Viruses Bio Factsheet

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B io Factsheet
September 1998 Number 32
Viruses made simple
Viruses lack the mitochondria necessary to derive energy and they cannot reproduce on their own. They are dependent on their host
cells and are only classed as living organisms when they infect host cells. After reproducing, viruses inevitably cause their host's
destruction. It is because of these characteristics that viruses are described as obligate intracellular parasites. This Factsheet will
describe the structure of different types of viruses, their life cycles and their pathogenic (disease causing) properties.
Classification of viruses. Replication of viruses in general
Viruses can be classified according to characteristics such as: Viruses identify the cells which they attack by recognising specific cell
(a) The disease they cause e.g. influenza surface receptors. For this reason viruses will usually only infect one
(b) Their tissue target e.g. neuronal species. In simple terms, when a virus infects a cell it injects its own
(c) The vector by which they are transmitted genetic material (either DNA or RNA) into the cell. The genetic material
of the virus then replicates and many new virus particles are assembled.
However, the most widespread classification schemes are based on viral The replicating virus uses the host cell's nutrients and energy sources,
structure, for example: preventing the cell carrying out normal metabolism. Finally, the cell is
destroyed and the virus particles are released and are able to infect new
(a) Their nucleic acid (DNA or RNA) cells. Viruses which infect bacterial cells are called bacteriophages, those
(b) Their size that infect animals and plants are called animal and plant viruses respectively.
(c) Their morphology (shape and structure)
Replication of bacteriophages
Common exam questions Bacteriophages, or phages as they are commonly called, undergo two types
1. Why can viruses be classified as living and non living? of replication: lytic and lysogenic. Fig 1(c) shows the T4 phage, an example
2. Explain why all viruses are considered to be parasites. of a lytic bacteriophage.
Size of viruses (a) Lytic replication e.g. T4 phage
Viruses range in size from the smallest picornaviruses (20 nanometres) to The first stage of lytic replication is the adsorption (attachment) of the
the largest poxviruses (350 nanometres). Note, however that they are phage to the bacterial cell. Following adsorption, the tip of the phage tail
much smaller than bacteria. becomes attached to a specific cell surface receptor. The sheath of the tail
then contracts, puncturing the bacterial cell wall and membrane, thus enabling
Virus morphology the phage genome (DNA) to be injected into the bacterium.
The virus core consists of nucleic acid, which may be associated with
accessory proteins such as enzymes. Surrounding the nucleic acid is a Following penetration, the DNA of the phage is transcibed (i.e. used as a
protein coat called a capsid. Before assembly with the nucleic acid and template for mRNA synthesis) and the bacterial DNA is degraded. The
accessory proteins, the empty protein shell is called a procapsid. After the mRNA is translated by the bacterial cell (i.e. proteins are synthesised
components have assembled, the structure is called a mature capsid. Viruses according to the instructions given by the mRNA template).
with no other features are referred to as naked capsid viruses. Alternatively
the capsid may be surrounded by a membrane composed of lipid, proteins Once the structural proteins and nucleic acid have been synthesised, the
and glycoproteins, in which case the particle is described as an enveloped phage components are assembled into mature capsids. After approximately
virus. 25 minutes some 200 phages are assembled, leading to lysis (disintegration)
of the bacterial host and the release of new viruses, which in turn infect
The morphology of the capsid can be described as follows: other bacteria. The cycle then repeats.
(a) Helical Exam Hint - Some Examination Boards require you to know the
(b) Icosahedral (a icosahedron is a 20 faced solid) difference between the lytic and lysogenic cycles. It is easy to confuse
(c) Composite (having helical and polyhedral components) the names but they are two very different processes.
Examples of such structures are shown in Fig 1.
Fig 1. Examples of capsid structures
(a) Helical (b) Icosahedral (c) Composite capsid head
e.g.T4 Phage contains DNA
helical tail
tail fibre
naked enveloped naked enveloped
1

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Viruses made simple Bio Factsheet
(b) Lysogenic replication e.g. Lambda () phages Replication of animal viruses
In addition to lysis, some types of viruses can replicate by incorporating Infection of the host with an animal virus leads to a variety of effects,
their DNA into the host DNA. This type of replication is called lysogeny.…read more

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Viruses made simple Bio Factsheet
Replication of plant viruses Retroviruses
All plant viruses are RNA viruses. Plant viruses enter their hosts through Retroviruses contain RNA and an enzyme called reverse transcriptase
damaged regions of the plant or by insect bites which transport the viruses which catalyses the production of single stranded DNA, called copy DNA
into the host cells. Once the virus has infected the host cell, replication is (cDNA) from the RNA template.…read more

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Viruses made simple Bio Factsheet
Treatment of viral infections Practice Questions
Treatment of viral infections is difficult because the infections take place
inside cells where it is difficult to reach. It is also difficult to find a drug 1. Fig 1. shows a diagram of a bacteriophage
which specifically targets the virus and not the host cells, since the virus
uses the host cell's nutrients and energy to replicate.…read more

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