Chapter 19 –
Viruses
(structure, reproduction,
pathogens)
Phages (TEM)
HIV infection (TEM) (CDC)
T4 bacteriophages infecting E. coli (colorized SEM)
Viral Evolution?
• They seem to exist in a gray area between life and biological
chemicals (“a kind of borrowed life”).
• If viruses are non-living, they don’t really die (they just go inactive).
• Yet, they do have genetic code so they have some evolutionary
connection to the living world. They can probably evolve very
quickly (constant pressure to dodge the host’s immune system).
• Most biologists believe that viruses originated from naked bits of
cellular nucleic acid (“mobile genetic elements”) such as:
– Plasmids (small circular DNA molecules that exist independently and
may be transferred between cells) or
– Transposons (chunks of DNA that can move from one location to
another in a cell’s genome).
Viruses have been used extensively in biological research.
• They showed evidence that genes are made up of
nucleic acids
• Helped us understand the molecular mechanisms of
DNA replication.
• Using virus research, we learned to manipulate genes
and transfer them from one organism to another
(agents of gene transfer in gene therapy).
Fig. 19-2. What causes
tobacco mosaic disease?
Fig. 19.3. Viral structure (colorized TEM).
Nucleic acid (DNA or RNA) enclosed in a protein coat (capsid).
(synthesis of RNA using a DNA template)
http://www.youtube.com/watch?v=Rpj0emEGShQ
Fig. 19.4. A simplified
viral reproductive cycle.
Fig. 19.5. The lytic
cycle of phage T4, a
virulent phage.
The entire cycle only
take 20-30 minutes.
Fig. 19.6. The lytic and lysogenic cycle of phage λ, a temporate phage.
Table 19.1. Classes of
animal viruses are
based on the nature of
the viral genome and
the presence/absence
of a viral envelope.
(Don’t memorize viral
classes and families).
Fig. 19.8. The
reproductive cycle of
HIV, the retrovirus that
causes AIDS.
Photos (left, TEM)
show HIV entering and
leaving white blood
cells.
DNA synthesized from
the viral RNA is
incorporated into the
white blood cell’s
chromosomal DNA
(provirus).
Viral diseases in plants
Thousands of viral
diseases in plants
destroying
agricultural and
horticultural crops.
Fig. 19.10. Viral infection of plants
Most viral plant
diseases have no
cure (yet).
Horizontal transmission: Infection from an external source of the virus. Most
likely when plants have been damaged (to allow entry for the virus)
Vertical transmission: Plant inherits viral infection from the parent.
Viroids and Prions
Viroids: Circular small RNA molecules that infect plants. A
single molecule can be an infectious agent that infects
plants.
Prions: Infectious proteins causing degenerative brain
diseases in animals. Example mad-cow disease spread by
eating infected food.
Alarming is that:
1. Prions act slowly with incubation periods on the
order of decades (sources of infection are not identified
until long after the first cases occur)
2. Prions are virtually indestructible!
A researcher lyses a cell that contains nucleic acid molecules
and capsomeres of tobacco mosaic virus (TMV). The cell
contents are left in a covered test tube overnight. The next day
this mixture is sprayed on tobacco plants. Which of the
following would be expected to occur?
A) The plants would develop some but not all of the symptoms of the
TMV infection.
B) The plants would develop symptoms typically produced by viroids.
C) The plants would develop the typical symptoms of TMV infection.
D) The plants would not show any disease symptoms.
E) The plants would become infected, but the sap from these plants
would be unable to infect other plants.