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Chapter 34: Viruses
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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Discovery of viruses
•
•
Unusual nature of viruses recognised in late 19th
century
Studies of tobacco mosaic disease showed that
– the pathogen could be transmitted in sap
– even when precipitated in ethanol the pathogen was still
capable of causing an infection
– it was capable of passing through filters with pores so
small they filtered out bacteria
•
Studies on other diseases of plants and animals
showed that these pathogens behaved similarly
(cont.)
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Discovery of viruses (cont.)
•
•
Dutch pathologist, W.M. Beijerinck, described the
tobacco mosaic pathogen as a ‘contagium vivum
fluidum’, a contagious living fluid
Later known as
– virus

Latin for slimy liquid or poison
– bacteriophage

bacterium eater
(cont.)
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Discovery of viruses (cont.)
•
•
1930s: serological tests detected novel antigens in
plants infected with tobacco mosaic
After treatment, sap from infected plants showed
streaming birefringence
– changed the plane of polarisation of light
– indicated that sap contained disc- or rod-shaped particles
•
•
Electron microscopy revealed rod-shape particles
Structure determined with X-ray diffractometry
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Virions
•
Virions are metabolically inert transmission phase
of virus
– virus genome in protective coat
When virion enters a host cell, the virion’s genome
hijacks the cell’s metabolic systems
• Metabolically active reproductive stage uses
•
– host’s metabolic systems to replicate
– host’s ribosomes to produce viral proteins
(cont.)
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Virions (cont.)
•
Virions have characteristic shapes imposed by the
covering of virus-coded virion or coat proteins
– enclose viral genome
•
•
Rod-shaped virions with helically coiled genome
Isometric virions with folded genome
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Fig. 34.4: Virion structure
(a) Rod-shaped virion
(b) Isometric virion
(cont.)
Copyright  2005 McGraw-Hill Australia Pty Ltd
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Fig. 34.4: Virion structure (cont.)
(c) Virion enclosed in a lipoprotein envelope
Copyright  2005 McGraw-Hill Australia Pty Ltd
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Bacteriophages
•
Complex bacteria-infecting virions
– rounded ‘head’ enclosing genome
– ‘tail’ of outer sheath and inner core
•
Terminal fibres of tail attach to host cell
• Tail core forced through host wall
• Viral genome enters cell
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Fig. 34.5 a + b: Virions of phages
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Viral genome
•
•
•
Virus genome consists of genes required for
infection
Usually contained in a single virion but some
viruses with divided genomes
Depending on species, genome may be
– RNA or DNA
– single stranded (ss) or double-stranded (ds)
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Replication
•
Viruses reproduce asexually
– may recombine with other viruses in mixed infections
– those with divided genome may reassort parts of genome
•
Double-stranded genomes
– replicate using pathways similar to those of host
•
Single-stranded genomes
– transcribed into complementary strand by replicase
complex
– enzyme includes host and viral proteins
– complementary strand transcribed repeatedly to produce
multiple progeny
(cont.)
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PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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Replication (cont.)
•
Plus-stranded genomes
– are translated directly or
– are translated from subgenomic mRNAs with same sense
as genome
•
Negative-stranded genomes
– must be transcribed into complementary plus strand
before translation
– virions carry viral replicase
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PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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Fig. 34.6: Life cycle of simple virus
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Retroviruses
•
•
•
•
•
ssRNA genomes transcribed into dsDNA on
infection of host
dsDNA incorporated into host chromosomes
DNA transcribed to produce ssRNA genomes
Replication strategy of retroviruses
RNA  DNA  RNA
Replication strategy of viruses with dsDNA
DNA  RNA  DNA
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Fig. 34.7: Life cycle of retrovirus
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Protein production
•
Viral proteins may be
– translated from mRNAs transcribed from genome
– produced as polyproteins and hydrolysed by virusencoded proteases
•
Many viral genomes
– have overlapping genes that are read with different
reading frames
– are ambisense, so that different parts of genome are
translated in opposite directions
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Viral ecology
•
Contagious viruses spread from host to host
– body fluids
– pollen
– seed
•
Viruses may be spread by vectors, which are often
organisms that feed on viral host
– mosquitoes and ticks are vectors for viruses that infect
animal hosts
– aphids, nematodes and fungi are vectors for viruses that
infect plant hosts
– some viruses are spread by plants to sap-feeding insects
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Viral infections
•
•
Symptoms of viral infections are specific to that
virus
Often cause enhanced chance of transmission
– coughing and sneezing expel fluids containing virions
– increased body temperature and CO2 production resulting
from fevers attract mosquitoes
– yellowing of infected plant leaves attracts sap-feeding
insects
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Epidemics
•
Epidemics (outbreaks of disease) occur when a
virus
– appears in a new locality (arrival of smallpox in North
America in 16th century)
– switches host (HIV moved from monkeys to humans)
– mutates (influenza moved from waterbirds to humans)
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Classification
•
Viruses are polyphyletic
– no common genes (unlike cellular organisms)
•
Despite free movement of genetic material, viruses
have well-defined species
– they have similar genetic sequences
– they have similar methods of transmission
– infection causes similar symptoms in host
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Selection acts on viruses
•
•
Studies of Qβ bacteriophages suggest that species
are maintained by selection
Qβ phage genome of ssRNA
–
–
–
–
•
high frequency of replication error
although new variants arise, population remains stable
one sequence favoured in competition—‘master copy’
uncertain whether master copy is real or average of all
variants
Stabilising selection maintains well-defined
species
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Viral groups
•
Viral genera/groups are composed of species that
possess the same
–
–
–
–
–
•
structure and composition of virions
replication strategy
biochemistry
mode of transmission
ecology
Related species may have differences in host
preference
– different species
– different parts of the same species
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Relationships
•
•
Viral genera have complex interrelationships
Many genes form gene families that produce same
proteins
– involved in genome replication
– proteases
– virion proteins
•
Some gene families are
– shared by viruses of plants, animals and bacteria
– related to host genes
– unique to virus groups or individual viruses
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Satellite viruses
•
Satellite viruses replicate in cells in the presence
of a helper virus
– prevent replication of the helper virus
– genomes of 1000–1400 nucleotides
•
Satellite virus (STNV) of tobacco necrosis virus
(TNV) encodes one protein (virion protein)
– other functions from helper virus and plant host
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Satellite nucleic acids
•
Satellite nucleic acids less complex than satellite
viruses
– transmitted within virions of helper virus
– helper viruses provides all proteins
•
Genome is ssRNA circle 300–400 nucleotides in
length
– transcribe complementary strand into long ssRNA strand
made up of multiple copies of genome
– cut into unit genomes by ribozymes
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Transposons
•
Repetitive sequences on genomes of cellular
organisms
– ‘junk’ or ‘selfish’ DNA
– may be up to 10 000 bp long
•
Repetitive DNA is complex
• Transposons are regions of DNA that exist in
multiple copies
– can move between host chromosomes
– may compose up to 40 per cent of host genome
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Retrotransposons
•
•
Retrotransposons (RTs) are related to retroviruses
Long interspersed elements (LINEs) are the most
common RTs in human genome
– DNA transcribed to RNA
– reverse transcribed to DNA by enzyme encoded by
LINES
– resulting DNA integrates into new site on host genome
– no virion protein gene
– do not produce virions
•
Short interspersed elements (SINEs) are also
common
– satellites of LINEs using LINE enzymes for replication
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Viroids
•
•
•
Viroids resemble viruses but do not encode
proteins
Possess circular ssRNA genome c. 350 bp long
Resemble satellite RNAs but are able to be
replicated in host without helper viruses
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Prions
•
Prions are proteinaceous infection particles
– encoded in host genome
– lack nucleic acids
– spread between species
•
Diseased prion proteins enter cells and cause
healthy cellular proteins to refold into diseased
form
– structural alteration causes proteins to accumulate as
amyloid plaques
– cells of nervous system
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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Virus control
•
Viruses are controlled most effectively by
preventing spread between hosts
• Biochemical methods of control are difficult
because viruses use host biochemistry
• Replication of some viruses can be retarded with
purine or pyrimidine analogues
• Immunisation with
– non-infectious (dead) virions
– live non-virulent virions
– isolated virion proteins
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Influenza
•
Influenza orthomyxovirus infects respiratory tract
– spreads by droplets produced by coughing
•
Virions with outer membrane of lipid with surface
proteins
– haemagglutinin (HA)
– neuraminidase (NA)
•
Influenza A genome in eight parts
– reassorts in mixed populations
– produce strains with novel combinations of HAs and NAs
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Other viruses
•
HIV-AIDS
–
–
–
–
•
human immunodeficiency lentivirus (HIV) is a retrovirus
causes acquired immunodeficiency syndrome (AIDS)
Lentivirus infects monkeys and other mammals
destroys immune system of host
Arboviruses
– arthropod-borne viruses transmitted between hosts by
ticks, mosquitoes and other arthropods
– Murray Valley encephalitis flavivirus (MVEV)
– West Nile flavivirus (WNV)
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