Structure of viruses

 Ø  Structurally viral particles are simple and minute in nature.

Ø  A wide diversity of shapes and sizes have been observed.

Ø  The viruses are measured in units of nanometre (nm) which is one-thousandth part of a micrometre or a one-millionth part of a millimetre. An Angstrom unit (A) is one-tenth of a nm.

Ø  Most viruses can be measured under electron microscope by comparing their size with that of known sized latex particles –

o   the poxviruses are measure around 330 × 230 × 100 nm,

o   Paramyxo and herpesviruses ranges from 120-300 nm

o   Whereas influenza and adenoviruses have diameter of 60-120 nm.

o   Smallest viruses are picorna and parvoviruses which measure between 20-40 nm.

Ø  Shapes vary – helical (TMV), Icosahedral (Rota virus), bullet shaped (Rabies), oval or spherical.

Ø  Helical shape - capsomer coiled around a central axis to form a helical structure. Common structure seen in single stranded RNA viruses.

Ø  Tobacco mosaic virus is a helical virus.

Ø  The virion (the virus particle) consists of a nucleic acid genome packaged into a protein coat (capsid) or a membrane (envelope).

Ø  The virion may also contain certain essential or accessory enzymes or other proteins to facilitate initial replication in the cell

Ø  Capsid or nucleic acid–binding proteins may associate with the genome to form a nucleocapsid, which may be the same as the virion or surrounded by an envelope.

Ø  The genome of the virus consists either of DNA or RNA. The DNA can be single or double stranded, linear or circular.

Ø  The virion (the virus particle) consists of a nucleic acid genome packaged into a protein coat (capsid) or a membrane (envelope)

Capsid

v  The capsid is a rigid structure

The capsid has two functions:

o   To protect the genome from external harmful factors or agents

o   To introduce the viral genome into the host cells.

v  The capsid is composed of a large number of capsomers that form its morphological units.

v  Viruses with naked capsids are generally resistant to drying, acid, and detergents, including the acid and bile of the enteric tract.

v  Many of these viruses are transmitted by the fecal-oral route and can endure transmission even in sewage.

v  Three kinds of symmetry are seen in capsid

o   Icosahedral capsid – a polygon with 12 to 20 corners.

§  Each side is in the shape of an equilateral triangle.

o   Helical symmetry – the capsomers and nucleic acids are wound together to form a helical symmetry.

 

Envelope

v  The envelope is a membrane composed of lipids, proteins, and Carbobydrates.

v  Envelopes confer chemical, antigenic and biological properties of viruses.

v  Some animal viruses are released from the host cell by an exocytosis process that coats the virus with a layer of the host cell’s plasma membrane; that layer becomes the viral envelope.

v  The envelope contains proteins determined by the viral nucleic acid and materials derived from normal host cell components.

v  The membranous structure of the envelope can be maintained only in aqueous solutions.

v  It is readily disrupted by drying, acidic conditions, detergents, and solvents such as ether, which results in inactivation of the virus.

v  As a result, enveloped viruses must remain wet and are generally transmitted in fluids, respiratory droplets, blood, and tissue.

o   Most cannot survive the harsh conditions of the gastrointestinal tract.

v  Depending on the virus, envelopes may or may not be covered by spikes.

v  The spikes are carbohydrate- protein complexes that project from the surface of the envelope.

v  The ability of certain viruses, such as the influenza virus, to clump red blood cells is associated with spikes.

o   Such viruses bind to red blood cells and form bridges between them.

v  The resulting clumping is called as haemagglutination and is the basis for several useful laboratory tests.

v  Viruses whose capsid is not covered by an envelope are known as naked viruses.

v  In naked viruses, it is the capsid that protects the nucleic acid from nuclease enzymes in biological fluids and that promotes attachment to susceptible host cell.

Nucleic acid

v  Contains only one type of nucleic acid, either single or double standard DNA or RNA.

v  Larger virions can hold a larger genome that can encode more proteins, and they are generally more complex.

v  Some of the virus may contain histone proteins in DNA.

v  Most of the viruses have genomes which consist of unbroken strands of nucleic acids with the exception of reoviruses which have 10 segments of dsRNA and influenza virus which consists of 08 segments of ssRNA.

v  Naked nucleic acids are more infectious - Picornaviridae, Togaviridae, Papovaviridae, Adenoviridae, and Herpesviridae.

 

 

Proteins

v  Two types: Structural and Non-structural or functional proteins.

v  Structural proteins are the proteins present in the coat of the virus whereas the enzymes and haemagglutinins are known as non-structural proteins.

v  Proteins make up the capsid.

v  Protects nucleic acids, also determines the antigenic specificity of the virus.

Lipids

v  Originate from the cell membrane of the host.

v  Lipids constitute about 35% of the dry weight of enveloped viruses

Enzymes

Four groups of enzymes have been detected in the viruses. These are:

A.     Neuraminidase

B.     RNA polymerase

C.     Reverse transcriptase

D.    Enzymes of cellular origin