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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 (T1-T7). 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 System7
• 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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The Lambda Phage Infecting the E. coli
Animation11
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. 12
• 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.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 18.8a13
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.
Image: NMAH
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• 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.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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• Can stunt plant growth and diminish crop yields.
• Most are RNA viruses with rod-shaped capsids
produced by a spiral of capsomeres.
Plant viruses are serious
agricultural pests
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 18.9a 16
• 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.
Viruses may have evolved from
other mobile genetic elements
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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• 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.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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• 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.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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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
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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