Viruses and Human Tumors
1. The relatively few human tumors caused by viruses are primarily caused by double-stranded DNA tumor viruses. (Table 14.11)
Retroviruses and Human Tumors
1. A rare tumor, a leukemia, is caused by a retrovirus.
14.7 Viral Host Range
1. Most viruses can infect only a single species and only certain cells within an organism.
2. Viruses causing zoonoses can multiply in widely divergent species such as birds, mosquitoes, and humans.
3. Viruses can modify their host range if two viruses with different host ranges infect the same cell.
Exchange of Protein Coats—Phenotypic Mixing (Figures 14.16,14.17)
1. In phenotypic mixing, the genome of one virus may be surrounded by the coat of the other virus. The virus achieves the host range of the virus whose coat it wears.
2. The genome of the resulting virus will code for its original coat, and therefore it reverts to its original host range following its release.
Genome Exchange in Segmented Viruses (Figure 14.18)
1. Segmented viruses like influenza can expand their host range through an exchange of genomes following infection of the same cell by two viruses with different host ranges, the process of genetic reassortment.
2. This type of exchange likely accounts for the pandemics caused by the influenza virus every 10 to 30 years.
14.8 Plant Viruses
1. Many plant diseases are caused by viruses. (Figure 14.19)
2. Virions do not bind to receptor sites on plant cells, but enter through wound sites.
Spread of Plant Viruses
1. Many plant viruses are very resistant to inactivation.
2. Viruses are spread in large part by humans, by planting seeds in contaminated soils, through transfer from infected plants by grafting, and through the parasitic plant dodder, which can establish connections between an infected and uninfected plant. (Figure 14.21)
3. Various vectors, like insects and worms, can also spread virions.
Insect Transmission of Plant Viruses
1. Viruses may be associated with the external mouthparts of an insect or they may multiply within the insect.
Review Questions 369
14.9 Other Infectious Agents
Prions (Figure 1.14, Table 14.12)
1. Prions consist of protein and no nucleic acid, and they cause a number of fatal neurodegenerative diseases called transmissible spongiform encephalopathies.
2. Uninfected cells synthesize a protein similar in amino acid sequence to the prion protein.
3. Prions replicate by converting the normal cellular protein to the prion protein. (Figure 14.22)
Viroids (Figure 1.13)
1. Viroids are plant pathogens that consist of circular, single-stranded RNA molecules; they are about one-tenth the size of the smallest infectious viral RNA known.
2. Many unanswered questions remain regarding their origin, how they multiply, and how they cause disease.
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