Sci-Tech Encyclopedia :
Virus
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Any of a heterogeneous class of agents that share three characteristics: (1) They consist of a nucleic acid genome surrounded by a protective protein shell, which may itself be enclosed within an envelope that includes a membrane; (2) they multiply
only inside living cells, and are absolutely dependent on the host
cells' synthetic and energy-yielding apparatus; (3) the initial step in
multiplication is the physical separation of the viral genome from its
protective shell, a process known as uncoating, which differentiates
viruses from all other obligatorily intracellular parasites. In essence, viruses are nucleic acid molecules, that is, genomes that can enter cells, replicate
in them, and encode proteins capable of forming protective shells
around them. Terms such as “organism” and “living” are not applicable
to viruses. It is preferable to refer to them as functionally active or
inactive rather than living or dead.
Alfalfa mosaic virus. (g) T4 bacteriophage. (h) M13 bacteriophage.">
Electron micrographs of highly purified preparations of some viruses. (a) Adenovirus. (b) Rotavirus. (c) Influenza virus (courtesy of George Leser). (d) Vesicular stomatitis virus. (e) Tobacco mosaic virus. (f) Alfalfa mosaic virus. (g) T4 bacteriophage. (h) M13 bacteriophage.
The
primary significance of viruses lies in two areas. First, viruses
destroy or modify the cells in which they multiply; they are potential
pathogens capable of causing disease. Many of the most important
diseases that afflictrabies, smallpox, poliomyelitis, hepatitis, influenza, the common cold, measles, mumps, chickenpox, herpes, rubella, hemorrhagic fevers, and the acquired immunodeficiency syndrome (AIDS) are caused by viruses. Viruses also cause diseases in livestock and plants that are of great economic importance. See also Acquired immune deficiency syndrome (AIDS); Plant pathology. humankind, including
Second,
viruses provide the simplest model systems for many basic problems in
biology. Their genomes are often no more than one-millionth the size
of, for example, the human genome; yet the principles that govern the
behavior of viral genes are the same as those that control the behavior
of human genes. Viruses thus afford unrivaled opportunities for
studying mechanisms that control the replication and expression of genetic material. See alsoHuman Genome Project.
Although
viruses differ widely in shape and size (see illustration), they are
constructed according to certain common principles. Basically, viruses
consist of nucleic acid and protein. The nucleic acid is the genome
which contains the information necessary for virus multiplication and
survival, the protein is arranged around the genome in the form of a
layer or shell that is termed the capsid, and the structure consisting of shell plus nucleic acid is the nucleocapsid. Some viruses are naked nucleocapsids. In others, the nucleocapsid is surrounded by a lipid bilayer
to the outside of which “spikes” composed of glycoproteins are
attached; this is termed the envelope. The complete virus particle is
known as the virion, a term that denotes both intactness of structure and the property of infectiousness.
Viral
genomes are astonishingly diverse. Some are DNA, others RNA; some are
double-stranded, others single-stranded; some are linear, others
circular; some have plus polarity, other minus (or negative) polarity;
some consist of one molecule, others of several (up to 12). They range
from 3000 to 280,000 base pairs if double-stranded, and from 5000 to
27,000 nucleotides if single-stranded. See also Virus classification.
Viral
genomes encode three types of genetic information. First, they encode
the structural proteins of virus particles. Second, most viruses encode
enzymes capable of transcribing their genomes into messenger RNAribosomes,
as well as nucleic acid polymerases capable of replicating their
genomes; many viruses also encode nonstructural proteins with catalytic
and other functions necessary for virus particle maturation and
morphogenesis. Third, many viruses encode proteins that interact with
components of host-cell defense mechanisms against invading infectious
agents. The more successful these proteins are in neutralizing these
defenses, the more virulent viruses are. molecules that are then translated by host-cell
The two most commonly observed virus-cell interactions are the lyticlysis
of the host cell; and the transforming interaction, which results in
the integration of the viral genome into the host genome and the
permanent transformation or alteration of the host cell with respect to
morphology,
growth habit, and the manner in which it interacts with other cells.
Transformed animal and plant cells are also capable of multiplying;
they often grow into tumors, and the viruses that cause such
transformation are known as tumor viruses. See alsoCancer (medicine); Oncology; Retrovirus; Tumor viruses. interaction, which results in virus multiplication and
There
is little that can be done to interfere with the growth of viruses,
since they multiply within cells, using the cells' synthetic
capabilities. The process, interruption of which has met with the most
success in preventing virus multiplication, is the replication of viral
genomes, which is almost always carried out by virus-encoded enzymes
that do not exist in uninfected cells and are therefore excellent
targets for antiviral chemotherapy. Another viral function that has been targeted is the cleavage
of polyproteins, precursors of structural proteins, to their functional
components by virus-encoded proteases; this strategy is being used with
some success in AIDS patients. See also Chemotherapy; Cytomegalovirus infection; Herpes; Influenza; Respiratory syncytial virus.
Antiviral agents on which much interest is focused are the interferons. Interferons are cytokines
or lymphokines that regulate cellular genes concerned with cell
division and the functioning of the immune system. Their formation is
strongly induced by virus infection; they provide the first line of
defense against viral infections until antibodies
begin to form. Interferons interfere with the multiplication of viruses
by preventing the translation of early viral messenger RNAs. As a
result, viral capsid proteins cannot be formed and no viral progeny results.
By
far the most effective means of preventing viral diseases is by means
of vaccines. There are two types of antiviral vaccines, inactivated
virus vaccines and attenuated active virus vaccines. Most of the
antiviral vaccines currently in use are of the latter kind. The
principle of antiviral vaccines is that inactivated virulent or active attenuated virus particles cause the formation of antibodies that neutralize a virulent virus when it invades the body. See also Animal virus; Plant viruses and viroids; Vaccination; Virus, defective.
