Chapter 19 – Viruses (structure, reproduction, pathogens). Phages (TEM). T4 bacteriophages infecting E. coli (colorized SEM). HIV infection (TEM) (CDC). Viral Evolution?. They seem to exist in a gray area between life and biological chemicals (“a kind of borrowed life”).
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T4 bacteriophages infecting E. coli (colorized SEM)
HIV infection (TEM) (CDC)
Nucleic acid (DNA or RNA) enclosed in a protein coat (capsid).
Fig. 19.4. A simplified viral reproductive cycle.
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 that causes AIDS.
Thousands of viral diseases in plants destroying agricultural and horticultural crops.
Most viral plant diseases have no cure (yet).
Fig. 19.10. Viral infection of plants
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: 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.