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Questions and Answers
Viruses are the largest infectious agents, ranging from 20 – 300 nm in diameter.
Viruses are the largest infectious agents, ranging from 20 – 300 nm in diameter.
False
Viruses can replicate outside of living cells.
Viruses can replicate outside of living cells.
False
The entire infectious unit of a virus is called a virion.
The entire infectious unit of a virus is called a virion.
True
The suffix '-viridae' indicates a virus family, while '-virus' indicates a genus.
The suffix '-viridae' indicates a virus family, while '-virus' indicates a genus.
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Viruses can infect only certain types of living organisms based on their host range.
Viruses can infect only certain types of living organisms based on their host range.
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Viruses contain both DNA and RNA as their genome.
Viruses contain both DNA and RNA as their genome.
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Chemotherapy is irrelevant for the treatment of viral infections.
Chemotherapy is irrelevant for the treatment of viral infections.
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Viral genomes do not provide any programming information for the host cell.
Viral genomes do not provide any programming information for the host cell.
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A capsid is a protein shell that encloses the viral genome.
A capsid is a protein shell that encloses the viral genome.
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Nucleocapsid refers to the viral capsid only, excluding any nucleic acid.
Nucleocapsid refers to the viral capsid only, excluding any nucleic acid.
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Structural proteins of viruses help in determining the antigenic characteristics of the virus.
Structural proteins of viruses help in determining the antigenic characteristics of the virus.
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Capsomeres are the largest components of the viral capsid.
Capsomeres are the largest components of the viral capsid.
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An envelope is present in all virus particles.
An envelope is present in all virus particles.
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Virus-encoded glycoproteins on the envelope are crucial for attaching the virus to target cells.
Virus-encoded glycoproteins on the envelope are crucial for attaching the virus to target cells.
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A virion refers to a non-infectious virus particle.
A virion refers to a non-infectious virus particle.
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The primary function of the capsid is to facilitate the transfer of the virus’s RNA from one host cell to another.
The primary function of the capsid is to facilitate the transfer of the virus’s RNA from one host cell to another.
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Non-enveloped viruses are also known as 'naked viruses'.
Non-enveloped viruses are also known as 'naked viruses'.
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Icosahedral symmetry is the arrangement typical of rod-shaped viruses.
Icosahedral symmetry is the arrangement typical of rod-shaped viruses.
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Poxviruses have a complex shape that resembles a brick.
Poxviruses have a complex shape that resembles a brick.
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All viruses contain either DNA or RNA but cannot use both in their replication cycle.
All viruses contain either DNA or RNA but cannot use both in their replication cycle.
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Positive-sense RNA genomes can function directly as mRNA within an infected cell.
Positive-sense RNA genomes can function directly as mRNA within an infected cell.
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Orthomyxoviruses and rhabdoviruses have infectious RNA genomes.
Orthomyxoviruses and rhabdoviruses have infectious RNA genomes.
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The Baltimore classification scheme categorizes viruses based on their replication method and mRNA relationship.
The Baltimore classification scheme categorizes viruses based on their replication method and mRNA relationship.
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Viruses can replicate independently without the assistance of living cells.
Viruses can replicate independently without the assistance of living cells.
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Viral replication initially involves the attachment of the virion to the host cell.
Viral replication initially involves the attachment of the virion to the host cell.
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During viral uncoating, the entire virion is expelled from the host cell.
During viral uncoating, the entire virion is expelled from the host cell.
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The replication of viral nucleic acids and proteins occurs before the assembly of capsids.
The replication of viral nucleic acids and proteins occurs before the assembly of capsids.
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Viruses are capable of synthesizing their own viral proteins without host cells.
Viruses are capable of synthesizing their own viral proteins without host cells.
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The Baltimore classification scheme consists of eight classes of viruses.
The Baltimore classification scheme consists of eight classes of viruses.
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The duration of the viral replication cycle for most bacterial viruses ranges from 20 to 60 hours.
The duration of the viral replication cycle for most bacterial viruses ranges from 20 to 60 hours.
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Uncoating occurs shortly after the penetration of the virus into the host cell.
Uncoating occurs shortly after the penetration of the virus into the host cell.
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Positive-sense single-stranded RNA viruses require a viral-encoded RNA-dependent RNA polymerase for replication.
Positive-sense single-stranded RNA viruses require a viral-encoded RNA-dependent RNA polymerase for replication.
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Enveloped viruses are considered infectious immediately upon release from the host cell.
Enveloped viruses are considered infectious immediately upon release from the host cell.
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The hepatitis B virus has a circular single-stranded RNA genome.
The hepatitis B virus has a circular single-stranded RNA genome.
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Cytopathic effects can be monitored in animal viruses when using cell cultures.
Cytopathic effects can be monitored in animal viruses when using cell cultures.
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Influenza viruses have a linear, segmented negative-sense RNA genome.
Influenza viruses have a linear, segmented negative-sense RNA genome.
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Rotavirus is classified as a double-stranded RNA virus and is a major cause of diarrheal illnesses.
Rotavirus is classified as a double-stranded RNA virus and is a major cause of diarrheal illnesses.
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SARS-CoV-2 uses the CD4 receptor for entry into cells.
SARS-CoV-2 uses the CD4 receptor for entry into cells.
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Papillomaviruses can lead to the development of various warts and are associated with cervical cancers.
Papillomaviruses can lead to the development of various warts and are associated with cervical cancers.
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Bacteriophages are typically cultivated in mammalian cell cultures.
Bacteriophages are typically cultivated in mammalian cell cultures.
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Ebola and Marburg viruses are classified as RNA viruses and can cause severe hemorrhagic fever.
Ebola and Marburg viruses are classified as RNA viruses and can cause severe hemorrhagic fever.
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The genome of poxviruses is linear, single-stranded RNA.
The genome of poxviruses is linear, single-stranded RNA.
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Hepatitis A virus is transmitted through direct blood exposure.
Hepatitis A virus is transmitted through direct blood exposure.
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Study Notes
Introduction to Medical Virology
- This is a course on Medical Virology, part of a larger Microbiology and Parasitology course.
- The academic session is 2024/2025.
- The presenter is Prof Dr Yeo Chew Chieng, from the Faculty of Medicine, UN SZA.
Learning Outcomes
- Describe the structure, characteristics, and classification of viruses.
- Describe the steps involved in viral replication.
- List common RNA and DNA viruses of clinical significance.
- Name common diseases caused by medically important viruses.
- Describe the chemotherapy of viral infections.
General Properties of Viruses
- Viruses are the smallest infectious agents, ranging from 20-300nm in diameter.
- They contain only one type of nucleic acid (either DNA or RNA) as their genome, encased in a protein shell (capsid).
- The entire infectious unit is a virion.
General Properties of Viruses (Examples)
- Adenovirus: 90nm
- Bacteriophage T4: 225nm
- Rabies virus: 170 x 70nm
- Rhinovirus: 30nm
- Bacteriophage M13: 800x10nm
- Chlamydia elementary body: 300nm
- Bacteriophages f2, MS2: 24nm
- Tobacco mosaic virus: 250 x 18nm
- Poliovirus: 30nm
- Prion: 200x20nm
- Vaccinia virus: 300x200x100nm
- E. coli (bacterium): 3000 x 1000nm
- Viroid: 300x10nm
- Ebola virus: 970nm
- Human red blood cell: 10,000nm in diameter.
- Plasma membrane: 10nm thick
General Properties of Viruses (Additional)
- Viruses are inert in extracellular environments; they replicate only in living cells.
- Viral genomes contain information needed to program host cells to synthesize virus-specific macromolecules.
- Viruses infect almost all living organisms.
- The host range for a given virus can be broad or extremely limited.
Taxonomy of Viruses
- Viruses are grouped into families based on morphology, genome structure, and replication strategies.
- Virus family names end in "-viridae".
- Within families, genera are categorized based on biological, genomic, physicochemical, or serological differences.
- Genus names end in "-virus".
Structure of Viruses
- Capsid: The protein shell or coat enclosing the nucleic acid genome.
- Nucleocapsid: The protein-nucleic acid complex, representing the packaged form of the viral genome.
Structure of Viruses (Additional)
- Structural proteins make up the viral capsid and have multiple functions: facilitate viral genome transfer between cells, protect the genome from host nucleases, and participate in attachment to susceptible cells.
- Structural proteins determine the virus's antigenic characteristics.
- Capsomeres: Smallest structural components visible in electron microscope images on icosahedral virus particles.
- Capsid is formed by assembly of capsomeres, arranged in a specific pattern, to give the distinctive shape.
Structure of Viruses (Additional)
- Envelope: A lipid-containing membrane surrounding some but not all virus particles.
- Viruses acquire envelopes during maturation, budding through host cell membranes.
- Virus-encoded glycoproteins are exposed on the surface of the envelope, enabling the virus to attach to target cells via receptors.
- Glycoproteins are important viral antigens, involved in interactions with neutralizing antibodies.
Enveloped Virus Budding
- Viral glycoproteins are involved in attachment.
- Viral capsid and portion of host cell membrane fuse together.
- Budding enveloped virus forms.
Structure of Viruses (additional)
- Virion: Complete infectious virus particle.
- Some viruses are just nucleocapsid (non-enveloped) and others include a nucleocapsid and envelope (enveloped).
Structure of Viruses (additional)
- Nucleocapsids are often constructed in highly symmetrical ways (symmetry refers to how capsomeres are arranged in the virus capsid).
- Two kinds of symmetry are recognized in viruses: helical and icosahedral.
- Helical symmetry is seen in rod-shaped viruses, usually arranged in an icosahedral shape.
Structure of Viruses (Additional)
- Some virus particles do not exhibit simple icosahedral or helical symmetries but are more complex in structure, like poxviruses.
Viral Genomes
- Viruses have either DNA or RNA genomes (single- or double-stranded, circular or linear, segmented or non-segmented).
- Some viruses use both DNA and RNA during their replication cycle.
- Classes of viruses are classified by the genome in the virion (e.g. ssRNA, dsDNA,ssDNA).
RNA Viral Genomes
- Positive sense RNA genomes (e.g., picornaviruses, togaviruses) can function as mRNA directly after infection.
- Negative-sense RNA genomes (e.g., orthomyxoviruses, rhabdoviruses) need to be transcribed into a complementary mRNA before translation.
The Baltimore Viral Classification Scheme
- Baltimore classification categorizes viruses based on their genome type, the relationship of the viral genome to its mRNA, and mode of replication.
- There are seven classes of viruses.
The Baltimore Viral Classification Scheme (Additional)
- Categorization includes classes with dsDNA, ssDNA, dsRNA, ssRNA(+), ssRNA(-), dsDNA(RT), and retroviruses (ssRNA->dsDNA).
- Specific virus family examples and common diseases are listed in a table.
Replication of Viruses
- Viruses multiply in living cells.
- Host cells provide energy and machinery for viral protein and nucleic acid synthesis.
- Viral genomes contain the necessary information for viral-specific macromolecule synthesis
Replication of Viruses (Additional)
- Viral replication generally involves these steps: attachment, penetration, uncoating, replication, assembly, and release.
Replication of Viruses (Additional Details)
- The viral replication cycle's duration varies, with some taking 20-60 minutes for bacterial viruses, and 8-40 hours for animal viruses.
Replication of Viruses: Attachment
- First step in viral infection.
- Interaction of the virion with specific receptors on the host cell surface.
- Receptors can be surface proteins or oligosaccharides.
Replication of Viruses: Penetration
- Virus particle is taken inside the host cell.
- Penetration can occur by fusion with the plasma membrane or receptor-mediated endocytosis.
Replication of Viruses: Uncoating
- Physical separation of the viral genome from other structural components
- Important for infectivity; viral genome is uncoated before replication.
Replication of Viruses: Synthesis and Assembly
- Non-enveloped double-stranded DNA viruses: viral DNA enters the nucleus for uncoating and replication, mRNA is transcribed, mRNA is translated in cytoplasm and proteins enter the nucleus, DNA and structural proteins assemble in nucleus.
Replication of Viruses: Synthesis and Assembly (Additional)
- Positive-sense single-stranded RNA viruses: viral RNA is uncoated, RNA is directly translated in the cytoplasm into viral proteins, a negative-sense RNA strand is synthesized from the positive-sense RNA, then used to produce more positive-sense RNA copies.
Replication of Viruses: Synthesis and Assembly (Additional)
- RNA viruses: all require viral RNA-dependent RNA polymerases, as host polymerases don't catalyze RNA synthesis from an RNA template. Specific details on dsRNA and ssRNA(-) viruses are present.
Replication of Viruses: Retroviruses
- retroviruses require reverse transcriptase. This enzyme synthesizes DNA from the ssRNA (+) genome, resulting in a dsDNA "provirus" that integrates into host DNA.
Replication of Viruses: Release
- Non-enveloped viruses: the host cell lyses to release viruses.
- Enveloped viruses: bud out of the host cell membrane, acquiring an envelope containing viral glycoproteins, before release.
Cultivation of Viruses
- Bacteriophages are cultivated in bacterial cultures.
- Animal viruses can be cultivated in living animals, chicken embryos (eggs), or cell cultures.
- Cell cultures are a common method for cultivating viruses today; plaques may be observed.
Cultivation of Viruses (Additional)
- When using cell cultures for animal viruses: observe cytopathic effects, such as cell lysis or necrosis, inclusion formation, and giant cell formation
Viruses of Clinical Importance
- The presentation outlines DNA and RNA viruses.
DNA Viruses (Specific examples)
- Adenoviridae (Adenoviruses) – Non-enveloped; icosahedral; linear dsDNA, causing respiratory diseases, conjunctivitis, and gastroenteritis.
- Papilomaviridae (Papillomaviruses) – Non-enveloped; icosahedral; circular dsDNA, causing genital and other cancers, warts.
- Herpesviridae (Herpesviruses) – Enveloped; icosahedral; linear dsDNA, causing herpes, chickenpox, mononucleosis, and Kaposi's sarcoma.
- Poxviridae (Poxviruses) – Enveloped; brick-shaped; linear dsDNA, causing smallpox (now eradicated), vaccinia, and other skin lesions.
DNA Viruses: Hepadnaviridae
- Enveloped; Pleomorphic; circular dsDNA with 3.2 kbp size, causing hepatitis B virus (HBV), acute and chronic hepatitis, and liver cancer.
RNA Viruses (Specific examples)
- Reoviridae (Reoviruses) – Non-enveloped; icosahedral symmetry; linear dsRNA; causing diarrheal illness in humans and animals.
RNA Viruses: Coronaviridae
- Enveloped; helical; single-stranded positive-sense RNA; 27–32 kb in size; causing common colds, severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and Coronavirus Disease 2019 (COVID-19).
RNA Viruses: Picornaviridae
- Non-enveloped; icosahedral symmetry; single-stranded (+) RNA; causing polio, coxsackievirus infections, and other enterovirus infections.
RNA Viruses: Flaviviridae
- Enveloped; icosahedral; single-stranded (+) RNA; causing dengue fever, yellow fever, West Nile virus, Zika fever, and Hepatitis C.
RNA Viruses: Orthomyxoviridae
- Enveloped; usually spherical-shaped, helical symmetry; segmented RNA (influenza); causing influenza.
- Genetic reassortment occurs in co-infection.
RNA Viruses: Paramyxoviridae
- Enveloped; pleomorphic; non-segmented, negative-sense RNA; causing measles, mumps, parainfluenza, and respiratory syncytial virus.
RNA Viruses: Filoviridae
- Enveloped; pleomorphic; long thread-like; non-segmented, negative sense RNA; causing Marburg and Ebola hemorrhagic fevers.
RNA Viruses: Retroviridae
- Enveloped; spherical; two copies of (+) single-stranded RNA; causing leukemia, sarcoma, and HIV/AIDS.
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Description
Test your knowledge on the characteristics and structures of viruses, including their replication, classification, and components. This quiz covers important concepts such as virions, capsids, and the role of viral genomes in infections. Dive in and see how much you know about these fascinating infectious agents!
