and uses it to make DNA
ⓒ. Viral nucleic acid is always double-stranded DNA
ⓓ. Viral nucleic acid may be DNA or RNA, and it carries the information needed to form new virus particles in a host
Correct Answer: Viral nucleic acid may be DNA or RNA, and it carries the information needed to form new virus particles in a host
Explanation: The viral genome is the hereditary blueprint that encodes viral proteins and controls replication steps inside the host. Depending on the virus, this genome can be DNA or RNA. Once inside a suitable host cell, the genome directs synthesis of capsid proteins and replication of the genome itself. This is why nucleic acid is essential for viral multiplication. The variation in genome type is a major reason viruses show diverse life cycles. Hence, viral nucleic acid can be DNA or RNA and carries information for forming new virus particles.
450. Which statement best supports “viruses have a single type of genetic material”?
ⓐ. A virus contains either DNA or RNA, and this single genome type defines it as DNA virus or RNA virus
ⓑ. A virus contains DNA in the nucleus and RNA in the cytoplasm
ⓒ. A virus contains only proteins, so genetic material is absent
ⓓ. A virus contains both DNA and RNA like a typical cell
Correct Answer: A virus contains either DNA or RNA, and this single genome type defines it as DNA virus or RNA virus
Explanation: Viruses are unique in having only one kind of nucleic acid as their genome. This means the genome is either DNA or RNA, not both, within a virus particle. This single-genome principle is used to categorize viruses into DNA viruses and RNA viruses. The genome type is central because it governs how the virus replicates and expresses its genes inside the host cell. It is therefore a fundamental classification and concept point. Hence, viruses have a single type of genetic material—either DNA or RNA.
451. The capsid of a virus is:
ⓐ. The protein coat that surrounds and protects the viral nucleic acid
ⓑ. A cytoplasmic membrane containing mitochondria
ⓒ. A peptidoglycan cell wall like bacteria
ⓓ. A cellulose wall like plants
Correct Answer: The protein coat that surrounds and protects the viral nucleic acid
Explanation: The capsid is a protein coat that encloses the viral genome (DNA or RNA) and protects it from physical and chemical damage. It also helps maintain the characteristic shape of the virus particle and provides stability outside the host. Capsid proteins are encoded by the viral genome and assembled inside the host cell. Since viruses lack cellular boundaries and organelles, the capsid acts as the major protective and structural layer of the virion. Hence, the capsid is the protein coat around viral nucleic acid.
452. Capsomeres are best described as:
ⓐ. Viral enzymes required for respiration
ⓑ. Host cell receptors used by viruses
ⓒ. Lipid molecules forming the nuclear membrane
ⓓ. Protein subunits that assemble to form the capsid
Correct Answer: Protein subunits that assemble to form the capsid
Explanation: The capsid is built from repeating protein units called capsomeres, which assemble in an orderly pattern to produce a stable protective shell. This repetitive arrangement provides strong protection using a limited set of proteins and gives the virus a definite symmetry and geometry. Capsomeres are part of the virus particle itself and are not derived from host cell structures. Their precise assembly is crucial for packaging and safeguarding the viral genome. Hence, capsomeres are protein subunits that form the capsid.
453. The most appropriate function of the viral capsid is to:
ⓐ. Perform protein synthesis using viral ribosomes
ⓑ. Carry out independent metabolism outside the host
ⓒ. Protect the genome and aid in attachment/entry into the host cell
ⓓ. Produce ATP for viral replication
Correct Answer: Protect the genome and aid in attachment/entry into the host cell
Explanation: The capsid protects viral nucleic acid from degradation and helps the virus remain stable outside the host. Capsid proteins also provide specific surfaces that recognize host cell receptors, enabling attachment and entry into susceptible cells. Because viruses lack organelles and independent metabolism, the capsid is central to both stability and infectivity. It additionally packages the genome into a compact, transmissible form, supporting efficient spread. Hence, the capsid protects the genome and helps in host attachment/entry.
454. Viruses are called obligate intracellular parasites mainly because they:
ⓐ. Can complete reproduction in sterile nutrient broth
ⓑ. Lack cellular machinery and depend on the host for replication and protein synthesis
ⓒ. Possess their own ribosomes and enzymes for independent life
ⓓ. Grow and divide by binary fission
Correct Answer: Lack cellular machinery and depend on the host for replication and protein synthesis
Explanation: Viruses do not have ribosomes, cytoplasm, or complete enzyme systems needed for independent metabolism and protein synthesis. As a result, they cannot replicate their genome or produce viral proteins on their own. To multiply, they must enter a living host cell and use the host’s machinery to make new viral components and assemble new virions. Outside host cells, they remain metabolically inactive, behaving like inert particles. Hence, viruses are obligate intracellular parasites due to their dependence on host cells.
455. Host specificity of many viruses is primarily determined by:
ⓐ. The match between viral surface proteins (capsid/envelope) and specific host cell receptors
ⓑ. The thickness of the host cell wall only
ⓒ. The number of mitochondria in the host cell
ⓓ. The presence of chloroplasts in the host cell
Correct Answer: The match between viral surface proteins (capsid/envelope) and specific host cell receptors
Explanation: Viral infection begins with attachment, which requires a specific interaction between viral surface proteins and receptors on the host cell membrane. Only cells that carry the appropriate receptors allow stable adsorption and entry, which is why many viruses infect only particular hosts or tissues. This receptor–ligand specificity acts like a lock-and-key mechanism and is a major determinant of host range. Without receptor compatibility, infection cannot proceed efficiently even if the virus reaches the organism. Hence, host specificity depends on viral protein–host receptor matching.
456. The term “virion” refers to:
ⓐ. A fungal spore with chitin wall
ⓑ. The host cell nucleus after infection
ⓒ. A bacterial cell infected by a phage
ⓓ. A complete infective virus particle with nucleic acid enclosed in a capsid
Correct Answer: A complete infective virus particle with nucleic acid enclosed in a capsid
Explanation: A virion is the fully assembled, infectious form of a virus outside the host cell. It consists of the viral genome (DNA or RNA) enclosed within a protective protein capsid, and in some viruses, an additional envelope may be present. The virion is the transmissible unit that initiates infection when it contacts and enters a susceptible host cell. It is not a cell and lacks cytoplasm and organelles, so it relies on the host after entry. Hence, a virion is the complete infective virus particle with genome and capsid.
457. A point that supports the non-living-like nature of viruses outside a host is that they can:
ⓐ. Perform respiration and produce ATP
ⓑ. Synthesize proteins using their own ribosomes
ⓒ. Be crystallized and remain inert until they enter a host cell
ⓓ. Grow on dead organic matter as saprophytes
Correct Answer: Be crystallized and remain inert until they enter a host cell
Explanation: Outside host cells, viruses show no metabolism, growth, or independent reproduction, and they can exist as stable particles. Under suitable conditions, they can be crystallized, demonstrating their inert, non-living-like state outside a host. Once the virion enters a living cell, it becomes active and directs replication using host machinery, showing living-like behavior. This contrast is a key reason viruses are described as lying at the boundary of living and non-living. Hence, crystallization and inertness outside a host support their non-living-like nature.
458. The capsid is important immunologically because it:
ⓐ. Converts host DNA into viral RNA directly
ⓑ. Acts as an antigenic surface that can be recognized by the host immune system
ⓒ. Functions as the host cell receptor during infection
ⓓ. Performs photosynthesis to supply energy
Correct Answer: Acts as an antigenic surface that can be recognized by the host immune system
Explanation: The capsid is made of viral proteins that are exposed on the surface of the virus particle, making them visible to the host immune system. These proteins can act as antigens and trigger immune responses such as antibody production. Antibodies often bind to capsid (or envelope) proteins and can neutralize the virus by blocking attachment or entry. Because immune recognition largely targets surface structures, capsid composition strongly influences immunity and vaccine design concepts. Hence, the capsid acts as an antigenic surface recognized by the host.
459. In a bacteriophage, the “head” is best understood as:
ⓐ. A protein capsid that encloses the viral nucleic acid
ⓑ. A mitochondrion-like energy unit
ⓒ. A cellulose-walled chamber for photosynthesis
ⓓ. A bacterial nucleus
Correct Answer: A protein capsid that encloses the viral nucleic acid
Explanation: The head of a bacteriophage is the capsid that encloses and protects the phage genome. It is designed to tightly package nucleic acid so the genetic material remains stable during transfer between bacterial cells. The head is not an organelle and does not perform metabolism or energy production; its role is structural protection and genome storage. During infection, the head retains the genome until it is injected through the tail apparatus into the host. Hence, the phage head is a capsid enclosing viral nucleic acid.
460. If the capsid of a virus is damaged or removed, the most direct outcome is that the virus will:
ⓐ. Start producing ATP independently
ⓑ. Convert into a bacterial cell
ⓒ. Begin dividing by binary fission
ⓓ. Lose protection of its genome and typically fail to establish infection effectively
Correct Answer: Lose protection of its genome and typically fail to establish infection effectively
Explanation: The capsid protects the viral genome from degradation and helps maintain the infective form of the virus outside the host. If the capsid is damaged or removed, the nucleic acid becomes highly vulnerable to environmental factors and is more likely to be destroyed. In addition, capsid proteins often play a key role in attachment and entry into host cells, so damage can reduce the ability to infect. Since successful infection depends on stable genome delivery into a host cell, infectivity typically drops sharply. Hence, loss of capsid leads to genome vulnerability and failure of effective infection.
461. In a typical head–tail bacteriophage, the genetic material is located in the:
ⓐ. Tail fibres
ⓑ. Base plate
ⓒ. Tail sheath
ⓓ. Head (capsid)
Correct Answer: Head (capsid)
Explanation: In head–tail bacteriophages, the head is a protein capsid that encloses and protects the phage nucleic acid. The genome is packed inside the head, allowing safe transfer between bacterial hosts. The tail apparatus mainly functions in attachment to the bacterial surface and delivery of the genome into the host cell, rather than storage. This division of roles is fundamental to phage structure and infectivity. Hence, the genetic material is located in the head (capsid).
462. The primary function of tail fibres in bacteriophages is:
ⓐ. Recognition and attachment to specific bacterial receptors
ⓑ. Packaging of DNA into the capsid
ⓒ. Synthesis of viral proteins
ⓓ. Formation of the bacterial cell wall
Correct Answer: Recognition and attachment to specific bacterial receptors
Explanation: Tail fibres are specialized structures that help a bacteriophage recognize and bind to suitable bacterial hosts. They attach to specific receptor molecules on the bacterial surface, and this step is essential to initiate infection (adsorption). Because receptor types vary among bacteria, tail fibre binding largely determines host specificity and tissue/strain preference. Without effective attachment, genome injection cannot proceed and infection fails. Hence, tail fibres function primarily in recognition and attachment to bacterial receptors.
463. The base plate of a bacteriophage is best described as the structure that:
ⓐ. Performs photosynthesis for the phage
ⓑ. Stores ATP for DNA synthesis
ⓒ. Helps anchor the tail fibres and assists attachment/injection steps
ⓓ. Produces ribosomes for protein synthesis
Correct Answer: Helps anchor the tail fibres and assists attachment/injection steps
Explanation: The base plate is a complex structure at the end of the phage tail that supports and anchors tail fibres. After the phage attaches to the bacterial surface, the base plate helps stabilize the phage and positions the tail apparatus for genome injection. This structural coordination is crucial during the early steps of infection because it ensures effective contact and successful DNA transfer. The base plate therefore plays a mechanical and functional role in attachment and injection. Hence, it helps anchor tail fibres and assists attachment/injection steps.
464. In many head–tail bacteriophages (e.g., T-even phages), DNA entry into the bacterium is facilitated by:
ⓐ. Budding from the bacterial membrane
ⓑ. Contraction of the tail sheath to drive the tail tube/core into the cell wall
ⓒ. Formation of an ascus around the DNA
ⓓ. Binary fission of the phage particle
Correct Answer: Contraction of the tail sheath to drive the tail tube/core into the cell wall
Explanation: In many head–tail phages, the tail sheath acts as a contractile structure. After adsorption, the sheath contracts and drives the inner tail tube/core toward the bacterial surface, creating a pathway for genome delivery. This mechanical action enables the phage DNA to pass from the head through the tail into the host cell. It is a key concept explaining how a non-cellular particle injects genetic material into a bacterium. Hence, contraction of the tail sheath facilitates DNA entry.
465. The “head–tail” design of bacteriophages is best interpreted functionally as:
ⓐ. Head for locomotion and tail for photosynthesis
ⓑ. Head for digestion and tail for respiration
ⓒ. Head for storage of enzymes and tail for energy production
ⓓ. Head for genome protection and tail for genome delivery to the host
Correct Answer: Head for genome protection and tail for genome delivery to the host
Explanation: The head is a capsid that protects and stores the phage genome outside the host. The tail apparatus is specialized for attachment to bacterial receptors and for delivering the genome into the host cell. This division of labor allows phages to remain stable in the environment and still inject genetic material efficiently once a suitable bacterium is encountered. It is a central functional interpretation of phage morphology. Hence, the head protects the genome and the tail delivers it to the host.
466. The “tail core/tube” of a bacteriophage is most directly involved in:
ⓐ. Providing a channel/pathway through which the genome is injected into the bacterium
ⓑ. Synthesizing the capsid proteins inside the host
ⓒ. Storing ribosomes for translation
ⓓ. Neutralizing host toxins before infection
Correct Answer: Providing a channel/pathway through which the genome is injected into the bacterium
Explanation: The tail tube/core forms the conduit that connects the head to the host surface during infection. After adsorption and, in many phages, tail sheath contraction, this tube provides the route through which the genome moves from the capsid into the bacterial cytoplasm. Its main role is mechanical delivery, ensuring efficient genome transfer into the host cell. Because genome injection is essential for infection, the tail tube/core is a critical structural component. Hence, it provides the channel for genome injection.
467. The first step of bacteriophage infection of a bacterium (in a typical lytic sequence) is:
ⓐ. Replication of viral DNA
ⓑ. Assembly of new phage heads
ⓒ. Adsorption (attachment) to the bacterial cell surface
ⓓ. Release (lysis) of host cell
Correct Answer: Adsorption (attachment) to the bacterial cell surface
Explanation: The infection process begins when the bacteriophage recognizes and attaches to specific receptors on the bacterial surface, a step called adsorption. This is essential because attachment positions the tail apparatus correctly for genome injection. Without adsorption, the phage cannot transfer its genetic material into the host, and the infection cycle cannot proceed. Host specificity is also established at this stage through receptor recognition. Hence, the first step in phage infection is adsorption to the bacterial cell surface.
468. A correct statement about the tail sheath in many bacteriophages is that it:
ⓐ. Is the site where DNA is permanently stored
ⓑ. Acts as a contractile structure that assists genome injection
ⓒ. Is made of peptidoglycan and forms the bacterial wall
ⓓ. Functions as the host receptor molecule
Correct Answer: Acts as a contractile structure that assists genome injection
Explanation: The tail sheath is a protein structure designed to contract during the infection process in many head–tail phages. After the phage attaches to the bacterium, sheath contraction pushes the tail tube toward the host surface and supports genome injection. The sheath does not store the genome; instead, it provides mechanical force to facilitate delivery. This contractile action is a defining functional feature of the tail sheath. Hence, the tail sheath acts as a contractile structure assisting genome injection.
469. A feature that best explains why bacteriophages show host specificity is:
ⓐ. Tail fibres binding only to specific bacterial surface receptors
ⓑ. Presence of chloroplast-like structures in the head
ⓒ. Existence of a cell wall made of cellulose
ⓓ. Ability to form spores in air
Correct Answer: Tail fibres binding only to specific bacterial surface receptors
Explanation: Host specificity in bacteriophages is largely determined by receptor recognition. Tail fibres have binding proteins that attach only to particular molecules on the bacterial surface, so only bacteria with the correct receptors can be infected. This precise interaction decides which bacterial species or strains a phage can adsorb to and inject its genome into. Because attachment is essential for infection, receptor-based recognition is the strongest basis for host specificity. Hence, tail fibres binding specific bacterial receptors explains host specificity.
470. A bacteriophage is best defined as a virus that:
ⓐ. Infects only plants
ⓑ. Causes only fungal diseases
ⓒ. Reproduces only by budding
ⓓ. Specifically infects bacterial cells
Correct Answer: Specifically infects bacterial cells
Explanation: A bacteriophage is a virus whose host is a bacterium. It attaches to bacterial surface receptors, injects its genetic material into the bacterial cytoplasm, and uses the bacterial machinery to produce new phage components. This host specificity distinguishes bacteriophages from viruses that infect plants, animals, or other organisms. Because the defining feature is bacterial infection as the target host, the term directly means “bacteria-infecting virus.” Hence, a bacteriophage is a virus that specifically infects bacterial cells.
471. In the lytic cycle of a bacteriophage, the final outcome for the host bacterium is:
ⓐ. Stable survival with no damage
ⓑ. Conversion into a fungal cell
ⓒ. Permanent integration of phage DNA without host death
ⓓ. Lysis (bursting) of the bacterium with release of new phage particles
Correct Answer: Lysis (bursting) of the bacterium with release of new phage particles
Explanation: In the lytic cycle, the phage takes over the host cell machinery to synthesize viral DNA and proteins, leading to rapid production and assembly of new phage particles inside the bacterium. At the end of the cycle, the bacterial cell wall ruptures and the host cell bursts, releasing many newly formed phages into the surroundings. The defining feature of this cycle is quick multiplication coupled with destruction of the host cell. Hence, the lytic cycle ends with bacterial lysis and release of progeny phages.
472. The lysogenic cycle is best identified by:
ⓐ. Immediate bursting of the host cell after infection
ⓑ. No replication of viral genetic material at any time
ⓒ. Integration of phage DNA into the bacterial genome as a prophage
ⓓ. Formation of ascospores inside asci
Correct Answer: Integration of phage DNA into the bacterial genome as a prophage
Explanation: In the lysogenic cycle, the phage genome becomes incorporated into the bacterial chromosome and is called a prophage. The bacterium continues to live and divide normally, copying the prophage along with its own DNA so the viral genetic material is inherited by daughter cells. This allows the phage to persist without immediately destroying the host. The key concept is stable integration and long-term maintenance within the host lineage. Hence, the lysogenic cycle involves integration of phage DNA as a prophage.
473. A bacterium carrying a prophage is termed:
ⓐ. A sporangium
ⓑ. A plasmid host
ⓒ. A lytic cell
ⓓ. A lysogen
Correct Answer: A lysogen
Explanation: A bacterium that contains integrated phage DNA (prophage) is called a lysogen. In this state, the host survives and reproduces normally while the prophage remains dormant within the bacterial chromosome. During each cell division, the prophage is replicated along with the host genome, ensuring inheritance across generations. This term is central for distinguishing lysogenic condition from immediate lytic infection. Hence, a bacterium carrying a prophage is called a lysogen.
474. The central contrast between lytic and lysogenic cycles is that:
ⓐ. Lytic leads to host cell death soon; lysogenic involves genome integration and host survival for many generations
ⓑ. Lytic involves host survival; lysogenic always causes immediate lysis
ⓒ. Lytic occurs only in plants; lysogenic occurs only in animals
ⓓ. Lytic uses RNA genome only; lysogenic uses DNA genome only
Correct Answer: Lytic leads to host cell death soon; lysogenic involves genome integration and host survival for many generations
Explanation: In the lytic cycle, phage replication is rapid, new particles assemble quickly, and the host cell ultimately bursts to release progeny phages. In the lysogenic cycle, the phage genome integrates into the host DNA and remains dormant as a prophage while the bacterium continues normal division. The prophage is inherited by daughter cells, so the infection persists without immediate host death. This destructive versus persistent strategy is the core conceptual difference between the two cycles. Hence, lytic kills the host soon, while lysogenic integrates and allows host survival.
475. The prophage in a lysogenic bacterium is replicated:
ⓐ. Independently in the environment without the host
ⓑ. Along with the host chromosome during bacterial cell division
ⓒ. Only after the bacterium forms spores
ⓓ. Only inside basidia
Correct Answer: Along with the host chromosome during bacterial cell division
Explanation: Once the phage DNA integrates into the bacterial chromosome, it behaves like a part of the host genome. When the bacterium replicates its DNA before dividing, the prophage DNA is copied at the same time and passed to daughter cells. This allows the phage genetic material to persist for long periods without producing new virions immediately. The defining point is vertical transmission through normal bacterial reproduction. Hence, the prophage is replicated along with the host chromosome during cell division.
476. A phage that can show both lytic and lysogenic cycles is commonly termed:
ⓐ. Virulent phage
ⓑ. Satellite virus
ⓒ. Temperate phage
ⓓ. Prion
Correct Answer: Temperate phage
Explanation: Temperate phages have the ability to follow two alternative pathways after infection. They may integrate their genome into the host chromosome as a prophage and remain dormant (lysogeny), or they may proceed to active replication leading to host lysis (lytic cycle). Environmental or cellular conditions can influence which pathway occurs and can also trigger a switch from lysogenic to lytic. This flexible life strategy distinguishes them from strictly lytic phages. Hence, a phage capable of both cycles is called a temperate phage.
477. A virulent phage is best described as one that:
ⓐ. Establishes lysogeny as its only life cycle
ⓑ. Follows only the lytic cycle and causes host cell lysis
ⓒ. Never attaches to bacteria
ⓓ. Converts bacteria into fungi
Correct Answer: Follows only the lytic cycle and causes host cell lysis
Explanation: Virulent phages characteristically proceed directly into active replication after infecting a bacterial cell. They do not establish stable integration as a prophage in the host genome. Their replication results in rapid assembly of new phage particles, followed by rupture of the host cell and release of progeny. This immediate destructive outcome is the defining feature of virulent phages. Hence, virulent phages follow only the lytic cycle and cause lysis.
478. In lysogeny, the phage genome is called a “prophage” because it:
ⓐ. Is a bacterial ribosome used by the phage
ⓑ. Represents an integrated, dormant form of the phage genome within the host DNA
ⓒ. Is a protein coat that floats in the cytoplasm
ⓓ. Is a spore produced by fungi
Correct Answer: Represents an integrated, dormant form of the phage genome within the host DNA
Explanation: During lysogeny, the phage DNA becomes incorporated into the bacterial chromosome and remains inactive in terms of immediate virion production. In this integrated state, it is copied along with host DNA during cell division and can persist through many generations. This stable, hidden viral genetic state inside the host genome is termed a prophage. The concept highlights persistence without immediate host destruction. Hence, a prophage is the integrated dormant form of phage DNA within host DNA.
479. The shift from lysogenic to lytic cycle is best described conceptually as:
ⓐ. Induction, where the prophage becomes active and enters the lytic pathway
ⓑ. Budding, where phage leaves the host membrane gradually
ⓒ. Fragmentation, where the prophage breaks into hyphae
ⓓ. Conjugation, where bacteria exchange spores
Correct Answer: Induction, where the prophage becomes active and enters the lytic pathway
Explanation: In lysogeny, the prophage remains dormant within the bacterial genome while the host divides normally. Under certain conditions, the prophage can become activated, exit the dormant state, and initiate viral replication and particle assembly. This switching process is termed induction and marks the transition from stable integration to active lytic development. Once induced, the infection typically proceeds to host cell lysis and release of progeny. Hence, the shift is induction—activation of the prophage into the lytic pathway.
480. A bacterium continues normal division for many generations after phage infection, while carrying phage DNA integrated in its chromosome. This describes:
ⓐ. Lytic cycle
ⓑ. Lysogenic cycle
ⓒ. Binary fission of virus particles
ⓓ. Sporangial formation
Correct Answer: Lysogenic cycle
Explanation: The defining feature of the lysogenic cycle is integration of phage DNA into the bacterial chromosome as a prophage, without immediate destruction of the host. The bacterium remains alive and continues normal cell division, copying the prophage along with its own DNA so the viral genome is inherited by daughter cells. Because no immediate bursting occurs, this creates a stable carrier state that can persist across many generations. This long-term persistence differentiates it from the lytic cycle. Hence, this situation describes the lysogenic cycle.
481. Viroids are best defined as:
ⓐ. Proteinaceous infectious particles without nucleic acid
ⓑ. DNA viruses with complex capsids
ⓒ. Small circular single-stranded RNA molecules that infect plants
ⓓ. Bacterial plasmids that replicate independently in soil
Correct Answer: Small circular single-stranded RNA molecules that infect plants
Explanation: Viroids are infectious agents composed only of a small, circular, single-stranded RNA molecule. They do not possess a capsid or any protein coat, which is a major feature distinguishing them from viruses. Their RNA alone is sufficient to establish infection and replicate inside susceptible plant cells using host machinery. They are primarily known as plant pathogens responsible for several crop diseases. Because they are RNA-only, circular, and plant-infecting, this definition accurately describes viroids.
482. The most characteristic structural feature that distinguishes viroids from viruses is that viroids:
ⓐ. Contain both DNA and RNA
ⓑ. Have complex head–tail morphology
ⓒ. Are larger than all viruses
ⓓ. Lack a protein coat and consist only of RNA
Correct Answer: Lack a protein coat and consist only of RNA
Explanation: A viroid is essentially naked RNA, meaning it has no capsid or envelope, and the RNA itself is the infective unit. Viruses always have a protein coat around their nucleic acid, so the absence of such a coat is a key diagnostic point for viroids. This also explains why viroids are structurally simpler than viruses. Despite lacking a protein coat, they can still replicate within plant cells and cause disease. Hence, viroids are distinguished by being RNA-only without a protein coat.
483. Viroids are primarily known to cause diseases in:
ⓐ. Plants
ⓑ. Animals
ⓒ. Archaebacteria
ⓓ. Fungi
Correct Answer: Plants
Explanation: Viroids are well-established as plant pathogens and are mainly discussed in the context of plant diseases. They infect plant tissues and can disrupt normal growth and development, often producing symptoms such as stunting, deformation, or reduced yield. Their spread is frequently associated with vegetative propagation and mechanical transmission in agricultural settings. The primary host range emphasis for viroids is therefore plant-centered. Hence, viroids primarily cause diseases in plants.
484. The genetic material in viroids is:
ⓐ. Double-stranded DNA
ⓑ. Single-stranded circular RNA
ⓒ. Double-stranded RNA enclosed in capsid
ⓓ. Single-stranded linear DNA
Correct Answer: Single-stranded circular RNA
Explanation: Viroids consist of a small RNA molecule that is single-stranded and circular, and this is the defining structural identity of the group. They do not contain DNA and are not enclosed in any protein coat. The RNA itself functions as the infectious genetic material and replicates inside plant cells using host enzymes. The circular nature is commonly emphasized because it relates to their replication and stability within the host. Hence, viroids contain single-stranded circular RNA as genetic material.
485. Potato spindle tuber disease is classically associated with infection by a:
ⓐ. Prion
ⓑ. Bacteriophage
ⓒ. Protozoan
ⓓ. Viroid
Correct Answer: Viroid
Explanation: Potato spindle tuber disease is a classic example used to illustrate viroid infection. The causative agent is a small, circular RNA molecule lacking a protein coat, highlighting that infectious agents can be simpler than viruses yet still cause serious plant disease. This example is important because it reinforces the concept of viroids as RNA-only plant pathogens. It also helps students remember viroids through a well-known crop disease association. Therefore, potato spindle tuber disease is linked to a viroid.
486. Compared with most viruses, viroids are generally:
ⓐ. Smaller and structurally simpler (RNA only)
ⓑ. Larger and more complex (RNA + capsid + tail)
ⓒ. Always DNA-based genetic entities
ⓓ. Composed only of proteins
Correct Answer: Smaller and structurally simpler (RNA only)
Explanation: Viroids are minimal infectious agents made only of a short RNA molecule and lack a capsid or any additional structural components. This makes them simpler in organization than viruses, which package nucleic acid within a protein coat and may also have envelopes. Viroids are also typically smaller than most viruses. Their simplicity is a key reason they are treated as a distinct category of infectious agents. Hence, viroids are smaller and structurally simpler because they are RNA only.
487. The most accurate statement about viroids is that they:
ⓐ. Multiply only in dead organic matter
ⓑ. Carry 70S ribosomes for protein synthesis
ⓒ. Replicate inside plant cells despite having no protein coat
ⓓ. Are free-living unicellular organisms
Correct Answer: Replicate inside plant cells despite having no protein coat
Explanation: Viroids do not have a protein coat, ribosomes, or metabolic machinery, so they cannot function independently outside living cells. However, once inside a plant cell, their RNA can replicate by using host enzymes and cellular resources. This host-dependent replication despite the absence of a capsid is a defining feature of viroids. Their RNA acts as the active infectious component responsible for disease development. Therefore, viroids replicate inside plant cells even though they lack a protein coat.
488. A key classification-level contrast is that prions differ from viroids because prions are:
ⓐ. Circular RNA pathogens of plants
ⓑ. DNA-based infectious agents with capsids
ⓒ. Bacterial endospores
ⓓ. Infectious proteins lacking nucleic acids
Correct Answer: Infectious proteins lacking nucleic acids
Explanation: Prions are infectious agents made only of protein and contain no nucleic acid, which makes them fundamentally different from viroids. Viroids are RNA-only agents, whereas prions represent protein-only infectivity. This chemical difference in the infectious unit is the key contrast tested in biology. Because prions lack nucleic acids, they form a distinct category of unusual pathogens. Hence, prions are infectious proteins lacking nucleic acids.
489. The viroid RNA molecule is typically:
ⓐ. Linear and double-stranded
ⓑ. Circular and single-stranded
ⓒ. Enclosed in a helical capsid
ⓓ. Associated with a bacterial cell wall
Correct Answer: Circular and single-stranded
Explanation: The hallmark structure of viroids is a small circular RNA molecule that is single-stranded and not enclosed by any capsid. This compact circular ssRNA is repeatedly emphasized as the defining feature that separates viroids from viruses. The RNA alone serves as the infectious genetic material and replicates in susceptible plant hosts using host machinery. Because this description matches the core identity of viroids, it is the correct characterization. Therefore, viroid RNA is circular and single-stranded.
490. The absence of a protein coat in viroids implies most directly that viroids:
ⓐ. Do not form a capsid and exist as naked RNA in the host
ⓑ. Must be bacterial cells with cell walls
ⓒ. Are multicellular organisms
ⓓ. Always contain both DNA and RNA
Correct Answer: Do not form a capsid and exist as naked RNA in the host
Explanation: Since viroids are composed only of RNA, they do not assemble a protein capsid and therefore exist as naked RNA rather than a nucleic acid packaged inside a protective protein coat. This lack of capsid is the most direct structural feature separating viroids from viruses. Inside plant cells, this RNA molecule functions as the infective unit responsible for replication and pathogenic effects using host resources. No cellular organization is present, only the RNA acting as the agent. Hence, viroids exist as naked RNA without forming a capsid.
491. Lichens are best described as:
ⓐ. A symbiotic association between a fungus and an alga/cyanobacterium
ⓑ. A parasitic association between fungus and bacteria only
ⓒ. A virus infecting algae
ⓓ. A plant root infected by bacteria
Correct Answer: A symbiotic association between a fungus and an alga/cyanobacterium
Explanation: Lichens are stable mutualistic associations formed by two partners: a fungal component (mycobiont) and a photosynthetic component (phycobiont), which may be an alga or a cyanobacterium. The fungus forms the main body, providing protection, structure, and a suitable microenvironment, while the photosynthetic partner produces carbohydrates that supply nutrition. This cooperation enables lichens to survive on nutrient-poor surfaces and in harsh environmental conditions where the partners alone would struggle. Hence, lichens are fungus + alga/cyanobacterium symbiotic units.
492. In lichens, the photosynthetic partner (phycobiont) primarily provides:
ⓐ. Organic food (carbohydrates) synthesized by photosynthesis
ⓑ. Chitin-based structural support
ⓒ. Spore dispersal structures
ⓓ. Host cell ribosomes for translation
Correct Answer: Organic food (carbohydrates) synthesized by photosynthesis
Explanation: The phycobiont is the photosynthetic member of the lichen association and is responsible for producing organic food. By capturing light energy and converting inorganic materials into carbohydrates, it supplies the primary nutritional input that supports the fungal partner. This food contribution is central to the success of lichens, especially on substrates such as rocks or tree bark where nutrients are limited. Hence, the phycobiont provides carbohydrates made by photosynthesis.
493. The fungal partner (mycobiont) in lichens mainly contributes:
ⓐ. Photosynthetic pigment for sugar formation
ⓑ. Protection, water retention, and anchorage to the substrate
ⓒ. Conversion of the lichen into a virus particle
ⓓ. Nitrogen fixation using heterocysts
Correct Answer: Protection, water retention, and anchorage to the substrate
Explanation: The mycobiont forms most of the lichen body and provides the structural framework that shelters the photosynthetic partner. It protects against desiccation and harsh environmental conditions, helps retain water and minerals, and anchors the lichen firmly to the surface on which it grows. This supportive role enables efficient photosynthesis and long-term survival in exposed habitats. Hence, the mycobiont provides protection, water retention, and anchorage.
494. The term that correctly identifies the algal/cyanobacterial partner in lichens is:
ⓐ. Mycobiont
ⓑ. Prophage
ⓒ. Phycobiont
ⓓ. Capsomere
Correct Answer: Phycobiont
Explanation: Lichens consist of two components: the fungal partner called the mycobiont and the photosynthetic partner called the phycobiont. The phycobiont specifically refers to the alga or cyanobacterium that carries out photosynthesis and provides carbohydrates to the association. This terminology helps clearly distinguish the functional roles of each partner in lichen symbiosis. Hence, the algal/cyanobacterial partner is called the phycobiont.
495. A correct ecological interpretation of lichens is that they:
ⓐ. Are always aquatic organisms
ⓑ. Can grow on bare rocks due to efficient mutualism between partners
ⓒ. Require chloroplasts in the fungal partner for survival
ⓓ. Are bacterial colonies with peptidoglycan walls
Correct Answer: Can grow on bare rocks due to efficient mutualism between partners
Explanation: Lichens often colonize bare rocks and other nutrient-poor substrates because their partners complement each other’s needs. The phycobiont produces organic food through photosynthesis, while the mycobiont provides protection, moisture retention, and efficient anchorage to the surface. This mutualistic combination allows lichens to tolerate extreme conditions and establish where many organisms cannot. Hence, lichens can grow on bare rocks due to strong mutualism.
496. Lichens are widely used as indicators of air pollution because they:
ⓐ. Need soil nitrogen fixation to survive
ⓑ. Are insensitive to atmospheric changes
ⓒ. Produce methane under anaerobic conditions
ⓓ. Are highly sensitive to pollutants such as $SO_2$ in the atmosphere
Correct Answer: Are highly sensitive to pollutants such as $SO_2$ in the atmosphere
Explanation: Lichens absorb water and dissolved substances directly through their body surface and lack protective barriers that many plants possess. Because of this, atmospheric pollutants can quickly affect their physiology and growth. Sulfur dioxide and other pollutants can inhibit lichen metabolism, damage cells, and lead to reduced abundance or disappearance from polluted regions. Therefore, the presence or absence of lichens is used as a practical biological indicator of air quality. Hence, lichens indicate air pollution due to high sensitivity to pollutants such as $SO_2$.
497. The most accurate statement about the nature of lichen association is that:
ⓐ. One partner is always killed by the other
ⓑ. Both partners benefit in a stable mutualistic relationship
ⓒ. It is a temporary association only during reproduction
ⓓ. It is a predator–prey interaction
Correct Answer: Both partners benefit in a stable mutualistic relationship
Explanation: In lichen symbiosis, the fungal partner benefits by receiving carbohydrates produced by the photosynthetic partner, while the photosynthetic partner benefits by receiving protection, moisture retention, and access to minerals provided by the fungus. This exchange of benefits is stable and long-lasting rather than accidental or temporary. The relationship improves survival of both partners in challenging environments, making it a classic example of mutualism. Hence, lichens represent a stable relationship where both partners benefit.
498. The best one-line functional summary of lichen symbiosis is:
ⓐ. Fungus provides protection; alga/cyanobacterium provides food
ⓑ. Fungus provides sugars; alga provides anchorage
ⓒ. Both partners fix nitrogen equally in all lichens
ⓓ. Both partners are viruses with protein coats
Correct Answer: Fungus provides protection; alga/cyanobacterium provides food
Explanation: The simplest functional summary of lichens is based on division of labor between the partners. The fungal component forms the protective framework, retains moisture, and anchors the lichen, while the photosynthetic component produces organic food through photosynthesis. This exchange is the core mechanism that maintains the lichen association and explains its ecological success. Hence, fungus provides protection and alga/cyanobacterium provides food.
499. If a lichen’s phycobiont is a cyanobacterium, an additional potential advantage is:
ⓐ. Photosynthesis can occur only in darkness
ⓑ. Production of basidiospores by the alga
ⓒ. Formation of peptidoglycan cell wall in the fungus
ⓓ. Ability to fix atmospheric nitrogen in some cases
Correct Answer: Ability to fix atmospheric nitrogen in some cases
Explanation: Cyanobacteria are capable of nitrogen fixation in many biological contexts, converting atmospheric nitrogen into usable forms. When a cyanobacterium serves as the phycobiont in a lichen, the association may gain access to fixed nitrogen, which can be especially valuable on nutrient-poor substrates. This benefit can improve growth and survival where nitrogen availability is limited. Hence, cyanobacterial phycobionts can provide nitrogen fixation in some cases.
500. The partner that forms most of the visible lichen thallus is generally the:
ⓐ. Mycobiont (fungal component)
ⓑ. Phycobiont (algal component)
ⓒ. Virion
ⓓ. Viroid
Correct Answer: Mycobiont (fungal component)
Explanation: The fungal partner contributes the majority of the lichen’s body mass and forms the main structural thallus. It creates the protective framework that houses the photosynthetic cells within it, controlling moisture conditions and offering stability. Because the fungus dominates the physical structure, most of what is visible in a lichen is fungal tissue, with the photosynthetic partner embedded inside. Hence, most of the visible lichen thallus is formed by the mycobiont.