Virology Quiz: Virus Classification and Characteristics

Medical Important Viruses

• Viruses are infectious agents with a minimal structure
• They consist of a genome (RNA or DNA, not both), and a protein-containing structure (capsid) that protects the genome
• Some viruses have additional features, such as a lipid-containing membrane (envelope)
• A complete virus particle is called a virion
• Viruses reproduce inside host cells
• Viruses can be in a variety of shapes and sizes, and the shape and size features are used to distinguish the types
• Viruses are classified into families, genera, and species based on characteristics like the type and structure of the viral nucleic acid, replication strategies, type of capsid symmetry (helical or icosahedral), and the presence/absence of a lipid envelope
• Viruses are much smaller than cells
• The size of viruses ranges from 20 nm in diameter for the smallest viruses (parvoviruses) to 300 nm for the largest (poxviruses)
• A complete virus particle is called a virion
• Viruses require host cells for replication
• Viral pathogenicity depends on many structural and functional characteristics
• Different viruses from one species can have very different clinical pathologies

Virus Structure

• Viruses are significantly smaller than cells
• Viruses are 100-1000 fold smaller than the cells they infect
• Virus sizes are similar to the size of bacteria

Viral Replication

• Virus replication involves steps like attachment, penetration, uncoating, gene expression, replication, assembly, and release of viral progeny

Icosahedral Symmetry

• Icosahedral symmetry is described as a complex structure that has numerous faces and thus is spherical or symmetrical
• Viral capsids are formed from subunits called capsomers, which are arranged in an icosahedral pattern
• Viral genomes found within this structure

Helical Symmetry

• Helical symmetry refers to a rod or coil shape (symmetric)
• The capsid is constructed of a single polypeptide that forms a helical structure around the nucleic acid to form the shape
• The complex of protomers and nucleic acid is called the nucleocapsid

Enveloped Viruses

• Enveloped viruses have a lipid-containing outer layer (envelope) surrounding their nucleocapsid
• Enveloped viruses are derived from host cell membranes and replaced by virus-specific proteins
• Envelope proteins confer virus-specific antigenicity
• Enveloped viruses are more sensitive to environmental conditions compared to naked viruses
• The size and shape are characteristic of enveloped shapes
• Naked viruses do not have these membranes; thus, they are not as sensitive to environmental changes

Baltimore Classification System

• This is used to classify viruses based on their genome structure and the form of mRNA they make
• The system classifies viruses into 7 groups; I (dsDNA), II (ssDNA), III (dsRNA), IV (+ssRNA), V (-ssRNA), VI (+ssRNA-RT), & VII (dsDNA-RT)

DNA Viruses

• These viruses have DNA genomes
• These are naked viruses with an icosahedral capsid; thus, they are non-enveloped
• The size increases in order-parvoviruses, papovaviruses, and adenoviruses
• There are enveloped viral families such as hepadnaviruses and herpesviruses
• The poxviruses are the largest and have complex internal symmetry

Papovaviruses

• These viruses are non-enveloped and circular DNA
• These viruses include papilloma, polyoma, and simian vacuolating viruses
• Usually cause cancer

Adenoviruses

• These viruses are non-enveloped
• Icosahedral symmetrical viruses with a double-stranded linear DNA
• Mostly cause mild diseases, including nose, mouth, and throat infections
• There are more than 40 subtypes

Hepadnaviruses

• Enveloped viruses with an icosahedral capsid
• Have a partially double-stranded DNA genome
• Characterized by reverse transcription
• Hepatitis B is a human pathogen from this family

Herpesviruses

• These viruses are enveloped with a double-stranded linear DNA
• Usually noted for causing latent infections (indefinite period of virus replication)
• Hepatitis B causes diseases like chicken pox and infectious mononucleosis, among others
• There are 5 important human pathogens—herpes simplex viruses (HSV types 1 & 2), varicella-zoster virus, cytomegalovirus, Epstein-Barr virus

Poxviruses

• These are the largest viruses and have a complex capsid
• Possess a bricklike shape, and their envelopes have unique appearances
• Most are characterized by skin lesions or "pocks"
• Smallpox virus and vaccinia virus are important members

RNA Viruses

• These viruses have an RNA genome and are classified into 8 families depending on their structures, presence or absence of an envelope, and type of polarity
• Coronaviruses, retroviruses, flaviviruses, reoviruses, togaviruses, Orthomyxoviruses, bunyaviruses, and paramyxoviruses are enveloped RNA viruses with single-stranded RNA with (+) or (-) polarity
• Naked RNA viruses include picornaviruses, caliciviruses, and reoviruses

Picornaviruses

• These are the smallest RNA viruses with (+) polarity
• These are non-enveloped with an icosahedral capsid
• Human pathogens like poliovirus, coxsackievirus, rhinovirus, and hepatitis A virus are included
• The size varies

Caliciviruses

• Naked RNA viruses with an icosahedral capsid and (+) polarity
• They cause diarrhea
• Human pathogens are noroviruses and sapoviruses

Reoviruses

• Naked RNA viruses with 2 icosahedral capsids; thus, they are non-enveloped
• Double-stranded RNA; thus, they have 10 segments
• Rotavirus is the main human pathogen and causes diarrhea mostly in infants

Flaviviruses

• Enveloped RNA viruses with (+) polarity and an icosahedral capsid
• Include dengue virus, yellow fever virus, West Nile virus, and Japanese encephalitis virus

Togaviruses

• Enveloped RNA viruses with (+) polarity and an icosahedral capsid
• Include alphaviruses (Eastern and Western encephalitis viruses) and rubella virus

Retroviruses

• Enveloped RNA viruses with (+) polarity and an icosahedral capsid
• Include human immunodeficiency virus (HIV) and human T-cell leukemia/lymphoma virus (HTLV)

Paramyxoviruses

• Enveloped RNA viruses with (-) polarity and a helical nucleocapsid
• Include measles, mumps, parainfluenza viruses, and respiratory syncytial viruses

Rhabdoviruses

• Enveloped RNA viruses with (-) polarity, helical nucleocapsid
• Include rabies virus

Filoviruses

• Enveloped RNA viruses with (-) polarity, helical nucleocapsid
• Ebola and Marburg viruses are included here

Coronaviruses

• Enveloped RNA viruses with (+) polarity, helical nucleocapsid
• Have a halo of spikes protruding from the envelope
• Include SARS-CoV, MERS-CoV, and SARS-CoV-2

Viral Diseases

• Important human viruses, including those classified as DNA viruses and RNA viruses, cause several diseases. Viral infections can affect various systems in the human body.

Molecular Tests for Virus Detection

• Nucleic acid detection tests are used to detect viral RNA or DNA (SARS-COV-2) in infected people

Antibody Tests for Virus Exposure

• These tests (serology) are used to detect an immune response against the virus—indicates previous exposure to a virus (SARS-COV-2)

COVID-19 Vaccines

• Several platforms exist, including mRNA, inactivated viral vectors, protein-based

Prevention, Symptoms, and Severity of COVID-19

• Prevention involves handwashing, avoiding contact with sick people, wearing masks, and avoiding crowded places
• Symptoms include fever, tiredness, cough, aches, nasal congestion, runny nose, sore throat, and diarrhea
• Severe symptoms include difficulty breathing and high blood pressure

SARS-CoV-2 Variants

• Different viral variants caused by mutations in the virus's genetic material have been labelled

References

• Information from the slides provides the references