Viruses and responses to viral infection WORLD OF MICROBIOLOGY AND IMMUNOLOGY582•
from DNA to RNA prior to packaging of the genetic material
into the new virus particles.
In the final stage, the viral particles are assembled and
exit the host cell. The assembly process can involve helper pro-
teins, made by the virus or the host. These are also called chap-
erones. Other viruses, such as tobacco mosaic virus, do not
need these helper chaperones, as the proteins that form the build-
ing blocks of the new particles spontaneously self-assemble. In
most cases, the assembly of viruses is symmetrical; that is, the
structure is the same throughout the viral particle. For example,
in the tobacco mosaic virus, the proteins constituents associate
with each other at a slight angle, producing a symmetrical helix.
Addition of more particles causes the helix to coil “upward”
forming a particle. An exception to the symmetrical assembly is
the bacteriophage. These viruses have a head region that is sup-
ported by legs that are very different in structure. Bacteriophage
assembly is very highly coordinated, involving the separate
manufacture of the component parts and the direct fitting
together of the components in a sequential fashion.
Release of viruses can occur by a process called bud-
ding. A membrane “bleb” containing the virus particle is
formed at the surface of the cell and is pinched off. For herpes
virus this is in fact how the viral membrane is acquired. In
other words, the viral membrane is a host-derived membrane.
Other viruses, such as bacteriophage, may burst the host cell,
spewing out the many progeny virus particles. But many
viruses do not adopt such a host destructive process, as it lim-
its the time of an infection due to destruction of the host cells
needed for future replication.See alsoHerpes and herpes virus; Human immunodeficiency
virus (HIV); Invasiveness and intracellular infectionVIRUSES AND RESPONSES TO VIRAL
INFECTIONViruses and responses to viral infectionThere are a number of different viruses that challenge the
human immune system and that may produce disease inhumans. In common, viruses are small, infectious agents that
consist of a core of genetic material—either deoxyribonucleic
acid(DNA) or ribonucleic acid(RNA)—surrounded by a shell
of protein. Although precise mechanisms vary, viruses cause
disease by infecting a host cell and commandeering the host
cell’s synthetic capabilities to produce more viruses. The
newly made viruses then leave the host cell, sometimes killing
it in the process, and proceed to infect other cells within the
host. Because viruses invade cells, drug therapies have not yet
been designed to kill viruses, although some have been devel-
oped to inhibit their growth. The human immune system is the
main defense against a viral disease.
Bacterial viruses, called bacteriophages, infect a variety
of bacteria, such as Escherichia coli,a bacteria commonly
found in the human digestive tract. Animal viruses cause a
variety of fatal diseases. Acquired Immunodeficiency
Syndrome(AIDS) is caused by the Human Immunodeficiency
Virus(HIV); hepatitisand rabiesare viral diseases; and hem-
orrhagic fevers, which are characterized by severe internal
bleeding, are caused by filoviruses. Other animal viruses
cause some of the most common human diseases. Often these
diseases strike in childhood. Measles, mumps, and chickenpox
are viral diseases. The common coldand influenzaare also
caused by viruses. Finally, some viruses can cause cancer and
tumors. One such virus, Human T-cell Leukemia Virus(HTLV),
was only recently discovered and its role in the development
of a particular kind of leukemia is still being elucidated.
Although viral structure varies considerably between
the different types of viruses, all viruses share some common
characteristics. All viruses contain either RNAor DNA sur-
rounded by a protective protein shell called a capsid. Some
viruses have a double strand of DNA, others a single strand
of DNA. Other viruses have a double strand of RNA or a sin-
gle strand of RNA. The size of the genetic material of viruses
is often quite small. Compared to the 100,000 genes that exist
within human DNA, viral genes number from 10 to about 200
genes.
Viruses contain such small amounts of genetic material
because the only activity that they perform independently of a
host cell is the synthesis of the protein capsid. In order to
reproduce, a virus must infect a host cell and take over the host
cell’s synthetic machinery. This aspect of viruses—that the
virus does not appear to be “alive” until it infects a host cell—
has led to controversy in describing the nature of viruses. Are
they living or non-living? When viruses are not inside a host
cell, they do not appear to carry out many of the functions
ascribed to living things, such as reproduction, metabolism,
and movement. When they infect a host cell, they acquire
these capabilities. Thus, viruses are both living and non-living.
It was once acceptable to describe viruses as agents that exist
on the boundary between living and non-living; however, a
more accurate description of viruses is that they are either
active or inactive, a description that leaves the question of life
behind altogether.
All viruses consist of genetic material surrounded by a
capsid; but variations exist within this basic structure.
Studding the envelope of these viruses are protein “spikes.”
These spikes are clearly visible on some viruses, such as theGrowth of virus causes clearing (plaques) in lawn of Escherichia coli
culture on agar.womi_V 5/7/03 11:00 AM Page 582