The characteristics of living cells were outlined early in this chapter. A common, but somewhat technical, definition of a living cell is as follows: Living cells contain both DNA and RNA and can use the genetic information encoded in these to synthesize proteins by using energy that they generate themselves. This definition is designed not so much to explain, positively, how a cell works as to exclude viruses from the realm of living cells. The essential features of a virus are shown in Figure 2.28. Viruses are packages of genes in protein coats and are much smaller than bacteria. Viruses are obligate parasites that must infect a host cell in order to replicate themselves. Whether viruses are alive or not is a matter of opinion; however, viruses are certainly not living cells. Virus particles (virions) do contain genetic information in the form of DNA or RNA, but are incapable of growth or division by themselves. A virus may have its parasite An organism or genetic entity that replicates at the expense of another creature virion A virus particle virus Subcellular parasite with genes of DNA or RNA which replicates inside the host cell upon which it relies for energy and protein synthesis. In addition, it has an extracellular form, in which the virus genes are contained inside a protective coat
A virus is composed of a protein coat and nucleic acid. Note that there are no ribosomes or phospholipid membranes and only one type of nucleic acid is present.
Bacterial Viruses Infect Bacteria 47
genome made of DNA or RNA, but only one type of nucleic acid is present in the virion of any given type of virus.
Viruses lack the machinery to generate their own energy or to synthesize protein. After invading a host cell, the virus does not grow and divide like a cell itself. The virion disassembles and the virus genes are expressed using the machinery of the host cell. In particular, viral proteins are made by the host cell ribosomes, using virus genetic information. In many cases, only the virus DNA or RNA enters the host cell and the other components are abandoned outside. After infection, virus components are manufactured by the infected cell, as directed by the virus, and are assembled into new virus particles. Usually the host cell is killed and disintegrates. Typically, several hundred viruses may be released from a single infected cell. The viruses then abandon the cell and look for another host. [Note that some viruses cause "chronic" or "persistent" infections where virus particles are made slowly and released intermittently rather than as a single burst. In this case the host cell may survive for a long time despite infection. In addition, many viruses may persist inside the host cell for a long time in a latent, non-replicating, state and only change to replicative mode under certain conditions—see Ch. 17.]
Some scientists regard viruses as being alive based on the viral possession of genetic information. The majority, however, do not accept that viruses are truly alive, since viruses are unable to generate energy or to synthesize protein. Viruses are thus on the borderline between living and non-living. Virus particles are in suspended animation, waiting for a genuine living cell to come along so they can infect it and replicate themselves. Nonetheless, a host cell whose life processes have been subverted by a virus does duplicate the viral genetic information and produces more virus particles. Thus viruses possess some of the properties of living creatures. Viruses are very important from a practical viewpoint. Firstly, many serious diseases are due to virus infection. Secondly, many genetic manipulations that are now used in genetic engineering are carried out using viruses.
Merely being a parasite does not prevent an organism from being a living organism. For example, rickettsias are degenerate bacteria that cause typhus fever and related diseases. They cannot grow and divide unless they infect a suitable host cell. However, rickettsias can generate energy and make their own proteins, provided they obtain sufficient complex nutrients from the animal cell they invade. Furthermore, rick-ettsias reproduce by growing and dividing like other bacteria. Viruses are subcellular parasites, totally dependent on other life forms for their energy, materials and even the equipment to manufacture their own components.
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