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The primary goal of virus classification is to identify the specific disease each virus causes, rather than organizing them into a taxonomic system.
Answer: False
Explanation: The fundamental purpose of virus classification is to name viruses and organize them into a taxonomic system. While disease causation is a characteristic considered, classifying solely by disease is less effective because different viruses can cause the same disease.
Francis O. Holmes's 1948 classification system was widely adopted due to its comprehensive account of morphological similarities among viruses.
Answer: False
Explanation: Francis O. Holmes's 1948 classification system was *not* widely adopted because it neglected to account for morphological similarities among viruses.
What is the fundamental purpose of virus classification?
Answer: To organize viruses into a taxonomic system and provide them with names.
Explanation: The fundamental purpose of virus classification is to systematically name viruses and organize them into a taxonomic system.
Which of the following is NOT a key phenotypic characteristic used to classify viruses?
Answer: The economic impact of the disease they cause
Explanation: Key phenotypic characteristics used for virus classification include morphology, nucleic acid type, mode of replication, host organisms, and disease type, but not the economic impact of the disease.
Which two major classification systems are employed for viruses?
Answer: The formal taxonomic classification system (ICTV) and the Baltimore classification system.
Explanation: The two principal classification systems for viruses are the formal taxonomic system overseen by the ICTV and the Baltimore classification system.
What was the basis of Francis O. Holmes's 1948 virus classification system?
Answer: The host organism (bacteria, plants, or animals).
Explanation: Francis O. Holmes's 1948 classification system categorized viruses into three groups based on their host: bacteria, plants, or animals.
Why was the Holmes classification system not widely adopted by the scientific community?
Answer: It neglected to account for morphological similarities among viruses.
Explanation: The Holmes classification system was not widely accepted because it neglected to account for morphological similarities among viruses.
The International Committee on Taxonomy of Viruses (ICTV) is the sole body responsible for maintaining a universal virus taxonomy.
Answer: True
Explanation: The ICTV is indeed the sole body authorized by the International Union of Microbiological Societies to develop, refine, and maintain a universal virus taxonomy.
In 2021, the ICTV mandated a trinomial format for naming new viral species, departing from the system used for cellular organisms.
Answer: False
Explanation: In 2021, the ICTV mandated a *binomial* format (genus species) for naming new viral species, aligning with the system used for cellular organisms, not a trinomial format.
As of 2022, the ICTV taxonomy listed over 11,000 named virus species, including satellite viruses and viroids.
Answer: True
Explanation: The 2022 ICTV taxonomy reported 11,273 named virus species, which includes satellite viruses and viroids.
The definition of a virus 'species' in virology is identical to Ernst Mayr's reproductive concept of species used for cellular organisms.
Answer: False
Explanation: The virological definition of a virus 'species' is a collective name for related viruses sharing common features, which contrasts with Ernst Mayr's reproductive concept of species used for cellular organisms.
The formal definition of a 'virus' was officially accepted by the ICTV Executive Committee in November 2020 and ratified in March 2021.
Answer: True
Explanation: The formal definition of a 'virus' was indeed accepted by the ICTV Executive Committee in November 2020 and ratified in March 2021.
The ICTV defines viruses *sensu stricto* as any mobile genetic element (MGE) that can replicate independently without encoding any virion proteins.
Answer: False
Explanation: The ICTV defines viruses *sensu stricto* as MGEs that encode at least one protein serving as a major component of the virion, and they do not replicate independently without encoding virion proteins.
Before 1982, the ICTV defined a virus species as 'a cluster of strains' with unique identifying qualities.
Answer: False
Explanation: The ICTV began defining a species as 'a cluster of strains' in 1982, not before. Prior to 1982, it was believed viruses could not fit Ernst Mayr's reproductive concept of species.
As of 2021, the ICTV defines a species as the lowest taxonomic level, a monophyletic group of MGEs distinguishable by multiple criteria.
Answer: True
Explanation: As of 2021, the ICTV defines a species as the lowest taxonomic level, a monophyletic group of mobile genetic elements (MGEs) whose properties can be distinguished from those of other species using multiple criteria.
ICTV Study Groups use criteria such as host range, pathogenicity, and genome relatedness to differentiate virus species within a genus.
Answer: True
Explanation: ICTV Study Groups indeed establish criteria like host range, pathogenicity, and genome relatedness to distinguish different species within a genus.
The ICTV actively governs the naming of individual virus strains, which exist at a taxonomic rank above that of a virus species.
Answer: False
Explanation: The ICTV does not govern the naming of individual virus strains, which exist at a taxonomic rank *below* that of a virus species, leaving it to international specialty groups.
SARS-CoV-1 and SARS-CoV-2 are distinct viruses classified within different virus species due to their differing disease outcomes.
Answer: False
Explanation: SARS-CoV-1 and SARS-CoV-2 are distinct viruses but are both classified within the same virus species, *Severe acute respiratory syndrome-related coronavirus*, despite their differing disease outcomes.
As of 2025, the ICTV system utilizes all levels of taxa, including subrealm, subkingdom, and subclass.
Answer: False
Explanation: As of 2025, the ICTV system utilizes all levels of taxa *except* subrealm, subkingdom, and subclass.
The universal use of italics for all taxonomic names in ICTV classification is a unique feature compared to other biological nomenclature codes.
Answer: True
Explanation: The universal application of italics for all taxonomic names in ICTV classification is a distinguishing feature compared to other biological nomenclature codes, which typically reserve italics for genus and species names.
As of mid-2023, all 11,273 virus species had been renamed to comply with the binomial nomenclature format.
Answer: False
Explanation: As of a mid-2023 review, approximately 80% (8,982 out of 11,273) of virus species had been renamed to comply with the binomial nomenclature format, with the remaining species expected to be renamed by the end of 2023, meaning not all were renamed yet.
What significant change did the ICTV implement in 2021 regarding viral species naming?
Answer: They mandated a binomial format (genus species) for naming new viral species.
Explanation: In 2021, the ICTV mandated a binomial format (genus species) for naming new viral species, aligning with the system used for cellular organisms.
As of 2022, approximately how many named virus species were listed in the ICTV taxonomy?
Answer: Over 11,000 species
Explanation: As of 2022, the ICTV taxonomy listed 11,273 named virus species, which is over 11,000.
How does the definition of a virus 'species' in virology differ from that in other biological fields?
Answer: Virology uses a broader definition, considering a species as a collective name for related viruses sharing common features, unlike the reproductive concept.
Explanation: In virology, a virus 'species' is defined as a collective name for a group of presumably related viruses sharing certain common features, which contrasts with the reproductive concept of species typically used for cellular organisms.
When was the currently accepted formal definition of a 'virus' officially accepted by the ICTV?
Answer: November 2020
Explanation: The currently accepted formal definition of a 'virus' was accepted by the ICTV Executive Committee in November 2020.
According to the ICTV's operational definition, what must a virus *sensu stricto* encode?
Answer: At least one protein that serves as a major component of the virion.
Explanation: The ICTV defines viruses *sensu stricto* as mobile genetic elements (MGEs) that encode at least one protein serving as a major component of the virion.
What was the ICTV's initial approach to defining a virus species in 1982?
Answer: They defined a species as 'a cluster of strains' with unique identifying qualities.
Explanation: In 1982, the ICTV began to define a species as 'a cluster of strains' that possessed unique identifying qualities.
Which of the following is NOT a criterion used by ICTV Study Groups to differentiate virus species within a genus?
Answer: The economic value of the host organism
Explanation: ICTV Study Groups use criteria such as host range, pathogenicity, and genome relatedness to differentiate virus species, but not the economic value of the host organism.
Which of the following taxonomic ranks are NOT actively utilized in the ICTV system as of 2025?
Answer: Subrealm, Subkingdom, Subclass
Explanation: As of 2025, all levels of taxa in the ICTV system are utilized, with the exceptions of subrealm, subkingdom, and subclass.
What is the significance of the universal use of italics for all taxonomic names in ICTV classification?
Answer: It is a distinguishing feature compared to other biological nomenclature codes where italics are typically reserved for genus and species names.
Explanation: The universal use of italics for all taxonomic names in ICTV classification is a distinguishing feature compared to other biological nomenclature codes.
As of mid-2023, what percentage of virus species had been renamed to comply with the binomial nomenclature format?
Answer: Around 80%
Explanation: As of a mid-2023 review, 8,982 out of 11,273 virus species (approximately 80%) had been renamed to comply with the binomial nomenclature format.
The Baltimore classification system categorizes viruses based on their morphology, host organisms, and the type of disease they cause.
Answer: False
Explanation: The Baltimore classification system primarily categorizes viruses based on their nucleic acid type, strandedness, sense, and method of mRNA synthesis, not morphology, host, or disease type. Classifying by disease or morphology is considered less effective.
The Baltimore classification is less effective than classifying by disease because different viruses can cause the same disease.
Answer: False
Explanation: The Baltimore classification is considered *more* effective than classifying by disease because it provides insight into a virus's replication strategy, leading to similar behavior within a category. Classifying by disease is less effective precisely because different viruses can cause the same disease.
Baltimore Group II viruses are characterized by a double-stranded DNA (dsDNA) genome.
Answer: False
Explanation: Baltimore Group II viruses are characterized by a *single-stranded DNA (ssDNA)* genome. Double-stranded DNA (dsDNA) viruses belong to Baltimore Group I.
Coronaviruses, Picornaviruses, and Togaviruses all belong to Baltimore Group IV, which consists of positive-sense single-stranded RNA viruses.
Answer: True
Explanation: Coronaviruses, Picornaviruses, and Togaviruses are indeed classified under Baltimore Group IV, which is defined by positive-sense single-stranded RNA ((+)ssRNA) genomes.
Retroviruses, such as HIV, are classified under Baltimore Group VI because they have a positive-sense RNA genome and replicate through a DNA intermediate.
Answer: True
Explanation: Retroviruses, including HIV, are classified in Baltimore Group VI, characterized by a positive-sense RNA genome that replicates through a DNA intermediate using reverse transcriptase.
The hepatitis B virus is an example of a Baltimore Group VII virus, characterized by a DNA genome that replicates using a reverse transcriptase enzyme.
Answer: True
Explanation: The hepatitis B virus is a member of Baltimore Group VII, which consists of double-stranded DNA reverse transcribing (dsDNA-RT) viruses that replicate using a reverse transcriptase enzyme.
Which of the following is a primary criterion used in the Baltimore classification system?
Answer: The type of nucleic acid and its method of mRNA synthesis.
Explanation: The Baltimore classification system primarily uses the type of nucleic acid, its strandedness, sense, and the method by which the virus synthesizes its messenger RNA (mRNA) to categorize viruses.
Why is the Baltimore classification considered more effective than classifying by disease or morphology?
Answer: It provides insight into a virus's replication strategy, leading to similar behavior within a category.
Explanation: The Baltimore classification is more effective because classifying viruses by their genome provides insight into their replication strategy, meaning viruses within a given category will behave similarly.
Which type of viruses belongs to Baltimore Group I?
Answer: Double-stranded DNA (dsDNA) viruses
Explanation: Baltimore Group I consists of double-stranded DNA (dsDNA) viruses.
Parvoviruses, such as human parvovirus B19, are examples of which Baltimore Group?
Answer: Group II (ssDNA)
Explanation: Parvoviruses, such as human parvovirus B19, are examples of Baltimore Group II, which consists of single-stranded DNA (ssDNA) viruses.
Which Baltimore Group includes Reoviruses and Rotavirus?
Answer: Group III (dsRNA)
Explanation: Reoviruses and Rotavirus belong to Baltimore Group III, which consists of double-stranded RNA (dsRNA) viruses.
Coronaviruses, Picornaviruses, and Togaviruses are all classified under which Baltimore Group?
Answer: Group IV ((+)ssRNA)
Explanation: Coronaviruses, Picornaviruses, and Togaviruses are all classified under Baltimore Group IV, which comprises positive-sense single-stranded RNA ((+)ssRNA) viruses.
Orthomyxoviruses (like influenza virus) and Rhabdoviruses (like rabies virus) belong to which Baltimore Group?
Answer: Group V ((-)ssRNA)
Explanation: Orthomyxoviruses and Rhabdoviruses belong to Baltimore Group V, which consists of negative-sense single-stranded RNA ((-)ssRNA) viruses.
What is a defining characteristic of Baltimore Group VI viruses?
Answer: They have a positive-sense RNA genome and replicate through a DNA intermediate.
Explanation: Baltimore Group VI viruses are single-stranded RNA reverse transcribing (ssRNA-RT) viruses, characterized by a positive-sense RNA genome that replicates through a DNA intermediate.
The hepatitis B virus is an example of a virus from which Baltimore Group?
Answer: Group VII (dsDNA-RT)
Explanation: The hepatitis B virus is an example of a Baltimore Group VII virus, which consists of double-stranded DNA reverse transcribing (dsDNA-RT) viruses.
The *incertae sedis* class recognized by the ICTV as of 2025 is *Naldaviricetes*.
Answer: True
Explanation: As of 2025, the *incertae sedis* class recognized by the ICTV is indeed *Naldaviricetes*.
Structure-based virus classification suggests a shared evolutionary history among viruses with similar virion assembly and structure, even across different host domains.
Answer: True
Explanation: Structure-based virus classification is founded on the principle that similarities in virion assembly and structure across viral groups, even those infecting different host domains, indicate a shared evolutionary history.
The ICTV has incorporated structural relationships into its higher-level taxonomy by defining realms based on the presence of a protein belonging to a specific structural family.
Answer: True
Explanation: The ICTV has integrated structural relationships into its higher-level taxonomy by defining realms based on the presence of a protein belonging to a specific structural family, as established in the 2019 release.
The *Riboviria* realm includes families such as *Coronaviridae*, *Picornaviridae*, and *Retroviridae*.
Answer: True
Explanation: The *Riboviria* realm indeed classifies numerous families, including *Coronaviridae*, *Picornaviridae*, and *Retroviridae*.
The *Monodnaviria* realm includes families like *Polyomaviridae*, *Papillomaviridae*, and *Parvoviridae*.
Answer: True
Explanation: The *Monodnaviria* realm includes families such as *Polyomaviridae*, *Papillomaviridae*, and *Parvoviridae*, among others.
What is the underlying principle of structure-based virus classification?
Answer: Similarities in virion assembly and structure across viral groups suggest a shared evolutionary history.
Explanation: Structure-based virus classification is based on the observation that similarities in virion assembly and structure across viral groups suggest a shared evolutionary history.
How has the ICTV integrated structural relationships into its higher-level taxonomy?
Answer: By defining realms based on the presence of a protein belonging to a specific structural family.
Explanation: The ICTV has integrated structural relationships into its higher-level taxonomy by defining realms based on the presence of a protein belonging to a specific structural family.
Which of the following families is classified under the *Adnaviria* realm?
Answer: *Lipothrixviridae*
Explanation: The *Adnaviria* realm includes families such as *Lipothrixviridae*.
Which realm includes families like *Alloherpesviridae* and *Caudovirales*?
Answer: *Duplodnaviria*
Explanation: The *Duplodnaviria* realm includes families such as *Alloherpesviridae* and numerous families under *Caudovirales*.
The *Varidnaviria* realm contains which of the following viral families?
Answer: *Poxviridae*
Explanation: The *Varidnaviria* realm includes families such as *Poxviridae*.
Which of the following is an *incertae sedis* family recognized by the ICTV as of 2025?
Answer: *Avsunviroidae*
Explanation: *Avsunviroidae* is listed as one of the *incertae sedis* families recognized by the ICTV as of 2025.