Download Introduction to viruses Viruses are non

Introduction to viruses
Viruses are non-cellular entities. Scientists do not agree on the point of whether
viruses are living or non-living. It would depend on your definition of what it takes
to be “alive.” The majority of scientists seem to think viruses are not alive. To me
they are alive. Here’s what we know: Viruses are ALL obligate parasites of
cells/organisms, and ALL organisms are host to viruses. There is a HUGE degree of
variation among the viruses….so much so that to lump them all into a single group is
quite a stretch.
How did viruses evolve? We’re not sure, but since they are totally dependent on
cells, it seems logical that cells came first. However, some scientists say that
bacteria and viruses evolved at the same time from some “pre-cellular” entity, and
while bacteria became more complex, viruses became simpler. Viruses use the same
genetic code as all cellular life. Viruses differ in their evolutionary histories. Some
appear to have evolved as renegade DNA or RNA that originated in cells. Others
(with very large genomes) may be the vestiges of cells that shed their cytoplasm and
all its contents and took up residence as endosymbionts. The evolution of most
viruses is unknown.
Where do viruses fit on the tree of life? If all cellular life can be seen as a tree
with a universal common ancestor at its base, the viruses would be like a cluster of
shrubs, vines, and epiphytes (plants that grow in trees like orchids and bromeliads).
How numerous are viruses? In terms of sheer numbers, there are more virus
“particles” than there are cells…by quite a bit. Some scientists have estimated that
there are 1031 virions on the planet. A single virus “particle” is called a virion.
Viral structure: A basic definition of a “virus” is a nucleic acid core (DNA or RNA)
surrounded by a protein coat, the capsid. Some viruses are wrapped in membranes
(envelopes) acquired from the host cell. Whereas ALL cells have double stranded
DNA (dsDNA) as their information molecule, viruses are known to have all kinds of
variations, including dsDNA, single stranded DNA (ssDNA), ssRNA, and dsRNA. The
majority of viruses that infect humans are ssRNA viruses. Even within these groups
the way viruses replicate varies and you can see that some are “negative sense” and
some are “positive sense.”
How do viruses enter cells? In most cells with a cell wall, the virus must use brute
force to enter the cell. They may also employ a vector, such as an insect with
piercing/sucking mouth parts. For cells lacking a cell wall, the virus typically enters
by way of endocytosis. This usually involves receptors on the viral surface that act
as key cards. The cell then brings the virus inside like the old Trojan Horse.
What do viruses do when they get into a cell? That depends. They may
incorporate their nucleic acid into that of the host and hang around for a while as
latent viruses (lysogenic cycle). Latent viruses can either pop out and take over the
cell’s machinery for nucleic acid replication and protein synthesis, and in so doing,
make thousands of copies of themselves and then bust out of the cells to go and
infect others (lytic cycle). Or they may lie dormant in the cell indefinitely. Genome
studies indicate that we have thousands of viral traces in our own DNA.
What role do viruses play in ecosystems? One significant role that viruses have
played is in the horizontal transfer of genes (HGT). This contributes to genetic
diversity in populations and contributes to evolutionary change. Another role is in
maintaining population size by way of disease.
How do viruses move from one host to another? If you can imagine a way for a
virus to move from one host to another, viruses have already figured that out.
Direct contact, body fluids, insects (mosquitoes), contaminated food and water, air
borne, etc. The strangest of all is rabies. The virus will kill the host, but before the
host dies, the virus gets into its brain and makes it bite another host.
How specific are host/virus relationships? The host range for any virus is very
limited, but some are less limited than others. Furthermore, because viruses evolve
so quickly, they can “spillover” from one host to another. A common example of this
is influenza, which spills over from birds and pigs (mostly) to humans. This
becomes a problem when the virus evolves the ability to spread from human to
human.
How big/small are viruses? Very, very small. With very rare exceptions, viruses
can only be seen with an electron microscope. The largest viruses are a wee bit
bigger than the smallest bacteria. Staphylococcus bacteria are small (800 nm/0.8
µm/0.0008 mm). By comparison, viruses range from 20 to 300 nm with rare
exceptions.
Who discovered viruses? Martinus Beijerinck (“buy your ink”) is generally
considered the father of Virology. Working with a viral disease of tobacco,
Beijerinck diluted the “filtered toxin” from diseased tobacco leaves (the filter was
small enough to prevent bacteria from passing). This filtered toxin caused disease
even when diluted. Even the very diluted samples caused the same degree of
disease, and this inferred reproduction.
Common examples: We don’t have a clue how many different viruses there are in
nature, but we are familiar with the ones that have attempted to keep human
population in check. Here is a very short list:
DNA Viruses of Humans: Smallpox, Chickenpox, Herpes, HPV, Mononucleosis
RNA Viruses of Humans: Hepatitis A, B, C; Polio, rhinovirus (colds), SARS, Yellow
Fever, Dengue Fever, West Nile, Rubella (German measles), HIV, Influenzas, Ebola,
Measles, Mumps, Pertussis (whooping cough), Rabies, etc, etc.