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BIOL08004 Introductory Microbiology University of the West of Scotland

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University of the West of Scotland

BIOL08004 Introductory Microbiology University of the West of Scotland

1. Introduction
Viruses are unlike all other living organisms in that they lack a cellular structure. They do not
undergo growth and division but are constructed from component parts produced within
organisms which they have infected. Viruses also lack the enzymes required to generate
metabolic energy. Viruses are obligate intracellular parasites i.e. they require a suitable host
cell to replicate within and are unable to do so on their own. Viruses are highly specific in the
host cells which they require and there are viruses of animals, plants, fungi, protozoa and
bacteria. Indeed, all other living organisms will have specific viruses which are capable of
infecting them.
(Chlamydia and Rickettsia grow and reproduce only within living host cells but are classed as
bacteria since they have a cellular structure (membranes, ribosomes, metabolic pathways etc.,
contain DNA and RNA and reproduce by binary fission)
One of the fundamental questions in virology is “are viruses alive?” This question is difficult
to answer since they lack many of the features of all other living organisms and one view is
that viruses are alive when they are located in a host cell (i.e. when they are infecting another
organism) and inert when they exist outside of a host cell. Other virologists use the terms
active and inactive to describe these two states.
2. Classification of Viruses
Based on the 2000 report from the ICTV (International Committee on Taxonomy of Viruses)
over 1,550 species of virus are now recognised. It is unknown how many other viruses are
undiscovered at this point.
Viruses are divided into 3 orders and within these orders specific families and species of
virus fall within these groups. Groups I – VII are used to divide viruses on the basis of their
Group Genome
I dsDNA viruses
II ssDNA viruses
III dsRNA viruses
IV (+) Sense RNA viruses
V (-) Sense RNA viruses
VI RNA Reverse Transcribing viruses
VII DNA Reverse Transcribing viruses
The subviral particles (satellites, viroids and prions) are placed in separate groups.
3. Virus Structure
Extensive studies using electron microscopy, immunology and biochemical analysis have
resulted in an increasingly clear picture of viral structure.
The virus particle (or virion) is composed of:
▪ DNA or RNA (the genome)
▪ A protein coat (the capsid)
▪ In some cases an additional outer layer (the envelope)
Virions range in size from 10nm (not much larger than a ribosome) to 300-400 nm (about the
size of the smallest bacteria). Some of the very largest viruses are visible using the light
microscope but most require the use of the electron microscope.
Viral Genome
The vast majority of viruses contain a genome of DNA or RNA. The 4 possibilities are
ssDNA (single stranded DNA), dsDNA (double stranded DNA), ssRNA (single stranded
RNA) or dsRNA (double stranded RNA). Animal viruses may contain any one of these types
of nucleic acid while plant viruses most often contain ssRNA and bacteriophages most often
contain dsDNA.
DNA viruses may possess linear or circular DNA.
RNA viruses may contain RNA:
▪ Plus or Positive strand RNA – RNA which has an identical code to that of viral mRNA
▪ Minus or Negative strand RNA – RNA which has a complementary sequence to that of
viral mRNA
RNA genes may be segmented whereby it is divided into separate parts which may even be
present in separate virions.
The smallest viral genomes are only large enough to code for 3-4 proteins while the largest
genomes code for over 100.
The nucleic acid and its surrounding protein coat are termed the nucleocapsid core. If a virus
has no envelope it is termed naked while a surrounding envelope renders the virus enveloped.
The capsid is composed of protein subunits called protomers. The protein capsid may have
one of 3 general morphologies or symmetries.
(i) Helical
(ii) Icosahedral
(iii) Complex
Helical Capsids
The protomers in a helical capsid are arranged in a helical or spiral arrangement. The genetic
material is wound in a spiral in the middle of the capsid or in a groove inside the capsid. The
capid’s overall morphology is a hollow tube with protein walls. An examples of a virus with
a naked helical capsid is tobacco mosaic virus (figure 1) which causes symptoms including
mottling, tissue death, stunting, yellowing and leaf curling in a range of plants (including
tobacco and tomato plants).
Figure 1: Tobacco mosaic virus and infected plant (images from
There are no identified naked helical viruses which infect animals (or humans) although it is
not clear why this should be the case other than it probably relates to host cell biology.
Icosahedral Capsids
Icosahedral capsids are regular polyhedrons with 20 equilateral triangle faces and 12 vertices
(points). Icosahedral capsids are composed of protein units called capsomers with each
composed of 5 or 6 protomers. Pentamers contain 5 protomers and hexamers contain 6
protomers. Pentamers are at the vertices of the polyhedron and hexamers form the edges and
triangular faces.
Icoasahedral capsids appear spherical when viewed at low magnification using an electron


BIOL08004 Introductory Microbiology University of the West of Scotland

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