Chapter 6

 

An Introduction to the Viruses

 

 

Viruses

Obligate intracellular parasites - non-living, infectious agents incapable of surviving without a host cell

work by invading a host, taking over cell functions, and directing it to produce more viruses

they must use structures and enzymes of cells to support their own reproduction

Can infect all forms of life, including Bacteria, Archaea, and Eukarya

Considered ultramicroscopic

  About 2000 viruses could fit into an average bacterial cell

Virion – a fully formed virus that can establish an infection in a host cell

 

 

General Structure

Viruses consist of nucleic acid (DNA or RNA) surrounded by a protective protein coat (capsid)

Also called a nucleocapsid

 

 

Capsid

Outer protein shell constructed of repeating protein subunits known as capsomeres

3 Common shapes:

Helical - composed of rod-shaped capsomeres arranged into hollow discs

Icosahedral - flat and rounded capsomeres arranged into a 20-sided figure

Complex - have an icosahedral head, helical tail, and attachment fibers

 

 

Envelope

Double layer of lipid that surrounds the capsid

Found on a majority of animal viruses

Naked viruses - viruses lacking an envelope

Created from portions of the host cell membrane

Host cell membrane proteins are replaced with viral proteins that connect the envelope to the capsid

Spikes - viral proteins that protrude from the surface and aid in attachment of the virus to a host cell

 

 

Functions of the Capsid/Envelope

Protects nucleic acid from enzymes and chemicals

Helps facilitate the movement of viral DNA/RNA into the host

Some parts stimulate the hosts immune system

 

 

Nucleic Acid

Directs the actions of the host cell once invasion has occurred

Can be DNA or RNA but not both  

DNA may be linear or circular, either double-stranded or single-stranded

RNA is often single-stranded but some viruses have double-stranded RNA

 

 

Enzymes

Viral polymerases - direct the duplication of DNA or RNA in the host

ex. reverse transcriptase

 

 

Viral Multiplication:  Bacteriophage Model

Adsorption - coming together of the virus and the host cell - use tail fibers

Attachment may occur on the cell wall, pili, or flagella

Penetration - the phage pushes an inner tube through the cell wall and injects nucleic acid into the host

protein coat remains on the outside

Transcription and Replication - the virus shuts down host cell machinery and directs the host to produce new viral mRNA and protein (transcription/ translation) and nucleic acid (replication)

Assembly - viral components assemble into new bacteriophages

Release

Lytic cycle - viral enzyme digests the host cell wall resulting in cell lysis and virus release

Lysogeny – viral DNA inserts into bacterial chromosome and enters inactive prophage state

  can be induced to enter lytic cycle

 

 

Viral Multiplication:  Animal Virus Model

Adsorption -virus attaches to specific membrane proteins that the host uses in its normal function

Uses spikes instead of tail fibers

Entry – entire virion is taken into the cell

Many enter via endocytosis

Some simply fuse with the host membrane and release nucleocapsid into the host

Uncoating - the envelope (if present) and capsid are separated from the nucleic acid

viral nucleic acid released into cytoplasm

Replication, Protein Synthesis & Assembly

Viral DNA/RNA stops the normal host function

The host organelles are used to produce the components of new viruses (protein and nucleic acid)

New viruses assemble themselves

Release

Enveloped viruses - released by budding and exocytosis

Nonenveloped viruses - released when the cell lyses

DNA viruses can integrate into the host chromosomes to form a provirus

 

 

Nomenclature

Family names end in –viridae. 

There are 14 families of RNA viruses and 7 families of DNA viruses that infect vertebrates

Genus names end in –virus

Species names generally reflect the disease the virus causes (ex. poliovirus)

 

 

Common DNA Viruses

Adenoviridae – adenovirus

Poxviridae – smallpox and cowpox virus

Herpesviridae

Herpes simplex type I (HSV-1)

Herpes simplex type II (HSV-2)

Varicella-zoster virus

Epstein-Barr virus

Cytomegalovirus (CMV)

Papillomaviridae – Human Papillomavirus (HPV)

Hepadnaviridae – Hepatitis B virus

Parvoviridae – parvovirus B19

 

 

Common RNA Viruses

Picornaviridae – poliovirus, coxsackievirus, rhinovirus and Hepatitis A virus

Togaviridae – encephalitis viruses and rubella virus

Rhabdoviridae – rabies virus

Arenaviridae - Lassa virus

Flaviviridae – West Nile virus, Dengue fever virus

Filoviridae – Ebola, Marburg virus

Paramyxoviridae – mumps virus

Bunyaviridae – Hantavirus

Reoviridae – rotavirus, measles virus, Respiratory syncytial virus (RSV)

Retroviridae – Human Immunodeficiency virus (HIV)

Coronaviridae – SARS virus

Orthomyxoviridae – influenza virus

Calciviridae – Norwalk virus

 

 

Medical Considerations

Nearly every cell in the body is susceptible to at least one virus

The outcome of viral infection depends on defense mechanisms of the host

Can develop a state of balanced pathogenicity with the normal host

 

 

Types of Virus Infections

Acute infections - short duration

host may develop long-lasting immunity

Persistent infections - persist for years, often without disease symptoms

host becomes a carrier

Types of Persistent Infections:

Latent infections - acute infection is followed by a symptomless period and then reactivation of the disease occurs

  Ex. Herpesvirus and Chickenpox   

Chronic infections - infectious virus detected at all times

  Ex. Hepatitis B

Slow infections - infectious agent gradually increases over a long time with no symptoms and eventually a lethal disease ensues

  Ex. AIDS and prions

 

 

Cytopathic Effects

Virus-induced damage to the cell that alters its microscopic appearance

Inclusion bodies – compacted masses of viruses or damaged organelles

important for diagnosis

Oncoviruses – cause the host cell to become cancerous

DNA viruses can integrate into the host DNA and mutate or activate genes that regulate cell growth

Virus does not kill the host cell but causes the cell to change its properties (transformation)

 

 

Viral Host Range

Most viruses can only infect a single species

Depends on attachment proteins on viral envelope or cellular factors for replication

The host range can be modified when two different viruses infect the same cell

Phenotypic mixing - exchange protein coats

Genetic reassortment - exchange genetic information (Influenza virus pandemics)

 

 

Prions

Consist of a small protein and no nucleic acid

Are abnormal versions of host proteins that replicate by interacting with and converting normal protein to abnormal protein

Cause fatal transmissible spongiform encephalopathies

Kuru, Creuzfeld-Jakob disease, and Mad Cow disease

 

 

Viroids

Small, single-stranded RNA molecules and no protein coat

All viroids identified to date infect plants