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| The components of an animal virus |
Throughout
human history, viral epidemics have caused us to become more aware of the
impact microbes have on our lives and on the course of history. In the previous
century, we need only look back at the swine flu influenza pandemic of 1918 and
1919, which killed half a million people in just 10 months. Fortunately, the
2009–2010 epidemic of it was milder, causing much concern, but resulting in
only 18,000 deaths in 214 countries.
The
media are full of both popularized and scientific writings on the “new” or
“reemerging” viruses. What factors are contributing to their increased impact?
Dengue fever, also known as “breakbone fever” for its very painful and
sometimes lethal symptoms, is rapidly spreading around the globe. Since 1970,
epidemics of the most deadly form of dengue have spread from 9 countries to
over 4 times as many countries. Global warming is allowing the Aedes mosquitoes
that spread dengue virus to survive winters further and further north.
Some
forms of cancer are definitely caused by viruses—viruses that we know are
transmitted person to person. What are your chances of “catching” cancer? This
is just one of the questions surrounding our knowledge of viral infections
today.
This article
briefly examines the origins of viruses. I hope, this article contibutes to
give us a better understanding of and appreciation for one of nature’s
tiniest-sized, but most dangerous, groups of microbes. Indeed, the name virus
itself comes from the Latin word meaning “poison.”
Viruses
are submicroscopic obligate intracellular parasites—they replicate only inside a
living host cell. Typical viral components of viruses are are a nucleic acid
core and a surrounding protein coat called a capsid. In addition, some
viruses have a surrounding lipid bilayer membrane called an envelope. A
complete virus particle, including its envelope, if it has one, is called a virion.
Viruses have polyhedral, helical, binal, bullet, or complex shapes and vary in size
from 20 to 30 nm (nanometre) in diameter.
Viruses
are clearly quite different from cellular microbes. Free viruses are incapable
of reproduction—they must infect host cells, uncoat their genetic material, and
then use the host’s machinery to copy or transcribe the viral genetic material.
Thus, some debate remains as to whether viruses are living or are nonliving
chemical aggregates. Because viruses cannot reproduce or metabolize or perform
metabolic functions on their own, some scientists say that they are not living.
Other scientists claim that because viruses have the genetic information for
replication, and this information is active after infection, they are living.
Much of the genetic regulation of viral genes is similar to the regulation of
host genes. In addition, viruses use the host’s ribosomes for viral replication
metabolism.
At
present, we cannot definitively say whether viruses are living or nonliving.
But we can ask, What are the origins of viruses? We do not know that either.
There are probably several different ways in which viruses arose. In fact, they
may appear and disappear continuously through time on our planet. However,
because viruses cannot replicate without a host cell, it is likely that viruses
were not present before primitive cells evolved.
One
hypothesis proposes that viruses and cellular organisms evolved together, with
both viruses and cells originating from self-replicating molecules present in
the precellular world. Another idea, sometimes referred to as reverse
evolution, is that viruses were once cells that lost all cell functions, retaining
only that information to replicate themselves by using another cell’s metabolic
machinery. A third hypothesis proposes that viruses evolved within the cells
they infect, possibly from plasmids, the independently replicating DNA
molecules found in many bacterial cells or from retrotransposons.
Plasmids
are selfreplicating and occur in both DNA and RNA forms. They do not, however,
have genes to make capsids. In fact, it has been proposed that plasmids evolved
from viroids. As some viroids moved from cell to cell, the viroid RNA may have
picked up several pieces of genetic information, including the information for
making a protein coat. Indeed, viruses, viroids, plasmids, and transposons all
are agents of evolution through lateral gene transfer. Viruses that insert
themselves into egg or sperm producing cells will be passed on from generation
to generation, becoming a permanent addition to that species’ genome.
In
trying to understand the origins of viruses, virologists—experts on virology, a
scientific field of study that only about 100 years old—have uncovered some
nucleotide sequence relationships common to certain viruses. On the basis of
this information, these viruses have been placed into families with similar
nucleotide sequences and genetic organization. However, they may have had
different origins. It may be possible to predict the potential disease effects
of newly discovered viruses by analyzing the nucleotide sequences of their
genomes and comparing them with sequences found in other, known viruses.