Chapter 17- Viruses
-What is a virus?
-Virus: an infectious particle incapable of replicating outside of a host cell
-Much smaller than bacteria (smaller than a ribosome)
-Cannot replicate or carry out metabolism outside of a host cell
-Genomes can be linear or circular and may consist of double stranded DNA, single stranded
DNA, double stranded RNA, or single stranded RNA
-Scientists don’t consider viruses to be alive, but classify them in a grey area between life forms
and chemicals.
-Structure of the virus
-Viruses consist of nucleic acid enclosed in a protein coat (and maybe surrounded by a
membranous envelope)
-Capsid: the protein shell enclosing the viral genome
-Capsomeres: the protein subunits that make up the capsid
-Viruses can be different shapes (helical, icosahedral, etc.)
-Viral envelope: membrane that surrounds the capsid (derived from the membrane of host cells)
-Bacteriophages (phages): the viruses that infect bacteria
-Mechanisms of viral replication
-Viruses can replicate only within a host cell because they lack the equipment needed for
making proteins and metabolic enzymes
-Each particular virus can infect cells of only a limited number of host species
-Host range: the limited number of species whose cells can be infected by a particular
virus
-Viral host ranges can be broad or narrow
-Viruses recognize host cells through protein interactions that take place on the surface
of the cell/virus
-Viral infection of multicellular eukaryotes is usually limited to particular tissues
-General features of viral replicative cycles
1. Viral infection begins when virus binds to a host cell and the viral genome makes its way
inside.
2. Once the viral genome is inside, the proteins it encodes can commandeer the host,
reprogramming the cell to copy the viral nucleic acid and manufacture viral proteins
3. After the viral nucleic acid molecules and capsomeres are produced by the host cell, they
spontaneously self-assemble into new viruses
4. Viral infections end with the exit of hundreds or thousands or viruses from the infected host
cell
5. Viral progeny that exit a cell has the potential to infect additional cells, spreading the viral
infection
-Replicative cycles of phages
-There are two mechanisms for phage replication: the lytic cycle and the lysogenic cycle
-Lytic cycle: a phage replicative cycle that culminates in the death of the host cell
-Virulent phage: a phage that replicates only by a lytic cycle
-Virulent phages have not been successful in destroying all bacteria because:
1. Bacteria have natural defenses to viruses
2. Viral DNA is often identified by restriction enzymes in the bacterial cell
-Restriction enzyme: an enzyme that recognizes and cuts DNA
molecules foreign to a bacterium
3. Many phages coexist with bacteria cells in a state called lysogeny
-Lysogenic Cycle: a phage replicative cycle that allows replication of the phage without
destroying the host
-Temperate phage: a phage that is capable of replicating by either a lytic or lysogenic
cycle
-During the lysogenic cycle, the viral DNA molecule is incorporated into the host DNA
-Prophage: a phage genome that has been inserted into a specific side on a
bacterial chromosome
-The phage genome is mostly silent within the bacterium
-Every time the host cell divides, it replicates the phage DNA along with its own and
passes the copies on to daughter cells
-An environmental signal (chemicals or radiation, etc) is usually what triggers the switch
over from lysogenic to the lytic mode
Replicative cycles of animal viruses
-Many animal viruses have an RNA genome and an envelope, which cause their replicative cycles
to vary from those of phages
-Animal viruses equipped with an envelope uses it to enter the host cell
-Glycoproteins protruding from the outer surface of the envelope bind to specific
receptor molecules on the surface of the host cell
-Viral progeny leaving the host cell use the host cell’s membrane to synthesize envelopes
-The viral capsids are wrapped in membrane as they bud from the host cell in a process
much like exocytosis
-Some of the molecules found in the envelope are not derived from the host cell, but
are specified by viral genes
-RNA as viral genetic material in animal viruses
-Viral RNA can be incorporated into animal cells in 3 ways:
1. The viral genome can directly serve as mRNA, so it can be translated
immediately into viral protein once in the host cell
2. The RNA genome serves as a template for mRNA synthesis
3. The RNA can be transcribed into DNA
-Reverse transcriptase: the enzyme that carries out RNA  DNA
transcription
-Retrovirus: an RNA virus that replicates by transcribing its RNA into
DNA and then inserting the DNA into a cellular chromosome
-A class of cancer-causing viruses
-HIV is a retrovirus
-Provirus: a viral genome that is permanently inserted into a host
genome
-Evolution of viruses
-Viruses cannot replicate or carry out metabolic activities independently, but their use of the
genetic code proves that they have an evolutionary connection to the living world
-Viruses infect every from of life; it seems that they evolved after the first cells appeared
-Most scientists believe that viruses originated from singular bits of cellular nucleic acids that
moved from one cell to another
-Viral genomes likely came from one of two sources that are both mobile genetic elements:
1. Plasmids: small, circular DNA found in bacteria and unicellular eukaryotes
-Plasmids are separate from a cell’s genome, can replicate independently from
the cell’s genome, and are occasionally transferred between cells
2. Transposons: DNA segments that can move from one location to another within a
cell’s genome
-There is an ongoing evolutionary relationship between viruses and their hosts that is becoming
very useful in biological research
-Viruses as pathogens
-Viral diseases in animals
-How much damage a virus causes depends on the ability of the infected tissue to
regenerate by cell division
Ex. Common cold: affects epithelium of respiratory tract; easily repaired
Ex. Poliovirus: affects mature nerve cells; not easily repaired
-Vaccine: a harmless derivative of a pathogen that stimulates the immune system to
mount defenses against the harmful pathogen.
-Vaccines may prevent certain viral illnesses, but medical technology can do little to cure
viral infections
-Antibiotics: kill bacterial infections only
-Antiviral drugs can interfere with viral nucleic acid synthesis
-Emerging viruses: viruses that suddenly become apparent
Ex. HIV
-Epidemic: a general outbreak
-Pandemic: a global epidemic
-Three sources of new viral diseases:
1. Mutation of existing viruses
-Viruses mutate frequently because their RNA genomes cannot
proofread or correct errors
2. Spreading of a viral disease from a small, isolated human population
3. The spread of existing viruses from other animals
-Emerging viruses are generally not new; they are existing viruses that mutate,
spread to new host species, or spread from isolated populations
-Plant viruses
-More than 2,000 viral plant diseases are known and they cause for an annual loss of
$15 billion worldwide.
-Symptoms of viral infections in plants: bleached/brown spots on leaves/fruits, stunted
growth, damaged flowers or roots, diminished yield
-Two major routes of viral disease spreading in plants:
1. Horizontal transmission: plant is infected from an external source like
herbivores
2. Vertical transmission: a plant inherits a viral infection from a parent because
of asexual reproduction
-Viral genomes spread throughout the plant by means of plasmodesmata (cytoplasmic
connections that penetrate the walls between adjacent plant cells)
-Plants are more likely to be infected by a virus if it has been damaged by wind, injury,
or herbivores