Chapter 6


An Introduction to the 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




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




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




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


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

can carry toxins



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

Penetration – 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


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




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 and generally reflect the disease the virus causes (ex. poliovirus)



Common DNA Viruses

Poxviridae – smallpox and cowpox virus


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

Flaviviridae – West Nile virus, Dengue fever virus

Filoviridae – Ebola, Marburg virus

Paramyxoviridae – mumps virus

Bunyaviridae – Hantavirus

Reoviridae – rotavirus, measles virus

Retroviridae – Human Immunodeficiency virus (HIV)

Coronaviridae – SARS virus

Orthomyxoviridae – influenza virus

Calciviridae – Norwalk virus



Virus Infections

Acute infections - short duration

host may develop long-lasting immunity

Persistent infections - persist for years, often without disease symptoms

Virus can remain latent in the host cell and periodically reactivate

host becomes a carrier


Cytopathic Effects (CPE) - virus-induced damage to the cell that alters its microscopic appearance

Inclusion bodies – compacted masses of viruses or damaged organelles

important for diagnosis

Syncytia – fusion of multiple host cells into single large cells with multiple nuclei

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)




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




Small, single-stranded RNA molecules and no protein coat

All viroids identified to date infect plants