Biomedical Engineering Reference
In-Depth Information
Chapter 38
Overview of Viruses
“The human genome is a living document of ancient and now extinct viruses.”
Michael Emerman and Harmit Malik [113]
In this topic, viruses and prions are referred to as “biological agents”; not as
living organisms. Viruses lack key features that distinguish life from non-
life. They depend entirely on host cells for replication; they do not partake in
metabolism, and do not yield energy; they cannot adjust to changes in their
environment (i.e. no homeostasis), nor can they respond to stimuli. Most
scientists consider viruses to be mobile genetic elements that can travel
between cells (much as transposons are considered mobile genetic elements
that travel within a cell). All viruses have a mechanism that permits them to
infect cells and to use the host cell machinery to replicate. At a minimum,
viruses consist of a small RNA or DNA genome, encased by a protective
protein coat, called a capsid. Some viruses carry one or several proteins.
Some viruses have an envelope extracted, in part, from host cell membranes.
For non-living organisms dependent entirely on host cells for their contin-
ued existence, viruses have done extremely well for themselves. Every class of
living organism hosts viruses. Viruses are literally everywhere in our envir-
onment, and are the must abundant life form in the oceans, in terms of numbers
of organisms [114]. At least 8% of the human genome is composed of frag-
ments derived from RNA viruses, acquired in our genetic past [115]. As far as
anyone knows, viruses are as ancient as the earliest forms of terrestrial life.
When did the first viruses appear on earth? Nobody knows, but much of
the current speculation on the origins of viral life centers on the so-called
RNA world, wherein the biogenic precursors of living organisms held
domain in ancient oceans. Before there were membrane-enclosed organisms,
there were volcanic rocks. When basalt melts and aggregates, small bubbles,
about the size of bacteria, form and interconnect with one another [116].
Examples of porous volcanic rocks include tufa, tuff, travertine, pumice,
stromboli basalt, and scoria. Any of these water-drenched rocks could have
been crucibles for the earliest biogenic molecules. RNA almost certainly pre-
ceded the appearance of DNA, as DNA bases require synthesis from RNA
