Biomedical Engineering Reference
In-Depth Information
nucleotides via the action of ribonucleotide reductase and thymidylate syn-
thetase. RNA molecules may have developed their biogenic properties, as
templates for synthesizing proteins, within rock bubbles. Silicates and hard
surfaces in rock may have served as structural catalysts, holding molecules
in place as they were lengthened (RNA synthesis), replicated (RNA replica-
tion), and translated (protein synthesis) from precursor molecules found in
sea-water. Because rock bubbles are interconnected, early RNA virus-like
molecules could move from bubble to bubble, exchanging genetic materials.
Sometime later, DNA may have appeared. DNA is a much more
stable molecule than RNA, less prone to replication error, and less suscepti-
ble to intrusion by RNA viruses that were freely commuting between rock
bubbles. The evolution of DNA modifications (adherent proteins and base
methylations, characterizing the early epigenome) may have developed as a
defense against infection by RNA viruses.
What came next? After DNA appeared, it was inevitable that DNA
viruses would emerge. DNA viruses, being more stable than RNA viruses,
could grow into large, complex entities, such as the megaviruses (Group I
double-stranded DNA Viruses, Chapter 39). Then came cell membranes. It is
known that phospholipids spontaneously form lipid bilayers in agitated
water. It seems plausible that rock-dwelling organisms, endowed with an
enclosing bilayer membrane assembled from phosphorylated small molecules
would eventually venture out into the ocean, searching for the best sources
of food. The late emergence of enclosing membranes, well after the initial
development of membrane-less organisms, is supported by the profound
structural differences in bacterial and archaean membranes [117]. If the two
classes of organisms had split off from a common, membrane-enclosed
ancestor, you might expect them to have similar membranes.
Hypothesizing on the origins of living and non-living organisms is all
good sport, but it should not be taken too seriously. Suffice it to say that the-
orists have proposed that all life on earth developed from ancient viruses
[118,119]. Motivated readers can delve into the subject and draw their own
conclusions.
At this time, the classification of viruses is somewhat crude. Anything
you choose as a classifying principle fails to biologically unify the sub-
classes. For example, if you classify viruses by their genomic molecules (i.e.
DNA or RNA, single strandedness or double strandedness), you will find
that subclasses of the same genomic type, will have dissimilar structures:
envelope, size, shape, proteins, capsid. When we list viruses based on
method of contagion, by persistence within host (i.e. acute, chronic, latent,
or persistent), toxicity (lytic, immunogenic), or by target cell specificity, no
consistent taxonomic correlation is found.
Though we cannot classify viruses phylogenetically, at this time, we can
usefully group viruses based on the physical characteristics of their genomes.
The Baltimore Classification divides viruses into seven groups based on
