Questions and Answers
Which of the following factors are used to classify viruses? (Select all that apply)
What are the major virus groups based on genome type? (Select all that apply)
What could small DNA viruses have evolved from?
Escaped, self-replicating fragments of cellular genome DNA
What is taxonomy in the context of viruses?
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Which observations about grouped viruses are correct? (Select all that apply)
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What advantages do viruses with dsDNA have? (Select all that apply)
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All RNA viruses have a linear genome.
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What is a disadvantage of (+)strand RNA viruses?
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What do (-)strand RNA viruses package into the virion?
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What are viroids?
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What could large DNA viruses have evolved from?
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Satellite viruses can replicate without a helper virus.
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Study Notes
Virus Classification
- Viruses are classified based on genome composition (RNA or DNA, single/double-stranded), topology (circular or linear), capsid symmetry, envelope presence, genetic relatedness, mRNA expression mechanisms, and host organisms.
Major Virus Groups
- Viruses are categorized into several major groups:
- ssDNA genome
- dsDNA genome
- (+)strand RNA genome
- (-)strand RNA genome
- dsRNA genome
- Use reverse transcriptase
- Include satellite viruses, satellite nucleic acids, and viroids.
Evolutionary Origins of Viruses
- Small DNA viruses potentially evolved from self-replicating fragments of cellular DNA.
- Large DNA viruses may have originated as degenerate intracellular parasites.
Taxonomy
- Taxonomy refers to the science of classification, which organizes life forms into species, genera, and families.
Grouped Virus Observations
- ssDNA viruses are generally small with few genes.
- Most plant viruses contain a (+)strand RNA genome.
- Fungal viruses predominantly have dsRNA genomes.
- Most bacteriophages are tailed and possess dsDNA genomes.
- Large dsDNA genomes (400-1200 kb) are primarily found in free-living unicellular algae and protozoa.
dsDNA Viruses
- dsDNA viruses exhibit a wide genome size range (5-1180 kb), with both circular and linear forms.
- Advantages include resistance to degradation and a low error rate in DNA replication.
- Mimivirus is the largest known virus infecting amoebae, and 95% of bacteriophages have dsDNA.
RNA Viruses
- All known RNA viruses contain linear genomes and are prone to degradation by nucleases.
(+)strand RNA Viruses
- Examples include togaviruses and flaviviruses, which infect vertebrates and invertebrates.
- They package RNA polymerase within their virion.
(-)strand RNA Viruses
- Characterized by helical nucleocapsids, many possess fragmented genomes.
- Responsible for severe diseases like measles and influenza.
- All (-)strand RNA viruses that infect vertebrates have envelopes; they face challenges with fragmented genomes.
dsRNA Viruses
- Typically contain multiple genome segments, each coding for single mRNAs and viral proteins.
- Have icosahedral capsids and are not overly dependent on host cellular environments, aiding adaptive evolution.
- Cystovirus infects bacteria.
Viruses with Reverse Transcriptase
- Can have RNA or DNA genomes at different replication stages; retroviruses carry an RNA genome.
- Packaged reverse transcriptase is unique to vertebrates and plants.
- Having two identical copies of the viral genome serves as a safeguard.
Satellite Viruses
- Require a helper virus for replication, encoding their own capsid proteins.
- Can only replicate in cells already infected with a helper virus.
Viroids
- Small, virus-like RNAs that replicate and cause disease in various plants.
- Composed of ssRNA with circular genomes and do not code for proteins; are not encapsulated.
Evolution: RNA World
- The RNA world theory proposes that self-replicating RNAs could bind to other ribonucleotides, facilitating self-replication without enzymes.
- Viroids and RNA viruses may have originated from this RNA world.
Evolution: Transition to DNA World
- During RNA world evolution, a self-replicating RNA could have evolved into a DNA-dependent RNA polymerase, leading to DNA's dominance in encoding information for protein synthesis.
Large DNA Viruses Evolution
- Large DNA viruses likely evolved from obligatory intracellular parasites, gradually losing unnecessary genes while gaining those responsible for packaging and transmitting viral genomes, exemplified by mimivirus.
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Description
Test your knowledge on the classification of viruses through these flashcards. Learn about the key characteristics that group viruses and the major virus categories based on genome type and other features. Perfect for anyone looking to deepen their understanding of virology.
