Читать книгу Basic Virology - Martinez J. Hewlett - Страница 25

Viruses are not the smallest or the simplest pathogens able to control their self‐replication in a host cell – that distinction goes to prions. Despite this, the methodology for the study of viruses and the diseases they cause provides the basic methodology for the study of all subcellular pathogens.

By the most basic definition, viruses are composed of a genome and one or more proteins coating that genome. The genetic information for such a protein coat and other information required for the replication of the genome are encoded in that genome. There are genetic variants of viruses that have lost information either for one or more coat proteins or for replication of the genome. Such virus‐derived entities are clearly related to a parental form with complete genetic information, and thus, the mutant forms are often termed defective virus particles.

Defective viruses require the coinfection of a helper virus for their replication; thus, they are parasitic on viruses. A prime example is hepatitis delta virus, which is completely dependent on coinfection with hepatitis B virus for its transmission.

The hepatitis delta virus has some properties in common with a group of RNA pathogens that infect plants and can replicate in them by still‐unknown mechanisms. Such RNA molecules, called viroids, do not encode any protein, but can be transmitted between plants by mechanical means and can be pathogens of great economic impact.

Some pathogens appear to be entirely composed of protein. These entities, called prions, appear to be cellular proteins with an unusual folding pattern. When they interact with normally folded proteins of the same sort in neural tissue, they appear to be able to induce abnormal refolding of the normal protein. This abnormally folded protein interferes with neuronal cell function and leads to disease. While much research needs to be done on prions, it is clear that they can be transmitted with some degree of efficiency among hosts, and they are extremely difficult to inactivate. Prion diseases of sheep and cattle (scrapie and “mad cow” disease) recently had major economic impacts on British agriculture, and several prion diseases (kuru and Creutzfeldt–Jacob disease[CJD]) affect humans. Disturbingly, the inadvertent passage of sheep scrapie through cattle in England has apparently led to the generation of a new form of human disease similar to, but distinct from, CJD. Details of this are covered in Part IV, Chapter 15.

The existence of such pathogens provides further circumstantial evidence for the idea that viruses are ultimately derived from cells. It also provides support for the possibility that viruses had multiple origins in evolutionary time.