Scientists have been able to coax crystal structures to change behavior when exposed to blue light. Though this does not mean the crystals are alive, they exhibit a response to a stimulus, just as you respond to having your name called or your leg pinched.
Everything alive shows a response to stimuli. Biologists, biology students, and thinking people all ponder the meaning of life. What is life? When is something alive, and when does it only appear to be alive? The list of characteristics of life is not finished. As humans explore beyond our reach, we find more puzzles. Extremophiles have been discovered living around hydrothermal vents on the seafloor where no light reaches.
How can they, or acid-loving, radiation-resistant organisms like exist? Individual cells carry on life processes themselves. The characteristics of a cell :. Cells differentiate and build on these basic functions by either not having a nucleus prokaryotic or having a nucleus eukaryotic. We mentioned that scientists coaxed crystals into showing response to a stimulus.
So are crystals alive? This is the danger of relying on only one of the eight characteristics of life. For something to be alive, it must exhibit all eight characteristics. Crystals accrete, which could appear to show growth and development, but they cannot reproduce, cannot metabolize resources, cannot excrete waste, and have no cell structures to store DNA.
Viruses have all of the characteristics of living things except cellular organization, homeostasis, metabolism, and response to stimuli. Currently, most biologists, virologists, doctors, and general scientists say viruses are not alive. That thinking may change as more evidence, such as the giant mimivirus , is revealed.
It may sound silly, but a good test of life's definition is to apply it to things we think of as non-living. How does a computer stack up? Once more, to the list! Since computers have only evolution, a kind of metabolism, and response to stimuli, they cannot be considered alive. They lack five of the eight characteristics of life. Get better grades with tutoring from top-rated professional tutors.
Several viruses of medical importance still remain unclassified. Some are difficult or impossible to propagate in standard laboratory host systems and thus cannot be obtained in sufficient quantity to permit more precise characterization. Hepatitis E virus, the Norwalk virus and similar agents see Ch.
The fatal transmissible dementias in humans and other animals scrapie in sheep and goat; bovine spongiform encephalopathy in cattle, transmissible mink encephalopathy; Kuru, Creutzfeldt-Jakob disease, and Gerstmann-Straussler-Scheinker syndrome in humans see Ch.
The agents causing transmissible subacute spongiform encephalopathies have been linked to viroids or virinos i. Some of the transmissible amyloidoses show a familial pattern and can be explained by defined mutations which render a primary soluble glycoprotein insoluble, which in turn leads to the pathognomonic accumulation of amyloid fibers and plaques.
The pathogenesis of the sporadic amyloidoses, however, is still a matter of highly ambitious research. Turn recording back on. National Center for Biotechnology Information , U. Show details Baron S, editor. Search term. General Concepts Structure and Function Viruses are small obligate intracellular parasites, which by definition contain either a RNA or DNA genome surrounded by a protective, virus-coded protein coat.
Classification of Viruses Morphology: Viruses are grouped on the basis of size and shape, chemical composition and structure of the genome, and mode of replication. Nomenclature Aside from physical data, genome structure and mode of replication are criteria applied in the classification and nomenclature of viruses, including the chemical composition and configuration of the nucleic acid, whether the genome is monopartite or multipartite.
Structure and Function Viruses are inert outside the host cell. Classification of Viruses Viruses are classified on the basis of morphology, chemical composition, and mode of replication.
Morphology Helical Symmetry In the replication of viruses with helical symmetry, identical protein subunits protomers self-assemble into a helical array surrounding the nucleic acid, which follows a similar spiral path.
Figure The helical structure of the rigid tobacco mosaic virus rod. Figure Fragments of flexible helical nucleocapsids NC of Sendai virus, a paramyxovirus, are seen either within the protective envelope E or free, after rupture of the envelope. Icosahedral Symmetry An icosahedron is a polyhedron having 20 equilateral triangular faces and 12 vertices Fig.
Figure Icosahedral models seen, left to right, on fivefold, threefold, and twofold axes of rotational symmetry. Figure Adenovirus after negative stain electron microscopy. Virus Core Structure Except in helical nucleocapsids, little is known about the packaging or organization of the viral genome within the core. Figure Two-dimensional diagram of HIV-1 correlating immuno- electron microscopic findings with the recent nomenclature for the structural components in a 2-letter code and with the molecular weights of the virus structural glyco- proteins.
Figure Schemes of 21 virus families infecting humans showing a number of distinctive criteria: presence of an envelope or double- capsid and internal nucleic acid genome. Virus Classification On the basis of shared properties viruses are grouped at different hierarchical levels of order, family, subfamily, genus and species.
JB Lippincott, Philadelphia, Gajdusek DC. Unconventional viruses and the origin and disappearance of kuru. Gelderblom HR. Assembly and morphology of HIV: potential effect of structure on viral function. Mattern CFT: Symmetry in virus architecture. Marcel Dekker, New York, Raven Press, New York, Springer-Verlag, New York, Elsevier, Amsterdam, Structure and Classification of Viruses.
In: Baron S, editor. Chapter In this Page. Related information. PubMed Links to PubMed. Similar articles in PubMed.
Review Retroviral Virions and Genomes [Retroviruses. Review Molecular mechanisms of virus spread and virion components as tools of virulence. A review. Acta Microbiol Immunol Hung. Review First glimpses at structure-function relationships of the nucleocapsid protein of retroviruses. J Mol Biol. Packaging of a unit-length viral genome: the role of nucleotides and the gpD decoration protein in stable nucleocapsid assembly in bacteriophage lambda.
Epub Sep 3. Review [The great virus comeback]. Forterre P. Biol Aujourdhui. Epub Dec Recent Activity. Clear Turn Off Turn On. Structure and Classification of Viruses - Medical Microbiology. Support Center Support Center. External link. Release often takes place in a destructive manner, bursting and killing the host cell.
Some viruses have a slightly more complicated replication cycle involving lytic and lysogenic phases. The lytic phase is similar to that described above, with virus particles infecting and being replicated. In the lysogenic phase, however, viral genetic material that has entered the host cell becomes incorportated in the cell and lies dormant. It is passed on to the progeny of the infected cells. Eventually, the lytic phase will start again, and cells that were never infected themselves, but carry the viral genetic material will begin to produce new virus particles.
SparkTeach Teacher's Handbook. Please wait while we process your payment. Sign up and get instant access to save the page as your favorite. Summary General Characteristics of Viruses. Structure Because most viruses are extremely well adapted to their host organism, virus structure varies greatly.
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