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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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chapter 19 viruses structure reproduction pathogens

Chapter 19 – Viruses(structure, reproduction, pathogens)

Phages (TEM)

T4 bacteriophages infecting E. coli (colorized SEM)

HIV infection (TEM) (CDC)

viral evolution
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
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. 19.3. Viral structure (colorized TEM).

Nucleic acid (DNA or RNA) enclosed in a protein coat (capsid).


(synthesis of RNA using a DNA template)

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.


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.

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 and prions

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.