Origin of Viruses
Iwanoski demonstrated that filtered sap from diseased
tobacco plants was transmissable - filterable bacterium
Beijernick - provided evidence that the transmissable
agent was not a bacterium
Stanley (1935) - isolated the infectious agent and called
it Tobacco Mosaic Virus (virus = latin for poison)
About the same time as Stanley's discovery, the electron
microscope was invented and this enabled viruses & bacteria to be studied
Viruses are the living dead and they are not cells. (See
discussion <A HREF="/courses/ss12bmi/microbes_are_cells.html" on microbes
are cells and organisms </A>).
From the above, there are three possibilities on the origin
If they are not cells and require living cells for replication,
than viruses must not have been present prior to cellular evolution and
could therefore have coevolved with cells
Were once cells but have lost all cellular functions retaining
only information to replicate themselves using hosts machinary
Viruses have evolved from plasmids (plasmids are self
replicating independent DNA) or from RNA viroids (see viroids section,
below). These "early" viruses did not contain genes for capsids. As viroids
moved from cell to another, it picked up such genes.
Viruses (poisons) are obligate parasites, filterable,
infectious particles. Demonstrated with plant sap.
20nm to 14000nm in size.
Most are smaller than bacteria (eg Adeno virus)
Vaccinia virus is the same size as the small bacteria
like Mycoplasma, Rickettsia & Chlamydia
Contain either DNA or RNA (never both) surrounded by a
protein coat (capsid) and sometimes covered by an envelope.
Obligate intracellular parasites & multiply inside
living cells by using cellular synthesis machinary
Most viruses infect specific host cells ie are host specific.
Host specificity is due to:
specific attachment sites on the host cells called receptors
Receptor sites for bacteriophage are found in bacterial
cell walls or fimbrae or flagella
Animal cell membranes contain receptors for animal viruses
availability of cellular factors required for viral multiplication
in the host cells.
Nucleic acid (DNA or RNA, never both)
may be linear or circular and
may be single or double stranded and
may exist as a single piece or multi-segmented pieces
The protein coat surrounding the nucleic acid is called
a capsid. A capsid
accounts for most of the mass of the viron
structure is determined by the viral nucleic acid
is made up of protein subunits called capsomers. Capsomers
maybe composed of
single type of protein
several types of proteins
arrangement of capsomers is characteristic of a particular
type of virus
Capsomers determine antigenicity of the viron
Some capsids are surrounded by an envelope. The envelope:
contains lipids, proteins & carbohydrates
is derived from
host cell membranes (can evade host defence mechanism)
and / or
may contain some proteins synthesised from viron genes
and / or
may contain some material derived from normal host cell
may be covered by spikes. Spikes:
are made up of carbohydrate & protein complexes &
project out into space from the surfaces of the envelope
used for attachment to host cells
Influenza viruses bind to RBC to form hemagglutination
Four morphological types of viruses based on the structure
of the capsid and envelope geometry.
Helical viruses -- nucleic acid is surrounded by capsids
which form long hollow rods (TMV, bacteriophage M13).
Polyhedral virsues -- capsids forming multiple (many)
sides. Icosahedron is a regular polyhedron with 20 triangular faces &
12 corners (Adeno virus).
Enveloped -- Pleomorphic to spherical shapes due to presence
of an envelope.
Enveloped helical virus - Influenza virus
Enveloped poyhedral virus- Herpes Simplex
Complex-- Complex stuctures in case of Lamda, T-odd &
polyhedral capsid + nucleic acid forms the head
helical tail (sheath, tail fibres + pin, and baseplate)
Naming and Taxonomy
Committee on Taxonomy of Viruses (ICTV)
established in 1973 oversees this function. A number of guidelines
are set down by the ICTV for naming and taxonomy of viruses. Please refer to
these links if you want in order to obtain details. Below I have outlined the
summary of these guidelnes.
2. Family suffix -VIRIDAE.
3.Viruses are never named after people but geographical
locations are okay. For example, Bunyaviridae (Bunyamwere in Uganda)
Picornaviridae (piccolo: Italian for very small),
Retroviridae (retro, latin for backward)
Parvoviridae (parvo, latin for small)
Togaviridae (toga or cloak)
Arbo (Arthropod Borne Virus)
Reoviridae (Respiratory Enteric Orphan):
not linked to a known disease
4. Some 4000 viruses belonging to 71 families, 11 subfamilies
and 175 genera are known (1995).
5. Taxonomy is based on a variety of criteria:
Host specificity (infective host)
Bacterial viruses (bacteriophages)
Fungal viruses (mycophages)
Nucleic acid type (RNA, DNA, SS, DS, circular, segmented
Human Immunodeficiency virus (HIV) is the species &
subspecies are designated by a number eg HIV-I
Methods of cultivating viruses
Animal viruses can be cultured in a variety of ways.
Live animals such as mice, rabbits, guinea pigs
Animal models are used for for diagnostic purposes: observed
for symptoms of disease & observed for tissue damage
some cannot be grown in animals. The problem of studying
AIDS is slow growth of virus in humans & therefore vaccine trials take
a very long to test
Simian AIDS (1986, infects green monkeys) & feline
AIDS (1987, domestic cats) are HIV related virus & the disease develops
within a few months, hence are good models to study growth of the virus
in different tissues.
Chimps can be infected with one sub-species of the AIDS
virus, viz HIV-1 (genus lentivirus) but no symptoms are produced.
Genetically engineered mice strains (containing genes
for human T cells & human gamma globin) developed in 1990 are suseptible
to AIDS infection
Suspension cell cultures
Adherent cell cultures
Convinient & cheap
Virus inoculated into 7 to 10 day chick embryonated eggs
into an area most appropriate for its growth
Chorioallantoic membrane (Pox virus)
Amniotic (Influenza virus)
Embryo cell damage
Formation of pocks / lesions on the membrane
Look for cytpathogenic effect (CPE) or plaques
Bacterial viruses can be grown on host specific bacteria.
Zones of clearing = plaques.
Viral multiplication can be divided into 5 phases. These
phases are dependent on the structure and the type on nucleic acid present
in the viron.
(e) Maturation and release
The following provides an overview of these different
phases citing examples of various common viruses.
(a) Adsorption and Attachment:
Attachment site of the viron specifically "connect"
to a complimentory receptor sites of a host cell.
Receptor sites are an inherited trait and can vary
from one individual to another. Therefore susceptibility to a viral infection
(and indeed to other infections) can vary from one person to another.
Bacteriophage (eg lamda
phage): A chance collision followed by weak chemical bonds forming
between a specific appendage, end of tail fibres and receptors on the bacterial
Animal viruses: Attachement
appendages are unlike lamda phage as these are
all over an animal viron and vary from viron to viron. The animal
are glycoproteins found on plasma membrane.
Non-enveloped dsDNA icosahedral adenovirus: fibres at corners of
Enveloped, 8 segmented linear ssRNA Influenza virus: Hemagglutinin spikes
attach to sialic acid of RBC plasma membrane (cause
hemagglutination, a hallmark of family Orthomyxoviridae (Influenza virus
A, B, C).
Enveloped retrovirus (dsRNA) Human Immunodeficiency Virus (HIV I &
causes Acquired Immune Deficiencies (AIDS): Envelope contains
glycoprotein spikes (gp120) which attach to CD4 receptors of T cells and
macrophages (CD4 absent in most other cells)
Direct the host sythesis machinary for its multiplication.
Bacteriophage: Lysogeny or lytic cycle.
Animal viruses: Lysogeny (eg cancer) or Budding.
Viruses in Australia
See "Australian Contributors to Virology" Chapter 6, pp
339-420, In History of Microbiology in Australia, Australian Society For
Microbiology, Frank Fenner, (Editor), Australian Print Group, Victoria.
The Infective Horse virus: In Sept 1994, a horse trainer
and 14 horses were killed due to a mystery viral illness in Brisbane. The
virus was identified as equine morbillivirus, a rare virus that can be
passed from animals (mammals such as cats, guinea pigs, horses) to humans.
The method of transmission is respiratory (ie via nose and mouth discharges).
Symptoms include breathing difficulties, very high fever and bloody foam
discharge from the nose and mouth. It is unknown what the natural host
is (perhaps native wild life) and how it spreads to the horses. (Sunday
Mail, 20th Aug, 1995).
Calicivirus and contrl of rabbit populations
1. All virues with the exception of members of the family
Parvoviridae have genetic
information for viral multiplication.
Parvovirus require "helper" viruses to supply
necessary components to produce
2. Viroids are much smaller than viruses, contain RNA
only and this RNA does not
code for proteins.
Discovered in 1971.
3. A plant pathologist discovered the first viroid
called Potato Tuber Spindle Viroid
which causes the disease Potato Tuber Spindle
4. There are 6 ways in which viroids are different
single circular RNA of low molecular weight (300
to 400 nucleotides).
often internally paired (3D) and is therefore protected against cellular
no capsids or envelopes
no requirement for helper viruses
do not produce proteins
always copied in the host cell nucleus
acn only be identified by nucleotide sequencing techniques
5. Viroids have so far been shown to infect plants
only. Viroids like plant viruses can
plant diseases eg potatoe spindle disease.
6. Plant cells are protected from diseases by an impermeable
cell wall. Viroids like
enter through wounds or via other parasites eg. plant sucking
nematodes, fungi. Cell death is perhaps due to RNA interference in
7. Do viroids have an old precellular origin or are
they a modern day example of a
extreme example of parasitism. Recently, base
sequence homology has
between viroids and introns (introns also do not code for
which has led to the speculation that viroids evolved from introns
1. Term proposed by Stanley Prusiner. Proteinaceous
particles (should be
"Proin" but "Prion" sounds better)
2. Prions cause 8 diseases in human and animal all
of which are neurological
progressive (dementing illnesses):
Creutzfeldt-Jacob Disease (CJD) - humans
Kuru Disease (PNG) - spread by canabilism in humans. Spread has ceased
due to lack of this practice.
Scrapie Disease in sheep
Bovine Spongiform Encephalopathy (BSE) or mad cows disease
3. Prions are infectious and infective proteins and
not DNA, RNA or toxins. Why?
Ressist inactivation by heat at 90oC.
Insensitive to radiation (damages DNA)
Insensitive to nucleic acid digesting enzymes (RNAse and DNAse)
Sensitive to protein denaturing agenst (phenol, urea)
4. There are 3 hypothesis for the infectious nature
(a) PrP (Prion
proteins) codes for normal proteins. PrP gene is located on
chromosome 20 in humans. Prion proteins are membrane proteins.
Abnormal mutant prion proteins PrPSC
fold incorrectly, and form fibres or
fibrils which stick together in cells ie they are not organised in the
membrane properly killing the cells.
The prion protein might contain a small but so far undetectable nucleic
that directs the synthesis of the abnormal PrPSC
infectious agent might be an unculturable or an unculture bacterium
which can pass through filters. A Spiroplasma-like bacterium has been seen
in brain tissues from CJD patients.
unknown but it is thought that prion proteins cause normal proteins
to fold incorrectly.
to cows via scrapie infected sheep that was used to supplement cow
feed in 1990.
to humans by the consuming the milk of affected cows has been
to mice by injection of prion proteins and disease symptoms
Some of the prion diseases are spread only in families ie it is inheritable
Please direct comments, suggestions & errors to: Dr Bharat Patel B.Patel@griffith.edu.au
[Created 06 Sept 1995]
[Modified 10 Oct 1997]