Download Topic 10 Viruses

Topic 10
Viruses
October 21, 2005
Biology 1001
What Are Viruses?
• Viruses are infectious particles consisting of nucleic acid
enclosed in a protein coat, and sometimes a membranous
envelope
• As obligate intracellular parasites, viruses are either the
most complex macromolecules or the tiniest, simplest
forms of life
The Discovery of Viruses
1883-1935
• Discovered as the causative agent of tobacco mosaic
disease
– Sap from diseased leaves causes the disease in new plants
– Ruled out a toxin, as the causative agent reproduced in the
new plants
– Could not isolate or culture a bacterium
– Concluded that the particle could only reproduce within a
host
– The infectious particle was subsequently shown to
crystallize and was observed with the electron microscope
The Structure of Viruses
• The viral genome
– The nucleic acid
component of the virus is
either dsDNA, ssDNA,
dsRNA, or ssRNA
depending on the virus
– A single linear or circular
molecule containing a
few to several hundred
genes
– (Details of table for
interest only)
The Structure of Viruses
• The protein coat of viruses is called the capsid
– The capsid is rod-shaped, polyhedral or complex in shape
– Capsids are built from a large number of one or a few
protein subunits called capsomeres
• TMV is a helical arrangement of >1000 molecules of a single protein
• Adenovirus is a polyhedral capsid of 252 identical protein subunits
• The most complex capsids belong to the phages, such as T4 or T2
• The viral envelope, if it exists, is a host-derived
phospholipid membrane
– Functions to help infect the host
– May contain viral proteins and glycoproteins
– Encases or cloaks the capsid
Figure 18.4
Viral Reproductive Cycles
• Viruses can reproduce only within a host cell because they
lack the enzymes and ribosomes necessary to make proteins
• The range of host cells that a particular virus can infect is
called its host range
– The host range can be broad (eg. rabies virus can infect humans, bats,
dogs, raccoons) or narrow (eg. poliovirus only affects humans)
• Host specificity results from the evolution of recognition
systems by the virus to receptors on the host cell
• In multicellular eukaryotes viruses infect specific tissues or
cell types (eg. rhinovirus – human cold virus - infects the
upper respiratory tract, poliovirus infects nerve cells, HIV
attacks certain white blood cells only)
Viral Reproductive Cycles
• Overview of a simplified viral
reproductive cycle, Fig. 18.5
– A viral infection begins when the
genome of a virus enters the host
cell
– Once inside, the viral genome
takes over the cell’s machinery –
enzymes, precursors, ribosomes
etc.
– New viral nucleic acid and new
viral proteins are synthesized
– New viral particles assemble and
exit the host cell, spreading the
infection to new host cells
Reproductive Cycles of Phage Viruses
• Bacteriophages are dsDNA viruses of bacteria
• They have two alternate reproductive mechanisms: the lytic
cycle and the lysogenic cycle
• The lytic cycle culminates in the death of the host and release
of hundreds of new phage particles
• Phages which can only reproduce lytically are called virulent
• During a lysogenic cycle, the phage DNA integrates into the
host’s genome where it is called a prophage, and reproduces
whenever the bacteria reproduces
• A virus such as λ that can enter a lysogenic cycle is called
temperate
• Ultimately the prophage exits the bacteria genome and initiates
a lytic cycle
Figure 18.6 The lytic cycle of phage T4, a virulent phage
Animation
Reproductive Cycles of Animal Viruses
• Animal viruses are varied in terms of reproductive cycle – one
key feature is type of genome, and another is the presence or
absence of a viral envelope (see Table 18.1)
• RNA as the genetic material
– Retroviruses are a type of ssRNA virus where the RNA acts as a
template for DNA synthesis (reverse flow of information)
– Retroviruses are packaged with the enzyme reverse transcriptase
– The DNA produced by a retrovirus is incorporated as a provirus into
the host genome; it is then transcribed into RNA that serves as the
mRNA as well as the genome for the next viral generation
• Role of the viral envelope
– Nearly all animal viruses with RNA genomes also have viral envelopes
– During infection, the viral envelope fuses with the host plasma
membrane to allow the virus to enter the cell
– Viral envelope glycoproteins are made in the host and transported to the
cell surface for packaging of new viruses
Figures 18.9 & 18.10 - The structure and reproductive
cycle of HIV, the retrovirus that causes AIDS
New human immunodeficiency
viruses exiting a helper T cell
Animation
Viruses and Disease
• Viruses cause disease in animals, plants, bacteria, and fungi
– Symptoms caused by damaging or killing cells, producing toxins, or
stimulating the host immune system
– Range from mild to severe, depending on reproductive cycle, type of
tissue or cell, etc.
• Major medical intervention is the vaccine, a harmless variant
that stimulates the host immune system to mount a preemptive defense
– Certain drugs such as acyclovir for herpes resemble nucleosides and
interfere with viral nucleic acid synthesis
Emerging Viruses
• Viruses that appear, or come to the attention of scientists,
suddenly
SARS
–
–
–
–
HIV – a retrovirus
SARS – a coronavirus
Ebola – a filovirus
“bird flu”1 – an orthomyxovirus
• Three factors contribute to emergence: mutation of existing
viruses, spread from one host species to another, spread from a
small isolated population
Hantavirus – a
group V ssRNA
virus
The Origin of Viruses
• Evolution of viruses
– Because viruses depend on cells for their own propagation, it is
reasonable to assume that they evolved after the first cells
– Most molecular biologists favor the hypothesis that viruses originated
from fragments of cellular nucleic acids that could move from one cell
to another
– Candidates for the original sources of viral genomes include plasmids
and transposons
– Plasmids are small circular DNA molecules, found in bacteria and
yeast, that replicate independently from the main chromosome(s) and
can be transferred between cells
– Transposons are DNA segments that can move from one location to
another within a cell’s genome
– Viruses, plasmids, and transposons are all mobile genetic elements
Are Viruses Alive?
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Cell as basic unit of structure?
DNA as hereditary material?
Growth and development?
Reproduction?
Regulation/homeostasis?
Emergent properties?
Evolutionary adaptations?
Order?
Energy processing?
Response to environment?