Classification and Replication of Viruses

Viruses

Submicroscopic infectious agents that replicate within living host cells

Composition - virus

Genetic material (DNA or RNA) inside a protein shell called a capsid, and sometimes an additional lipid envelope.

Replication process - virus

Involves attachment, entry, uncoating, replication, assembly, and release of new virions

Variety -virus

Exhibit a wide range of shapes and structures

Living status - virus

Not considered living organism due to their inability to reproduce or metabolize independently

What is a prion?

a misfolded protein

Prions are not

a virus, or any other recognized infectious agent (bacteria, fungi, parasite)

All known prion diseases in mammals affect:

– the structure of the brain or other neural tissue;

– all are progressive,

– have no known effective treatment, and

– are always fatal.

All known mammalian prion diseases were caused

by the prion protein (PrP) until 2015, when a prion form of alpha-synuclein was hypothesized to cause multiple system atrophy (MSA)

Prion aggregates are

stable

Structural stability of prions means they are

resistant to denaturation by chemical and physical agents: they cannot be destroyed by ordinary disinfection or cooking.

A prion disease

is a type of proteopathy, or disease of structurally abnormal proteins.

Capsid

made of capsomeres (glycoproteins)

Viral proteins are

lock and key to host cell receptors, initiating infection.

Structure of virus

• Capsid

• Nucleic acids (DNA or RNA)

• Envelope

Both the capsid and the envelope are

antigenic and specific sites are recognized by the host immune system.

Capsid and envelope (when present) serve

to protect the genome (RNA & DNA).

Virus structure w/ symmetry

• Isometric

• Helical

• Spherical

Isometric (= icosahedral, polyhedral)

• Capsid constructed of 20 equilateral triangular faces

e.g. herpesviruses

Helical

• Tubular construction with the subunits arranged a coil

• Bullet-shaped

e.g. rabies

Spherical

• Capsid constructed like a circle

e.g. influenza, coronaviruses

Viral structure - no symmetry

• Complex

• Filamentous

Complex

e.g. smallpox virus and phage

Filamentous

Pleomorphic

e.g. ebolavirus

Function of capsid

protect the fragile nucleic acid genome and play a role in initiating infection by delivering the genome from its protective shell in a form in which it can interact with the host cell.

Viral Taxonomy is based upon:

• Morphology of virion, capsid, and envelope

• Genome (RNA, DNA, SS, DS etc)

• Serological relationships (Serotypes)

• Replication strategy

Capsomeres are assembled

from proteins, and these can now be crystalized and studied for receptor binding (Spike proteins

Capsid Proteins (structural)

• Important for viral stability and attachment

• Antibodies are generally formed against the structural proteins.

Non-structural - Functional (other proteins)

• Enzymes involved in viral replication

• When antibodies are found against the non-structural proteins this may help in differentiating animals vaccinated with inactivated recombinant vaccines from those naturally infected (DIVA principle)

The capsid and envelope of the virus is responsible for

recognition of the host cell (receptor binding).

cellular receptor molecule

Binds to a specific virus-attachment protein

There are __ classes of viral genomes.

7

Names suggest something about the viral family

• Herpes =creeping

• Corona = crown

• Picorna = small rna

• Retro = backwards

• Calici = cup

Viruses classified into Families and Genera

• Either DNA or RNA

• Single strand or double strand

Promiscuous

capable of infecting several species (Rabies virus)

Plastic

exhibiting adaptability to change or variety in the environment (Influenza virus)

A strain

a well characterized virus

Virulence

Different strains may have different properties

An isolate

refers to the virus recovered from a specific host or location.

Serotype

is a distinct variation within a virus based on surface antigens causing host to produce distinct antibodies - means that immunity is not conferred by previous exposure to a different type (foot-and-mouth disease)

Viruses must

replicate in living cells

Cytopathic effects =

cell transformation in cell culture