WHAT ARE VIRIONS?

VIRIONS are virus particles: they are the INERT CARRIERS of the genome, and are ASSEMBLED inside cells, from virus-specified components: they do not GROW, and do not form by DIVISION.


HIV	Phage Φ29	Phage P22	Influenza	Smallpox

Tobacco mosaic	Filamentous phage	T4 phage	Adenovirus	Coronavirus
All pictures copyright Russell Kightley www.rkm.com.au

They may be regarded as the EXTRACELLULAR PHASE of the virus: they are exactly analogous to "spacecraft" in that they take viral genomes from cell to cell, and they protect the genome in inhospitable environments in which the virus cannot replicate.

Note the strong resemblance between a bacterial virus - T2, T4 or T6 phage of E coli, which evolved possibly billions of years ago - and the only human-crewed spacecraft to have landed on another planet.


T-Even phage structure - copied from Wikipedia	Lunar excursion module (LEM) of Apollo 16: courtesy NASA

Basically, all virions have:

genomic nucleic acid: this may be RNA or DNA, ss or ds, and may be all or part of the genome (see also here)
protein associated with the genome: this may be in the form of a helically-assembling or isometrically-assembling SIMPLE NUCLEOCAPSID only, or
more complex structures with two or more layers of protein, with nucleoprotein innermost.

If the virions are simple NUCLEOPROTEINS - that is, contain only nucleic acid and protein - then they are usually composed ONLY of virus-specified components. However, certain host components may be "trapped" within virions, such as POLYAMINES: these are polycationic compounds which serve to neutralise charge on the viral nucleic acid as it is packed into the CAPSID, or protein coat. PAPOVAVIRUSES may in addition encapsidate host histones associated in NUCLEOSOME complexes with virus genomic DNA.

The essentials of virion structure may be seen by clicking on the images below.


Simple helical structures	Simple isometric structures	Compound structures	Enveloped and complex virions

Excellent electron micrographs of virions - and an explanation of their basic structure - may be seen at the University of Cape Town's Medical Microbiology Division teaching pages.

For other excellent representations of virus structure, go to the University of Wisconsin's Institute for Molecular Virology, where Jean-Yves Sgro has assembled a gallery of 3-D image reconstructions of different viruses, together with explanations of structure.

	Tutorial Contents
"What is a Virus?"