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Basics of Viruses

Many people are exposed to viruses and determine whether an organism is alive.
become sick, but for the majority of the The two biggest blocks to viruses being
population, they have no idea what a accepted as 'living' organisms is their
virus actually is. A virus is something lack of a cell membrane and their
that causes an illness and it is inability to metabolize on their own.
contagious. At its most basic level a Another criteria for living organisms
virus is just a strand of DNA surrounded that viruses lack is adaptation, or the
by a protein shell, which is what makes ability to change and evolve depending on
it different from other fragments of DNA their environment. Viruses lack any hard
such as prions and viroids. A virus parts that can fossilize such as bones
cannot reproduce outside of a host cell and they are too small to leave imprints
and is considered by many scientists as in sand, like some ancient jellyfish.
not being truly 'alive'. Viruses can Thus there are no fossil records of
infect plants and animals and some are viruses, which makes showing
even able to infect bacteria.Humans have relationships between different viruses
dealt with viral diseases, such as rabies very difficult.Because viruses do not
and smallpox, for many centuries. As leave any fossil remains, the best way to
early as the 18th century, people were hypothesize their origins has been to use
observed who had been exposed to smallpox molecular techniques, that is to study
or cowpox, a similar virus and had the genetic materials of different
subsequent immunity to smallpox. In the viruses and try to determine any
late 19th century a porcelain filter, relationships by finding similarities
developed by Charles Chamberland, was between them. There are two main
used to indirectly study the tobacco hypotheses concerning virus origins. The
mosaic virus. Dmitri Ivanowski performed first hypothesis, involving the small
experiments on crushed tobacco leaf viruses with only a few genes, is that
extracts that showed infected plants were these small viruses may be runaway
still infectious after bacteria were snippets of a living organisms DNA.
filtered out. Many others performed These small DNA fragments could have come
experiments that showed similar from plasmids or transposons, which are
filterable disease causing agents. Felix transferable genetic elements and are
d'Herelle and Frederick Twort, working prone to entering or exiting genomes.
independently, found that bacteria could The second hypothesis involves the larger
be infected by viruses, not just animals viruses. These large viruses may have
and plants. In the 1930s, Wendell been small parasitic cells at one time
Stanley showed that the tobacco mosaic and, through the process of
virus was mostly protein and shortly reverse-evolution, lost any genes not
after, it was further separated into needed by their parasitic lifestyle.
genetic material and a protein shell.A There are a few bacteria, Rickettsia and
virus is not considered truly alive Chlamydia, that support this hypothesis
because it is missing several of the main because they cannot reproduce outside of
qualities that scientists use to a host cell, like viruses.




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