Virus Mis Behavior

Among the more basic varieties of life are viruses. Their simple structure and activities have prompted questions as to whether they are actually "alive," or are merely chemical robots which multiply and reside within truly living organisms. Viruses form spontaneously as their simple subunits assemble into complex three dimensional structures. The assembly process, driven by the increased stability or lowered energy of the completed virus, flows against the second law of thermodynamics which dictates that order proceeds to disorder. Data from the tobacco mosaic virus (Chapter 6) show that hydrophobic interactions which exclude water from unassembled subunits offset the increase in order (negative entropy) of the assembled virus.

Figure 9.1: Phage virus landing and injecting genetic material into host cell. The cylindrical collar undergoes a collective conformational contraction associated with the injection. By Paul Jablonka.

Generally, viruses enter cells and pirate their genetic machinery to cause virus multiplication; they then escape to begin another cycle. Some viruses quietly and harmlessly reside within host cells for extremely long periods of time whereas the sojourn of other viruses involves the total commandeering of cell machinery and can have extremely damaging consequences for the host. Virus induced human diseases range from trivial common colds and influenza to chicken pox, measles, hepatitis, polio, and deadly diseases such as smallpox, rabies, yellow fever, cancer, and acquired immunodeficiency syndrome ("AIDS"). Some viral diseases such as smallpox have been controlled by modern medicine but others such as AIDS remain significant threats to human health.

Once assembled inside a host cell, viruses can escape in several ways. One route involves causing the death and disintegration of the host cell, allowing the newly formed viruses to spill out and carry their infection elsewhere. Viruses which are covered by lipid membranes can also escape by "budding" or reverse endocytosis ("pinocytosis"), a mechanism similar to secretion or extrusion of neurotransmitter vesicles from synaptic boutons.

An important subclass of viruses are retroviruses which contain RNA without DNA. When arriving within a host cell, the retrovirus brings an enzyme known as reverse transcriptase which converts the virus single stranded RNA into a double stranded DNA copy. This allows the virus to commandeer the host cell genetic machinery. Among the human retroviruses is HTLV-III (or "HIV"), carrier of AIDS.

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