Principles of Virology, part 1.
Richard Condit, Department of Molecular Genetics and Microbiology, University of Florida.
In this lecture, we will explore
some fundamental principles underpinning the discipline of virology.
We will begin by defining what a virus is,
followed by a brief history of virology.
We will then outline the general principles of
virus structure, classification, and replication.
Lastly, we will briefly introduce fundamental concepts
in virus pathogenesis and emerging virus infections.
First, a brief overview definition of a virus.
A virus is defined as an obligate intracellular parasite.
In its simplest form,
a virus consists of a segment of genetic material,
either RNA or DNA,
wrapped in a protein coat.
A virus particle thus constituted cannot multiply on its own outside of a cell.
Instead, the virus coat protein mediates
the transmission of the viral genetic material to the inside of a cell,
where the existing cell machinery decodes the genetic information.
The resulting viral gene products,
again in combination with host cell machinery,
bring about the replication of the viral genetic material,
synthesis of a new code,
and assembly of new virus particles.
As we will see, the variations on
this basic theme are seemingly boundless as viruses have
evolved to fill an infinity of ecological niches defined primarily by their hosts.
We now consider a very brief history of virology.
The history of virology is rich with discovery and reaches far beyond virology itself,
shaping the diverse fields of immunology and
molecular biology and more recently, gene therapy.
Interestingly, the first major breakthrough in virology,
the development of the smallpox vaccine by Jenner in 1796
precedes the description of the true nature of viruses by nearly 100 years.
The first experiments that recognized viruses as
entities distinct from other microorganisms were conducted in
the 1890s by three different groups of
investigators studying tobacco mosaic disease and foot and mouth disease.
In 1892, Ivanovsky discovered that tobacco mosaic disease could be induced by extracts
of diseased plants that had been filtered through
porcelain filters designed to retain the smallest of known bacteria.
In 1898, Beijerinck studying tobacco mosaic disease and Loeffler and
Frosch studying foot and mouth disease in cattle
also described the filterable nature of the disease agents,
and furthermore determined that they could be grown
or amplified only on their host of origin.
The first human virus was described in 1901 when Reed and
Carroll determined that yellow fever could be
transmitted from human to human using filtered serum.
In 1915 and 1917,
Twort and d'Herelle independently discovered
bacterial viruses dubbed bacteriophage by d'Herelle.
In his studies of bacteriophage,
d'Herelle developed many of the fundamental techniques and
concepts unique to virology that are still used today,
such as the plaque assay.
In 1931, Woodruff and Goodpasture
developed a technique of growing viruses on embryonated chicken eggs,
greatly facilitating the laboratory manipulation of viruses.
The invention of the electron microscope in 1933 by Ruska and
Knoll allowed scientists for the first time to visualize viruses directly.
Modern bacteriophage research was launched in
1939 with a description of the one-step growth experiment by
Ellis and Delbruck and persisted through
the 1970s through the inspirational leadership of Luria and Delbruck,
spawning most of the central concepts of both virology and molecular biology.
Modern animal virology was born in the late 1940s and
early 1950s with the development of techniques for virus growth in cell culture,
pioneered by Enders who in 1948,
grew polio virus in cell culture and by Dulbecco,
who in 1952 adapted
the bacteriophage plaque assay to work with polio virus in cell culture.
In 1970, molecular biology was turned on its head with
the discovery of retroviral reverse transcriptase by Temin in Baltimore.
1979 saw virologies and perhaps humanity's greatest triumph with
the declaration by the World Health Organization
that smallpox had been eradicated worldwide.
Only to be followed shortly in 1983 with the description by
Montagnier and Gallo of one of today's deadliest infectious agents,
human immunodeficiency virus or HIV,
the causative agent in AIDS.
Today, the discipline of virology is as vigorous as ever,
facing modern challenges of existing and emerging virus diseases in a shrinking world.
We now turn to a consideration of the elements of virus structure.