Virology Basics Quiz

Medical Important Viruses

• Viruses are infectious agents
• Composed of either RNA or DNA, but not both
• Also includes a protein-containing structure (capsid) to protect the genome
• Some viruses have additional features like an envelope with a lipid bilayer
• A complete virus particle is called a virion
• Viruses need host cells to reproduce

Virion

• A virion is a delivery system for the viral genome
• It protects the genome and enables the virus to bind to host cells
• May contain enzymes for initial steps in viral replication
• The pathogenicity of a virus depends on structural and functional characteristics
• Different species of viruses in a group can have varying clinical pathologies

Virus Structure

• Viruses are much smaller than cells; 100-1000 times smaller
• Smallest viruses are 20nm, and largest are about 300nm; overlapping with bacterial cells
• Pass through filters used for bacteria, supporting viral etiology

Virus Classification

• Viruses are categorized into families, subfamilies, genera, and species
• Based on viral nucleic acid type and structure
• Strategy of viral replication
• Symmetry of the viral capsid (structure of the protein shell)
• Presence or absence of a lipid envelope

Viral Genome

• Genomes can be DNA or RNA, either single or double stranded
• Common forms in nature are single-stranded RNA (ssRNA) and double-stranded DNA (dsDNA)
• Single-stranded RNA (ssRNA) genomes are subdivided into positive and negative polarity
• Positive polarity is used as a template for protein synthesis; negative polarity requires conversion

Capsid Symmetry

• The protein shell enclosing the genome has two geometric configurations
• Helical symmetry; rod or coiled shape
• Icosahedral symmetry; spherical or symmetric

Envelope

• An important structural feature used to categorize viruses
• The envelope is a lipid-containing membrane surrounding the nucleocapsid
• Viruses without envelopes are called naked viruses
• The envelope is derived from the host cell membrane
• Replaced by virus-specific proteins, imparting virus-specific antigenicity

Viral Replication

• Virus replication steps begin with attachment, penetration and uncoating of viral genome
• Gene expression and replication are followed by assembly and release of progeny viruses
• Not all the stages are the same across all types; some can change

Baltimore Classification

• This classifies medically important viruses into 7 groups
• Based on DNA or RNA, double or single stranded
• Positive (+) polarity or negative (-) polarity
• Other features like using reverse transcriptase also included

DNA Viruses

• Three naked, icosahedral families presented in order of increasing particle size
• Hepadnaviruses and herpesviruses are enveloped icosahedral viruses
• Poxviruses have a complex internal structure These families have several virus types (Parvoviruses, Papovaviruses, Adenoviruses)

Papovaviruses

• Naked icosahedral viruses with double-stranded circular DNA
• The name is an acronym of papilloma, polyoma, and SV40 viruses

Adenoviruses

• Naked icosahedral virus with double-stranded linear DNA
• Cause mild diseases like nose, mouth, pharynx infections

Hepadnaviruses

• Double-shelled viruses with icosahedral capsids enclosed by an envelope
• DNA is a double-stranded circle, unusual because complete strand is missing-
• Hepatitis B virus is the key pathogen in this family

Herpesviruses

• Enveloped viruses with icosahedral nucleocapsids, double-stranded linear DNA
• Cause latent infections
• Examples are Herpes simplex virus, Varicella-zoster virus, Cytomegalovirus, Epstein-Barr virus

Poxviruses

• Largest viruses with brick-like structures
• Envelope with unusual appearance, containing complex capsid symmetry
• Important members include smallpox and vaccinia viruses

RNA Viruses

• Three naked icosahedral families listed first, followed by enveloped icosahedral viruses
• Remaining families are enveloped helical viruses
• First five have single-stranded linear RNA, last three have single-stranded circular RNA

Picornaviruses

• Smallest RNA viruses (28nm in diameter)
• Single-stranded, linear, nonsegmented, positive-polarity RNA
• Naked icosahedral capsid
• Pathogens include poliovirus, coxsackievirus, echovirus, hepatitis A virus, and rhinoviruses
• Transmitted with contaminated food and water

Caliciviruses

• Naked viruses (38 nm diameter) with icosahedral capsids
• Single-stranded, linear, nonsegmented, positive-polarity RNA
• Common cold, and hepatitis E virus; contaminated water/food

Reoviruses

• Naked viruses (75nm in diameter); two icosahedral capsid coats
• Double-stranded linear RNA
• Respiratory enteric orphan viruses
• The main human pathogen is rotavirus, leading to diarrhea in infants

Flaviviruses

• Enveloped viruses, icosahedral capsid, single-stranded linear RNA
• Positive-polarity RNA
• Pathogens like hepatitis C, yellow fever, dengue, West Nile virus, St. Louis and Japanese encephalitis

Togaviruses

• Enveloped viruses, icosahedral capsid, single-stranded linear RNA
• Positive-polarity RNA
• Examples are alphaviruses and rubiviruses (Eastern and Western encephalitis viruses, rubella)

Retroviruses

• Enveloped viruses, icosahedral nucleocapsid, single-stranded linear RNA
• Positive-polarity RNA (but reverse transcriptase)
• Oncoviruses (sarcoma and leukemia viruses) and lentiviruses ("slow viruses") important pathogens

Orthomyxoviruses

• Enveloped viruses with helical nucleocapsids
• Eight segments of linear, single-stranded, negative-polarity RNA
• Major pathogen is influenza virus

Paramyxoviruses

• Enveloped viruses with helical nucleocapsids; single-stranded linear RNA
• Negative-polarity RNA
• Measles, mumps, parainfluenza, and respiratory syncytial viruses

Rhabdoviruses

• Enveloped viruses in bullet shape with helical nucleocapsids; single-stranded linear RNA
• Negative-polarity RNA
• The most important human pathogen is rabies virus

Filoviruses

• Enveloped viruses with helical nucleocapsids; single-stranded linear RNA
• Negative-polarity RNA
• Pathogens are Ebola and Marburg virus

Coronaviruses

• Enveloped viruses with helical nucleocapsids and single-stranded, linear, nonsegmented positive-polarity RNA
• The spikes around the surface give the appearance of a 'corona'
• Cause common cold, SARS, MERS and COVID-19.

Diseases

• Coronaviruses are a significant cause of common cold, often trailing in frequency only to rhinoviruses
• 2002: Severe acute respiratory syndrome (SARS) emerged
• 2012: Middle East respiratory syndrome (MERS) emerged
• 2020: SARS-CoV-2 and COVID-19 emerged

Prevention

• Washing hands frequently with water and soap or hand sanitizer
• Avoid contact with sick people
• Avoid touching eyes, nose, and mouth with unwashed hands
• Wear a mask in public situations
• Avoid crowded places

Molecular Tests

• Nucleic acid detection utilizing swabs or blood samples for diagnosis
• Antibody tests to detect immune response (serology) following exposure

COVID-19 Vaccines

• Multiple types of vaccines (mRNA, inactivated virus, viral vector, protein-based)
• Wide accessibility due to significant global research and collaboration

COVID-19 Variants

• Genetic variations arising in the virus over time

Summary of Viral Classifications and characteristics

• Viruses are classified based on their genome type (DNA or RNA), strandedness (single or double), the presence or absence of an envelope, and the symmetry of their capsid

References

• Various books on microbiology (e.g., from Lippincourt, Jawetz Melnick, and Adelbergs) and others

Test your knowledge on the fundamental concepts of virology including virus structure, classification, and key features that distinguish different virus families. This quiz covers essential topics that are crucial for understanding viruses and their interactions with host cells.