Virus alive

In biology, the trouble with definitions is far worse than in mathematics. Few biologists ever use the word proof — it is far too exact. Doctors rarely use the word cure. Who knows, after all Remission is a more comfortable word, if only because it means very little "the illness seems to have gone away for now, for all I can see I have no idea if it will come back". Nature deftly sidesteps our clumsy swings at a definition. How does one define life Reproduction and some form of metabolism seem to be musts, but then is a virus alive It has no metabolism of its own and so falls outside most definitions of life. If you can define life to encompass a virus, then how about the infectious prion, which is simply a protein ... 

Consequences of virus infection in animal cells Viruses can have varied effects on cells. Lytic infection results in the destruction of the host cell. However, there are several other possible effects following viral infection of animal cells. In the case of enveloped viruses, release of the viral particles, which occurs by a kind of budding process, may be slow and the host cell may not be lysed. The cell may remain alive and continue to produce vims over a long period of time. Such infections are referred to as persistent infections. 

Observation of living systems shows them to be complex mixtures of different chemicals each of which functions to support one or more of the necessary tasks required to keep the organism alive . At one end of the spectrum of complexity lie the viruses and related simple proteins, such as prions, which cannot replicate by themselves but reproduce with the aid of other, higher, life forms. They therefore fulfill the definition of life to a very limited extent. At the other end of the spectrum there are the multicellular organisms such as humans with a vast diversity of processes that relate directly, and often more subtly, to the basic requirements of life. Even here, though, there is often a dependence on another living system. For example, humans cannot survive without vitamin Bi2 but are unable to synthesize it and must rely on external agents for its production. 

Virus particles are only about 0.1 to 0.01 microns in size (1 micron=0.001 millimeter). They can only be seen with the aid of an electron microscope. The typical irus can hardly be considered alive, consisting not of a cell but merely of nucleic acid iRNA or DN.Ai surrounded by a protein sheath. Viroids are even simpler—each is just a strand of RNA. 

Because viruses cannot grow or reproduce on their own, they are not considered to be alive. To survive, a virus must infect a host cell and take over its internal machinery to synthesize viral proteins and in some cases to replicate the viral genetic material. When newly made viruses are released, the cycle starts anew. Viruses are much smaller than cells, on the order of 100 nanometer (nm) in diameter in comparison, bacterial cells are usually > 1000 nm (1 nm= 10 meters). A virus is typically composed of a protein coat that encloses a core containing the genetic material, which carries the information for producing more viruses (Chapter 4). The coat protects a virus from the environment and allows it to stick to, or enter, specific host cells. In some viruses, the protein coat is surrounded by an outer membrane-like envelope. 

All living entities are comprised of cells. The cell is the basic living unit of all organisms. Smaller systems exist, for example viruses, but these can only become alive when they enter and usurp the cell s normal fimction. 

The Variola major virus is very stable and survives in exudates from patients for many months. The virus is unlikely to survive in dried crusts for more than a year [266]. Yet, it can be preserved in sealed ampules at 4°C for many years, and indefinitely by freeze-drying. This leads to the presumption that the virus still may be preserved alive within bodies of victims buried in permafrost. Much more feasible, however, the practical employment of smallpox virus for bioterrorism purposes is remarkably worrisome and reckoned to be a prime threat. 

Viruses are unable to grow or reproduce outside a host cell, but infection of viruses causes disruption of healthy homeostasis, resulting in cell death. Thus, viruses have to keep the host cell alive long enough for viral replication to occur. What is the mechanism by which viruses prevent premature cell death during infection Normally, replication of baculovirus in host insect cell results in the formation of polyhedra at around 24-h post-infection. However, infection of p35 mutant virus causes cell death beginning at around 9-12 h post-infection, and no polyhedra are formed [102]. This discovery opened a new avenue to identify a family of proteins called inhibitors of apoptosis (lAPs). 

Biomolecules are molecules that are specifically found inside of living things and have some function related to life. This includes molecules found in plants, animals, insects, bacteria, or even viruses (which are often considered not to be alive in a technical sense). Biomolecules span a wide range of sizes, some weighing only about 50 atomic mass units (amu), while others weigh millions of amu (see Atoms and Molecules to review the definition of amu). They are in our hair, skin, tissues, organs, and just about everywhere else in our bodies too. 

Many patients infected with the human immunodeficiency virus (HIV) that causes acquired immune deficiency syndrome (AIDS) are treated with a class of drugs called protease inhibitors. Before these drugs were developed in the early 1990s, HIV-infected individuals would die within a few years of diagnosis. Today, protease inhibitors, when given in combination with other drugs, can reduce HIV to undetectable levels and keep AIDS patients alive. 

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