Virology Quiz: Virus Characteristics and Types

Medical Important Viruses

• Viruses are infectious agents composed of a genome (RNA or DNA, but not both) and a protein capsid that protects the genome.
• Many viruses have additional structures, like a lipid envelope, further distinguishing them.

Virion

• A complete virus particle is called a virion.
• It serves as a delivery system, protecting the viral genome and enabling the virus to attach to host cells.
• The virion may include enzymes necessary for viral replication, which is intracellular.
• Pathogenicity varies within related virus groups, leading to different clinical pathologies.

Virus Structure

• Viruses are significantly smaller than the cells they infect (100-1000-fold).
• Virion sizes vary: the smallest are around 20 nm (parvoviruses) and the largest about 300 nm (poxviruses).
• Viruses pass through filters designed to trap bacteria.

Virus Classification

• Viruses are categorized into families, sometimes subfamilies, based on the type and structure of the viral nucleic acid, viral replication strategy, and symmetry of their capsids (helical or icosahedral), and the presence or absence of a lipid envelope.
• Within a virus family, variations in properties like host range, serologic reactions, and amino acid sequences of viral proteins serve as the basis for further division into genera and species.

Virus Genome

• Viral genomes can be either DNA or RNA, and either single-stranded (ss) or double-stranded (ds).
• Single-stranded RNA viruses are categorized into positive- or negative-polarity.
• Positive-polarity RNA serves directly as a template for protein synthesis, while negative-polarity RNA requires an additional step to produce a viable template.
• There are both RNA and DNA virus families.

Viral Capsid Symmetry

• The protein shell enclosing the viral genome (capsid) has either helical or icosahedral symmetry.
• The capsid's structure results from multiple copies of a single polypeptide or a few different polypeptides.

Helical Symmetry

• Capsids with helical symmetry (rod-shaped or coiled) are comprised of repeated, identical protein units (protomers) linked together.
• These protomers bind to one another via hydrogen bonds. forming a helical structure.
• This structure encloses the viral nucleic acid, creating a helical nucleocapsid.

Icosahedral Symmetry

• Capsids with icosahedral symmetry (spheres) are more complex, with multiple polypeptide types grouped into subassemblies called capsomers.
• The capsomers form a 20-faced structure (icosahedron) with a central void.
• The nucleic acid genome is found within the icosahedral structure's internal space.

Viral Envelope

• An envelope, a lipid-containing membrane, frequently surrounds the nucleocapsid (in enveloped viruses).
• The virus envelope is derived from host cell membranes, but the virus-specific proteins confer virus-specific antigenicity on the virus particle.
• Enveloped viruses have a flexible and coiled nucleocapsid within the envelope, typically appearing roughly spherical.

Viral Replication

• Viral replication follows a cycle of attachment, penetration/uncoating, gene expression/replication, assembly and finally, release of viral progeny.

Baltimore Classification of Medically Important Viruses

• Viruses are categorized into 7 groups based on their DNA or RNA structure/replication mechanisms.

DNA Viruses

• The three naked (non-enveloped) icosahedral virus families (parvoviruses, papovaviruses, and adenoviruses) are categorized by increasing size.
• The hepadnavirus family (including hepatitis B virus) and herpesviruses are enveloped icosahedral viruses.
• The poxviruses, the largest, have a complex internal symmetry.

Papovaviruses

• These are naked icosahedral viruses with double-stranded circular DNA.
• Three human papovaviruses: papilloma, polyoma, and SV40 viral infections, cause various human diseases.

Adenoviruses

• Adenoviruses are naked icosahedral viruses with double-stranded linear DNA.
• They cause mild respiratory diseases, pharyngitis, and infections in other locations (ocular illnesses).

Hepadnaviruses

• These are enveloped viruses with an icosahedral DNA core surrounded by an envelope.
• Their unusual DNA structure, mostly double-stranded but with a single-stranded region, includes hepatitis B.

Herpesviruses

• These are enveloped viruses with an icosahedral capsid and double-stranded linear DNA.
• Notable examples of human pathogens include herpes simplex virus types 1 and 2, varicella-zoster virus, cytomegalovirus, and Epstein-Barr virus

Poxviruses

• These are the largest viruses, possessing a brick-like shape, an envelope, and complex capsid symmetry.
• A prominent example is the smallpox virus.

RNA Viruses

• The three naked icosahedral families (picornaviruses, caliciviruses, and reoviruses) are listed first, followed by three enveloped icosahedral families.
• The other eight families are enveloped helical viruses, further subdivided into groups with single-stranded linear (first 5) or single-stranded circular (last 3) genomes.

Picornaviruses

• Smallest RNA viruses (28 nm).
• Naked, icosahedral, and single-stranded RNA.
• Poliovirus, coxsackievirus, echovirus, hepatitis A virus, and rhinoviruses are human pathogens.

Caliciviruses

• Naked, icosahedral RNA viruses (38 nanometers). Single-stranded, linear, non-segmented, positive-polarity RNA.
• Contagious diarrhea (often called Norwalk virus) can be caused by these viruses. Hepatitis E virus is a related pathogen.

Reoviruses

• Naked viruses with icosahedral capsid coats and double-stranded, linear RNA.
• Rotavirus, causing diarrhea, is one example.

Flaviviruses

• Enveloped viruses with an icosahedral capsid, single-stranded, linear, non-segmented, positive-polarity RNA.
• These include hepatitis C, yellow fever, dengue, West Nile, St. Louis encephalitis, and Japanese encephalitis viruses.

Togaviruses

• Enveloped viruses with icosahedral capsids and single-stranded, linear, non-segmented, positive polarity RNA.
• Alphaviruses cause encephalitis (western and eastern), and rubella (German measles) virus is also in this family.

Retroviruses

• Enveloped viruses with icosahedral capsids and two identical, single-stranded, linear, positive polarity RNA strands.
• They use reverse transcriptase to convert their RNA genome to DNA for integration into the host genome.
• Examples include human T-cell leukemia virus (HTLV) and human immunodeficiency virus (HIV).

Orthomyxoviruses

• These enveloped viruses have helical nucleocapsids and eight segments of nonsegmented, negative polarity RNA.
• Influenza viruses.

Paramyxoviruses

• Enveloped RNA viruses with helical nucleocapsids and nonsegmented, negative polarity RNA.
• Measles, mumps, parainfluenza, and respiratory syncytial viruses cause significant illnesses.

Rhabdoviruses

• These bullet-shaped viruses have envelopes and helical nucleocapsids of nonsegmented, negative polarity RNA.
• Rabies virus is the single most notable pathogen.

Filoviruses

• Enveloped viruses with helical nucleocapsids and single-stranded RNA.
• Ebola and Marburg viruses cause severe hemorrhagic fevers.

Coronaviruses

• Enveloped viruses with helical nucleocapsids and single-stranded, linear, nonsegmented, positive polarity RNA.
• Coronaviruses like SARS-CoV-2 (COVID-19), SARS-CoV, and MERS-CoV are significant.

Viral Diseases

• Coronaviruses are a significant cause of the common cold.
• Emerging viral outbreaks include Severe Acute Respiratory Syndrome (SARS) in 2002, Middle East Respiratory Syndrome (MERS) in 2012, and Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) in 2019.

COVID-19 Symptoms

• Common symptoms include fever, tiredness, dry cough, aches, pains, nasal congestion, runny nose, sore throat, and diarrhea.
• Severe symptoms include difficulty breathing and involve individuals with underlying health conditions (asthma, compromised immune systems, and pre-existing health conditions).

COVID-19 Transmission

• The virus can be transmitted through the air, via droplets or through contaminated surfaces.

COVID-19 Prevention

• Frequent hand-washing, avoidance of contact with sick individuals, use of facemasks. avoiding crowded spaces, are ways to prevent the spread of the virus.

COVID-19 Molecular and Antibody Tests

• Nucleic acid tests detect viral genetic material (RNA).
• Antibody tests detect the presence of antibodies indicating a prior infection.

COVID-19 Vaccines

• Multiple vaccines (mRNA, inactivated virus, viral vector) are currently available to prevent infection.

Virus Variants

• Virus variants (e.g., Omicron, Alpha, Beta, Gamma, Delta) emerge from mutations; some display increased transmission potential, different virulence, variations in symptoms, or a different response to therapies or vaccines.

Study Notes and References

• This is a summary of viral information from various sources.
• Specific references for further study are also available from the provided materials.

Test your knowledge on various viruses and their characteristics with this quiz. From adenoviruses to hepadnaviruses, explore the different types, structures, and replication cycles of these pathogens. Ideal for students studying virology and infectious diseases.