Читать книгу Basic Virology - Martinez J. Hewlett - Страница 26

1 Viruses are a part of the biosphere. However, there is active debate concerning whether they should be treated as living or nonliving.Briefly describe one feature of viruses that is also found in cell‐based life forms.Briefly describe one feature of viruses that distinguishes them from cell‐based life forms.

2 Why is it likely that viruses have not evolved from free‐living organisms?

3 Give examples of infectious agents that are smaller self‐replicating systems than viruses.

4 Ebola virus is a deadly (90% case‐fatality rate for some strains) infectious agent. Most viruses, however, are not nearly as lethal. Given the nature of viruses, why would you expect this to be so?

5 Given that viruses are a part of the biosphere in which other organisms exist, what might be the kinds of selective pressure that viruses exert on evolution?

6 Viruses were originally discovered because of their size, relative to known bacterial cells. Tobacco mosaic virus was called a “filterable infectious agent” by this criterion. Why is size not a good defining feature for viruses? What is a better definition?