One the major problems which had to be overcome in order to fully understand the nature of viruses was to find an efficient means to cultivate these infective agents. While bacteria could be grown easily on artificial media, viruses could not. Viral particles appeared as crystaline when purified, so their exact nature was unknown, and the name contagium vivum fluidum (contagious living fluid) was proposed. Two French scientists, Frederick Twort and Felix d'Herelle separately discovered that some viruses called bacteriophages, could infect and destroy bacterial cells, and two Americans, Robert Nye and Frederick Parker found that they could successfully raise animal viruses on cultures produced by cultivating eukaryote cells in artificial media. Such cell cultures could be used to study viruses of many types. Other workers would discover that some viruses could be cultured in chicken eggs (Goodpasture et al., 1931), bacteriophages pass DNA to their host cells (Hershey and Chase, 1952), and tobacco mosaic virus passes RNA to the host cell it infects (Fraenkel-Conrat et al., 1957). All of these discoveries, as well as those of other workers, would enhance our understanding of viruses and ultimately lead to the development of vaccines against many viral diseases, specific in-vitro tests to determine the presence of viruses in the body of an infected individual, and the synthesis of synthetic antiviral agents used to combat such diseases as herpes, influenza, and AIDS.
All viruses have two major components, a protein coat called a capsid, which is composed of individual protein subunits called capsomeres, and a nucleic acid core composed of either DNA or RNA, but not of both together. The combination of the capsid and nucleic acid core is called the nucleocapsid. Some viruses, such as influenza and HIV, have an external viral envelope, which is composed of cell membrane modified by the virus prior to release from the host cell. This envelope is responsible for the pathogenicity of the virus it surrounds, since it is permiated by protein "spikes" which serve as sites which bond to receptors on the cell membrane of potential new host cells. Nonenveloped viruses, such as the adenovirus, carry their protein spikes on the capsid itself.
Since the envelope is composed mostly of nonpolar lipids, it can be removed by contact with the nonpolar solvent ether. Enveloped viruses are thus referred to as ether soluble, since they can be inactivated, or attenuated in this way. Attenuated viruses can still elicit the production of antibodies by a host organism, so they can be used to produce vaccines against those viruses of the same type which still retain their envelope. Nonenveloped forms, however, are not inactivated by ether, so they still remain infective.
1. Attachment (Adsoption)- the virus attaches itself using
either its tail fibers (bacteriophage) or
protein "spikes" which bind to receptor-sites on the surface of the host cell wall or membrane.
Viruses are nonmotile, thus attachment depends on random brownian motion to bring the virus
into contact with the potential host cell.
2. Penetration and Uncoating- Once the virus has attached itself,
it can then penetrate the host cell
wall or membrane. The protein coat is not necessary for viral replication and must be removed
through the process called uncoating. Uncoating occurs along with attachment in bacteriophage
replication, since the bacteriophage "injects" its nucleic acid into the host cell. The empty protein
coat then will either remain attached to the host cell, or will fall off. In viruses which parasitize
eukaryotic cells, the process of uncoating depends upon the type of virus. Some nonenveloped
viruses such as the adenovirus simply attach, then physically force their nucleic acid into the cell,
while others trigger the host to pull them in via phagocytosis. Enveloped viruses incorporate their
envelope with the cell membrane of the host, allowing the virus to enter and be enzymatically
stripped of its protein coat.
Once uncoating has occurred, the nucleic acid of the virus remains in the cell for a period of time known as the eclipse phase. During this period, it is physically impossible to tell that the cell has actually been infected. Some varieties of virus have or produce DNA which incorporates with the genome of the host cell to become a prophage (bacteriophage) or provirus (eukaryotic viruses). This set of viral genes may remain untranscribed for long periods of time, during which the cell continues to live and metabolize normally, even to replicate and give rise to new viral infected cells. When this occurs, the viral cycle is said to have become temperate, or to have entered the temperate or lysogenic phase. If viral DNA is incorporated into the chromosome of some bacteria, these can gain the ability to become pathogenic (i.e. to switch on latent genes which encode for pathogenic properties such as the production of toxins). This phenomenon is called lysogenic conversion, and can occur in such bacterial genera as Staphylococcus, Bacillus, Haemophilus, and Clostridium.
3. Maturation and Assembly- When the viral nucleic acid
does begin to express itself, either
through the activity of viral enzymes, or transcription of incorporated viral DNA, the normal
metabolic activities of the cell cease, and the cell's own biosynthetic processes are used in the
production of early proteins, which are subunits of the virus protein coat and/or portions of the
viral envelope, as well as new viral nucleic acids (either DNA or RNA, depending upon the type
of virus infecting the cell). Early proteins are assembled inside the cell, becoming late proteins,
which surround the new viral nucleic acids. While this maturation process is occurring, clusters
of new virions, called inclusion bodies, become visible within the infected cell. Inclusion
bodies (example- Cowdry bodies in cells infected with the rabies virus), malformed cells, and
damaged or destroyed cells are called cytopathic effects.
4. Release- In the cycle of the bacteriophage and
many of the eukaryote-infecting viruses, release
occurs when the cell fills with new virions and lyses, spilling the new viruses into the surrounding
environment. Some forms, however, take a much slower approach. Enveloped viruses must
wrap a bit of modified host-cell membrane around themselves, so release occurs by a viral
enzyme-mediated process called budding, releasing just a few viruses at a time, until the host
cell runs out of ATP and dies.
When a bacteriophage contacts a host cell, it attaches itself by protenaceous tail fibers, which change their conformation on cell contact, allowing a base plate to come into contact with the host cell wall. Small protein spikes penetrate the wall (G+ host) or membrane (G- host) to hold the virus in place, then the sheath contracts, bringing the tube into contact with the cell. Lysozyme digests a small hole at the point of contact, so the tube can penetrate between the cell wall and the inner cell membrane. The viral nucleic acid is then injected into this space, where it is transported passively into the cytoplasm of the host. If the bacteriophage injects DNA, this serves as a template for mRNA, which codes for the production of early proteins. These early proteins include viral structural components and enzymes which enable the replication of new viral DNA, and lysozyme. The new virions begin to assemble or amplify within the host cell, until there is no more room in the cytoplasm. Lysozyme then causes lysis of the cell wall, and the new virions are freed to infect new host cells.
Some bacteriophages contain RNA which is designated
as either + or - stranded. + (plus) stranded
RNA serves immediately as mRNA, so that early viral proteins can be
produced within the host cell cytoplasm. - (minus) stranded
RNA serves as a template to produce new + stranded RNA, which then
can be used to produce early proteins. Other bacteriophages contain
double-stranded RNA, which when in the cytoplasm of the host cell
separates into + and - strands. In all of the RNA bacteriophage types,
assembly and release of new virions occurs in a fashion which is similar
to that of the T-even DNA bacteriophages, by lysis of the host cell.
In some instances, however, phenotypic
properties of a prophage-infected host are altered owing to the presence
of viral DNA. This phenomenon is called lysogenic conversion, and
can result in the production of structures or substances which enhance
the pathogenicity of some bacteria. Examples of species which become
pathogenic via lysogenic conversion include Corynebacterium diptheriae,
the agent of diptheria, Clostridium botulinum, which causes botulism,
Staphylococcus aureus, an inhabitant of the nasal cavity
which causes food intoxication, toxic shock syndrome, scalded skin syndrome
in infants, as well as boils and other forms of localized inflammations,
and Streptococcus pyogenes, the agent of strep-throat, scarlet fever,
and necrotizing faciatus. Each of these organisms produces exotoxins
only when infected by a prophage.
The synthesis of early and late proteins, as well
as the synthesis of new nucleic acids dependes on the nature of the type
of animal virus nucleic acid. Viruses which contain double-stranded
DNA take several hours to uncoat, then enter the host nucleus and stop
normal biosynthetic activities. These viruses serve as templates
for their own replication, and are released by lysis. Single-stranded
DNA viruses uncoat, then utilize host cell components to synthesize their
missing complementary DNA nucleoside. After the double strand is
produced, it can serve as a template for replication. Release is
also by host cell lysis. RNA viruses can also be double or single
stranded. Double-stranded RNA viruses do not enter the nucleus of
the host cell; they contain their own RNA polymerase which is used for
self-replication of viral nucleic acids upon uncoating in the host cell
cytoplasm. Once the two pieces of RNA have been separated, the +
strand serves as mRNA to build new early proteins. These are assembled
into capsids surrounding the plus strand and integral RNA replicase then
is used to synthesize - strands. The new virions are released by
host cell lysis. Single-stranded RNA viruses contain either + or
- strands. The poliovirus, which is a + stranded form, uses its RNA
as mRNA to produce a large polypeptide which is enzymatically cleaved to
produce RNA polymerase. This is then used to synthesize - RNA strands
and early capsid proteins. The assembly of new virions triggers the
breakdown of host lysosomes, leading to host cell autolysis. This releases
the newly formed viruses into the interstitial fluids, where they can infect
new cells. The influenza virus, a - stranded form, replicates after
uncoating by using its RNA as a template to make new + strands, which serve
to produce early proteins. Some of these early proteins serve as
capsid components, while others, neuriminidase and hemagglutinin,
are incorporated into the cell membrane of the host. The new virions
are releasd by budding, meaning that a portion of the host cell membrane
surrounds each new capsid as it is released from the cell. This membrane
becomes the viral envelope, which enables the new virus to mimic the antigenic
structure of its host. The placement and composition of neuraminidase
and hemagglutinin spikes on the envelope of an influenza virus is highly
variable, with as many as eight different types of - strands. This
is due to two phenomena called antigenic drift and antigenic shift.
Antigenic drift is caused by mutation in viral genes which code
for the production of both of these spikes in newly replicated viruses.
Antigenic shift occurs due to gene reassortment when two different
strains of an influenza virus infect a single host cell, leading to recombination
of the viral genome prior to synthesis of early proteins and synthesis
of new virions. Drift and shift of viral antigens are important sources
of variation in the influenza virus and can lead to the development of
strains which are not as likely to be recognized and targeted by host immune
responses, increasing pathogenicity.
1. Type of nucleic acid- DNA or RNA.
2. Type of nucleic acid- single or double stranded.
3. Capsid morphology- helical, bullet-shaped, isocohedral, circular, pleomorphic.
4. Presence or absence of an envelope.
There are five classes of RNA viruses, and three classes of DNA viruses
which infect animal cells.
2. Suppose you were asked to develop a chemotheraputic compound
to treat individuals
suffering form viral infections. How would you attack this problem? Could there be more than
one way to prevent a virus from infecting a host cell, or would you attempt to stop the virus
from replicating once infection has taken place? Defend your answer(s).
3. In terms of the ability to infect animal cells, do enveloped
viruses have any advantage(s)
over nonenveloped viruses? Why do you think plant viruses such as the tobacco mosaic virus
(TMV) have not evolved an envelope?
Test Yourself- Use this quiz to test
your understanding of viruses.