The science of virusesvirology


What is virology

Virology, often considered a part ofmolecular machinery traditionally
microbiology or of pathology, is theassociated with bacteria. Is it a
study of biological viruses and virussimplified version of a parasitic
like agents: their structure andprokaryote, or did it originate as a
classification, their ways to infect andsimpler virus that acquired genes from
exploit cells for virus reproduction,its host?
the diseases they cause, the techniquesWhile viruses reproduce and evolve, they
to isolate and culture them, and theirdon't engage in metabolism and depend on
potential uses in research and therapy.a host cell for reproduction. The
A researcher in virology is aoften-debated question of whether they
virologist.are alive or not is a matter of
Virus structure and classificationdefinition that does not affect the
A major branch of virology is virusbiological reality of viruses.
classification. Viruses can beViral diseases and host defenses
classified according to the host cellOne main motivation for the study of
they infect: animal viruses, plantviruses is the fact that they cause many
viruses, fungal viruses, andimportant infectious diseases, among
bacteriophages (viruses infectingthem the common cold, influenza, rabies,
bacteria, which include the most complexmeasles, many forms of diarrhea,
viruses). Another classification useshepatitis, yellow fever, polio, smallpox
the geometrical shape of their capsidand AIDS. Some viruses, known as
(often a helix or an icosahedron) or theoncoviruses, contribute to certain forms
virus's structure (e.g. presence orof cancer; the best studied example is
absence of a lipid envelope). Virusesthe association between Human
range in size from about 30 nm to aboutpapillomavirus and cervical cancer. Some
450 nm, which means that most of themsubviral particles also cause disease:
cannot be seen with light microscopes.Kuru and Creutzfeldt-Jakob disease are
The shape and structure of viruses cancaused by prions, and hepatitis D is due
be studied with electron microscopy,to a satellite virus.
with NMR spectroscopy, and mostThe study of the manner in which viruses
importantly with X-ray crystallography.cause disease is viral pathogenesis. The
The most useful and most widely useddegree to which a virus causes disease
classification system distinguishesis its virulence.
viruses according to the type of nucleicWhen the immune system of a vertebrate
acid they use as genetic material andencounters a virus, it produces specific
the viral replication method they employantibodies which bind to the virus and
to coax host cells into producing moremark it for destruction. The presence of
viruses:these antibodies is often used to
DNA viruses (divided intodetermine whether a person has been
double-stranded DNA viruses and the muchexposed to a given virus in the past,
less common single-stranded DNAwith tests such as ELISA. Vaccinations
viruses),protect against viral diseases, in part,
RNA viruses (divided into positive-senseby eliciting the production of
single-stranded RNA viruses,antibodies. Specifically constructed
negative-sense single-stranded RNAmonoclonal antibodies can also be used
viruses and the much less commonto detect the presence of viruses, with
double-stranded RNA viruses),a technique called fluorescence
reverse transcribing virusesmicroscopy.
(double-stranded reverse-transcribingA second defense of vertebrates against
DNA viruses and single-strandedviruses, cell-mediated immunity,
reverse-transcribing RNA virusesinvolves immune cells known as T cells:
including retroviruses).the body's cells constantly display
In addition virologists also studyshort fragments of their proteins on the
subviral particles, infectious entitiescell's surface, and if a T cell
even smaller than viruses: viroidsrecognizes a suspicious viral fragment
(naked circular RNA molecules infectingthere, the host cell is destroyed and
plants), satellites (nucleic acidthe virus-specific T-cells proliferate.
molecules with or without a capsid thatThis mechanism is jump-started by
require a helper virus for infection andcertain vaccinations.
reproduction), and prions (proteins thatRNA interference, an important cellular
can exist in a conformation whichmechanism found in plants, animals and
induces other protein molecules tomany other eukaryotes, most likely
assume that same conformation).evolved as a defense against viruses. An
The latest report by the Internationalelaborate machinery of interacting
Committee on Taxonomy of Viruses (2005)enzymes detects double-stranded RNA
lists 5450 viruses, organized in overmolecules (which occur as part of the
2,000 species, 287 genera, 73 familieslife cycle of many viruses) and then
and 3 orders.proceeds to destroy all single-stranded
The taxa in virology are not necessarilyversions of those detected RNA
monophyletic. In fact, the evolutionarymolecules.
relationships of the various virusEvery lethal viral disease presents a
groups remain unclear, and threeparadox: killing its host is obviously
hypotheses regarding their origin exist:of no benefit to the virus, so how and
1. Viruses arose from non-living matter,why did it evolve? Today it is believed
separately from and in parallel to otherthat most viruses are relatively benign
life forms, possibly in the form ofin their natural host; the lethal viral
self-reproducing RNA ribozymes similardiseases are explained as resulting from
to viroids.an "accidental" jump of the virus from a
2. Viruses arose from earlier, morespecies in which it is benign to a new
competent cellular life forms thatone that is not accustomed to it (see
became parasites to host cells andzoonosis). For example, serious
subsequently lost most of theirinfluenza viruses probably have pigs or
functionality; examples of such tinybirds as their natural host, and HIV is
parasitic prokaryotes are Mycoplasma andthought to derive from the benign monkey
Nanoarchaea.virus SIV.
3. Viruses arose as parts of the genomeWhile it has been possible to prevent
of cells, most likely transposons or(certain) viral diseases by vaccination
plasmids, that acquired the ability tofor a long time, the development of
"break free" from the host cell andantiviral drugs to treat viral diseases
infect other cells.is a comparatively recent development.
It is of course possible that differentThe first such drug was interferon, a
alternatives apply to different virussubstance that is naturally produced by
groups.certain immune cells when an infection
Of particular interest here isis detected, thus stimulating other
mimivirus, a giant virus that infectsparts of the immune system.
amoebae and carries much of the



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