Unit 4: Viruses
Intro Video
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Anatomy of a Virus
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• The tiniest viruses are
20 nm in diameter.
(smaller than a
ribosome)
Virus Size
• They consist of nucleic
acids enclosed in a
protein coat and
sometimes a
membranous envelope.
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• The virus genomes (sets of genes) may be
–
–
–
–
Double stranded DNA
Single stranded DNA
Double stranded RNA
Single stranded RNA
• Called either a DNA or RNA virus
depending on type of nucleotide.
• May be linear or circular
• Smallest have only 4 genes; largest have
several hundred.
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• Capsid – protein shell that covers viral
genome. May be:
– Rod-shaped
– Polyhedral
– More complex
• Capsids built from large numbers of protein
subunits called CAPSOMERES
• Most complex capsids found in viruses that
infect bacteria – BACTERIOPHAGES (T1T7). They have protein tail piece with tail
fibers that attach to the bacterium
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Reproduction
• Viruses are obligate
intracellular parasites
that can reproduce only
within a host cell.
• They do not have:
– Enzymes for metabolism
– Ribosomes
– Equipment to make
proteins
• Animation
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Each type of virus can infect and parasitize a
limited range of host cells called its HOST
RANGE.
• Some are broad based while others are not.
– Swine flu virus infects swine or humans
– Rabies infects many mammals
– Some can parasitize only E. coli
• Eukaryote viruses usually tissue specific
• Viruses use a “lock and key” fit to identify
hosts.
• Virus Entry System
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• Regardless of the type of virus, the parasite
diverts the host cell’s resources for viral
production.
• The host cell provides:
• Nucleotides for nucleic acid production
• Enzymes
• Ribosomes
• tRNA
• Amino acids
• ATP
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Replication occurs using
lytic or lysogenic cycles
• The Lytic Cycle
– Ends in death of host
cell
– Virulent viruses
• Tobacco Mosaic,
HIV, T4
• The Lysogenic Cycle
– Replication of the viral
genome w/out destroying
host cell.
– Temperate virus may
reproduce by either cycle.
• Lambda virus:
resembles T4 but only
has a single short tail
fiber
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Animation
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The Lambda Phage Infecting the E. coli
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Animation
RETROVIRUSES
• Most complicated
• Genetic information flows in
the reverse direction
• Have the enzyme reverse
transcriptase
– Transcribes DNA from an
RNA template
• The newly made DNA then
integrates as a provirus into the
nucleus of the animal cell
• The host’s RNA polymerase
transcribes the viral DNA into
RNA molecules.
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• Several very dangerous “emergent viruses” have
risen to prominence.
– HIV, the AIDS virus, seemed to appear suddenly in
the early 1980s.
– Each year new strains of influenza virus cause
millions to miss work or class, and deaths are not
uncommon.
– Ebola virus has caused
hemorrhagic fevers
in central Africa
periodically since
1976.
Fig. 18.8a
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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Viral Diseases in Animals
• The damage caused depends on the ability of
infected tissue to regenerate by cell division.
– Cold virus – skin and mucous cells
• Easy to repair
– Poliovirus – motor neurons
• Permanent damage
• Vaccines: harmless variants
of pathogens that stimulate
the immune system defenses.
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Image: NMAH
• First vaccine developed in the late 1700s by
Edward Jenner to fight smallpox.
• Vaccines can help prevent but do little to cure
most viral infections.
• Antibiotics are powerless against viruses
• Some recently-developed drugs do combat some
viruses, mostly by interfering with viral nucleic
acid synthesis.
– AZT interferes with reverse transcriptase of HIV.
– Acyclovir inhibits herpes virus DNA synthesis.
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Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Plant viruses are serious
agricultural pests
• Can stunt plant growth and diminish crop yields.
• Most are RNA viruses with rod-shaped capsids
produced by a spiral of capsomeres.
Fig. 18.9a
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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Viruses may have evolved from
other mobile genetic elements
• Viruses are in between life and nonlife.
• An isolated virus is biologically inert and yet it
has a genetic program written in the universal
language of life.
• Although viruses are obligate intracellular
parasites that cannot reproduce independently, it
is hard to deny their evolutionary connection to
the living world.
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Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Because viruses depend on cells for their own
propagation, it is reasonable to assume that they
evolved after the first cells appeared.
• Most molecular biologists favor the hypothesis
that viruses originated from fragments of cellular
nucleic acids that could move from one cell to
another.
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Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Candidates for the original sources of viral
genomes include plasmids and transposons.
– Plasmids are small, circular DNA molecules that are
separate from chromosomes.
– Plasmids, found in bacteria and in the eukaryote
yeast, can replicate independently of the rest of the
cell and are occasionally be transferred between cells.
– Transposons are DNA segments that can move from
one location to another within a cell’s genome.
• Both plasmids and transposons are mobile
genetic elements.
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Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Virus, Viroids, and Prions
Virus
• DNA or RNA with protein coat (some w/membrane)
• Most infect specific types of cells in one host
Viroid:
• circular, single-stranded infectious RNA; no protein coat
• Cause plant diseases
Prions:
• Infectious proteins
• Inherited and transmissible by ingestion, transplant, & surgical
instruments
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Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Video
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