Virology Quiz on Virus Classification

Medical Important Viruses

• Viruses are infectious agents, minimally constructed of a genome (RNA or DNA, not both) and a protein capsid to protect the genome.
• Some viruses have additional features like a lipid-containing envelope.
• A complete virus particle is called a virion.
• Viruses can only reproduce within host cells.
• A virion acts as a delivery system for the viral genome.
• A virion's payload is its viral genome, which can include enzymes needed for viral replication.
• Viral pathogenicity varies greatly within a closely related virus group and depends on structural and functional characteristics. This can lead to distinct clinical pathologies within the same family.

Virus Structure

• Viruses are significantly smaller than the cells they infect (100-1000x smaller).
• The smallest viruses are in the nanometer range (20nm in diameter), while some larger ones reach 300nm.
• Smallpox virus, a large virus, overlaps the size of bacterial cells.
• Viruses generally pass through filters designed to trap bacteria, making filterability useful for detecting viral etiology.

Virus Classification

• Viruses are divided into related groups (families).
• Subfamilies are further categorized based on factors such as the structure and type of viral nucleic acid, replication strategy, capsid symmetry (helical vs. icosahedral), and the presence of a lipid envelope.
• Within a virus family, additional properties like host range, serological response, and amino acid sequences of viral proteins help further classify into genera and species.

Viral Genome

• A virus genome can be either DNA or RNA, single-stranded, or double-stranded.
• The most common viral genomes are ssRNA and dsDNA.
• Single-stranded RNA genomes are further classified into positive and negative polarity.
• Positive-polarity RNA (mRNA-sense) can be used as a template for proteins.
• Negative-polarity RNA (asense) can't be directly used as a protein template but requires a complimentary RNA strand.

Viral Capsid Symmetry

• The protein shell (capsid) that encloses the virus genome can have one of two geometric configurations.
• Spherical (icosahedral) or rod-shaped (helical)
• The capsid is built of multiple copies of one or a few different polypeptide types, which requires minimal genetic information.

Helical Symmetry

• A helical symmetry structure is common in some virus families.
• They consist of repeated polypeptide subunits (protomers); these self-assemble into a helical cylinder.
• The protomer is hydrogen-bonded to neighboring protomers forming a nucleocapsid.

Icosahedral Symmetry

• In icosahedral structures, the capsid is made of multiple copies of one or a few different polypeptide types.
• These form capsomers, clustered into an icosahedron (a 20-sided geometrical figure with 12 vertices).

Viral Envelope

• A virus's envelope is an important structural characterization feature determined by the presence or absence of a lipid-containing membrane that surrounds the nucleocapsid.
• Enveloped viruses have a flexible and coiled nucleocapsid inside their envelope, appearing roughly spherical.
• The envelope is derived from host cell membranes.
• Virus-specific proteins replace cellular proteins, resulting in virus-specific antigenicity.

Viral Replication Cycle

• Viral replication is a series of steps, starting from virus attachment to host cells to releasing viral progeny.
• The steps include attachment, penetration, uncoating, gene expression and replication, assembly, and release.

Baltimore Classification

• There are 7 groups of medically important viruses according to the Baltimore Classification system.
• These are based on whether the genome is DNA or RNA and whether it is single-stranded or double-stranded, linear or circular.

DNA Viruses

• There are three naked (non-enveloped), icosahedral families, which include parvoviruses, papovaviruses, and adenoviruses. A fourth enveloped, icosahedral group is the hepadnaviruses, family that includes hepatitis B virus.
• The poxviruses are enveloped viruses with a complex internal symmetry, and they are the largest of the DNA viruses.

RNA Viruses

• The three naked icosahedral virus families are presented first followed by the three enveloped icosahedral viruses.
• All but three of the remaining families are enveloped helical viruses; the first five have single-stranded linear RNA, and the last three have single-stranded circular RNA.

Specific Virus Families (examples)

• Picornaviruses: Smallest RNA viruses with single-stranded, linear, positive-polarity RNA within an icosahedral capsid. Common human pathogens are poliovirus, rhinovirus, and coxsackievirus.
• Caliciviruses: Naked RNA viruses (38 nm) with single-stranded, linear positive-polarity RNA in an icosahedral capsid. Common culprits for foodborne outbreaks, like Norwalk virus.
• Reoviruses: Naked viruses with double-stranded RNA, linear segmented genetic material in double icosahedral capsids. Rotavirus is a major cause of diarrhea in infants.
• Flaviviruses: Enveloped, icosahedral, single-stranded, linear, positive-polarity RNA. Common viruses include hepatitis C, yellow fever, dengue, West Nile, and Japanese encephalitis viruses.
• Togaviruses: Enveloped viruses with single-stranded, linear, positive-polarity RNA in an icosahedral capsid. Alphaviruses and rubiviruses are major human pathogens (e.g., eastern and western equine encephalitis, rubella virus).
• Retroviruses: Enveloped viruses contain single-stranded linear, positive, nonsegmented RNA. This class includes human immunodeficiency virus (HIV) and leukemia viruses.
• Orthomyxoviruses: Enveloped, helical, single-stranded RNA genome with nonsegmented, negative-polarity RNA. Influenza viruses.
• Paramyxoviruses: Enveloped viruses with a helical nucleocapsid and nonsegmented negative RNA. Includes measles, mumps, and respiratory syncytial viruses.
• Rhabdoviruses: Bullet-shaped, enveloped viruses with a helical capsid containing nonsegmented, negative-polarity RNA. Rabies is an important human pathogen.
• Filoviruses: Enveloped viruses, highly pleomorphic, filamentous morphology. Contains single-stranded, negative, nonsegmented RNA. Ebola and Marburg are notable human pathogens.
• Coronaviruses: Enveloped, helical, nonsegmented, positive-polarity RNA. Common cold, SARS, MERS, COVID-19 are significant human diseases caused by coronaviruses.

Viral Diseases (examples)

• SARS, MERS, COVID-19 are noteworthy disease caused by coronaviruses.
• Measles, mumps, parainfluenza, and respiratory syncytial viruses are major human pathogens caused by paramyxoviruses.
• Rabies is a deadly human illness caused by the rhabdovirus.

COVID-19-Specific Information

• COVID-19, caused by SARS-CoV-2, is a severe acute respiratory syndrome coronavirus (SARS-CoV-2)
• SARS-CoV-2 was identified in Wuhan, China, and characterized as a pandemic.
• Incubation period: 1-14 days.

Prevention and Diagnosis

• Handwashing, avoiding sick people, wearing masks, and avoiding crowded places are helpful preventative measures.
• Molecular tests like PCR and antibody tests are used to detect active infections and prior exposure to the virus.

References

• Various publications (see slide 57)
• Websites (see slides 45,49, and 53)

Test your knowledge on various virus classifications and characteristics with this engaging quiz. Questions cover a range of virus types, including hepeviruses, reoviruses, flaviviruses, and retroviruses. Perfect for students and enthusiasts of virology seeking to deepen their understanding of viral biology.