LECTURE 5: viruses

What is a virion?

A single, complete infectious virus particle. 

What is a capsid?

The protein coat surrounding the viral genome that provides protection and shape. 

What is a viral envelope?

A lipid membrane surrounding the capsid that is derived from the host cell membrane. 

What is host range?

The collection of species that a virus can infect.

What is tropism?

A virus's preference for a particular host tissue or cell type.

What is CPE (cytopathic effect)?

Visible damage or changes in infected cells caused by viral replication. Each virus may produce a characteristic CPE.

What is a prion?

An infectious misfolded protein that causes disease without containing DNA or RNA.

What is a TSE?

Transmissible Spongiform Encephalopathy, a group of fatal prion diseases that produce sponge-like damage in brain tissue.

Why are viruses classified as nonliving microbes?

They are acellular, cannot reproduce independently, have no metabolism of their own, and must use a host cell to make new viruses. 

What does "obligate intracellular pathogen" mean?

A pathogen that must be inside a host cell to reproduce. 

How do viruses differ from cells?

Viruses are acellular particles containing genetic material, whereas prokaryotes and eukaryotes are living cells.

What are the functions of a capsid?

Protection of the genome and maintenance of viral shape.

What are viral envelopes made of?

Lipids derived from the host cell membrane. 

What is the function of a viral envelope?

Helps viruses enter host cells and assists with release by budding.

What are viral spikes?

Viral proteins or glycoproteins that extend from the virus surface. 

What are the functions of viral spikes?

Host-cell attachment, entry, and determination of host specificity. 

Why do viruses evolve rapidly?

Fast replication, large numbers of virions produced, and frequent mutations. 

Why do RNA viruses evolve faster than DNA viruses?

RNA replication is more error-prone, resulting in more mutations.

What can beneficial viral mutations accomplish?

Escape immune detection, broaden host range, increase infectivity, and create new variants. 

What is antigenic drift?

Small, gradual mutations that accumulate over time in viral genes.

What is antigenic shift?

A major genetic change caused by reassortment of viral genome segments.

Which process usually causes pandemics?

Antigenic shift.

Which process causes seasonal influenza changes?

Antigenic drift.

How are viruses classified?

By nucleic acid type, presence or absence of an envelope, genome arrangement, and capsid shape. 

What suffix do viral families use?

"-viridae" (example: Herpesviridae). 

How are viruses commonly named?

By disease caused, host infected, tissue infected, location discovered, or viral characteristics.

What determines whether a virus can infect a host?

Presence of the correct receptor on host cells. 

What does "No receptor = No infection" mean?

If a host cell lacks the required receptor, the virus cannot attach or infect it.

Give an example of viral tropism.

HIV targets T cells; Hepatitis B targets liver cells.

What are the six steps of animal virus replication?

Attachment → Penetration → Uncoating → Replication → Assembly → Release.

What happens during attachment?

Viral proteins bind specific host cell receptors. 

How do enveloped viruses enter cells?

Membrane fusion or receptor-mediated endocytosis.

How do naked viruses enter cells?

Receptor-mediated endocytosis.

What is uncoating?

Breakdown of the capsid to release the viral genome. 

What occurs during replication?

Viral genome replication and synthesis of viral proteins. 

What occurs during assembly?

Formation of new virions.

How are enveloped viruses released?

Budding

How are naked viruses released?

Lysis or exocytosis. 

What is the major structural difference between enveloped and naked viruses?

Enveloped viruses have a lipid envelope; naked viruses do not. 

Which viruses typically exit by budding?

Enveloped viruses. 

Which viruses commonly cause host-cell lysis?

Naked viruses. 

What is an acute viral infection?

Rapid viral replication, host-cell lysis, symptoms, and eventual clearance by the immune system.

What is a chronic persistent infection?

Continuous production and release of virions over a long period without killing the host cell. 

What is a latent infection?

Viral genome remains in host cells but virions are not produced until reactivation occurs. 

What is the hallmark of latent infections?

 Dormancy followed by periodic flare-ups.

What is an oncogenic virus?

A virus capable of contributing to cancer development.

What is viral transformation?

Conversion of a normal cell into a tumor cell due to persistent viral infection. 

Name some oncogenic viruses.

HPV, Epstein-Barr virus (EBV), Hepatitis B virus, Hepatitis C virus.

What cancers are associated with HPV?

Cervical cancer and several other anogenital cancers.

What cancers are associated with Hepatitis B and C?

Liver cancer.

Why is routine microscopy not commonly used for viruses?

Most viruses require electron microscopy, which is not available in most clinical labs. 

How can animal viruses be cultured?

Using living mammalian cell cultures. 

What is a cytopathic effect (CPE)?

Virus-specific damage observed in cultured cells.

What immunologic methods are commonly used?

Antibody testing. 

What molecular methods are commonly used?

PCR and other nucleic acid detection tests. 

How do antiviral drugs work?

They target viral replication processes.

Do antiviral drugs usually cure infections?

No. They usually control or limit infections rather than eliminate them completely.

What is post-exposure prophylaxis?

Laboratory-prepared antibodies given early after exposure to help prevent disease. 

What is the virome?

The collection of all viruses associated with a particular organism or environment.

What is an oncolytic virus?

A virus that preferentially infects and kills cancer cells.

What is Lambda (λ) phage known for?

Integrating into the bacterial chromosome and establishing lysogeny.

What is CTXφ known for?

Carrying the cholera toxin genes in Vibrio cholerae.

What is φSa3ms known for?

Carrying virulence genes that can enhance the pathogenicity of Staphylococcus aureus.

What are prions made of?

Misfolded proteins with no nucleic acid.

How do prions cause disease?

They induce normal proteins to misfold, leading to brain damage.

What are examples of TSEs?

Creutzfeldt-Jakob disease (CJD), Kuru, and Mad Cow Disease (BSE).