Biomedical Engineering Reference
In-Depth Information
7.2
DNA structure
It is generally recognized that the main unifying theme of modern biology is
the theory of evolution, elegantly stated by Charles Darwin in '
The Origin
of the Species
'. In it, he postulates that the evolutionary changes seen in liv-
ing organisms are the result of
descent with modifi cations.
The mechanism
which acted on these modifi cations and causes the great diversity of liv-
ing creatures is
natural selection
. It is by this process that organisms, having
inherited some advantageous modifi cation, acquire a selected advantage
in dealing with the many challenges that nature imposes. These fortunate
organisms are thus better able to survive and are more successful in pass-
ing on these advantages to future offspring, giving rise to the survival of the
fi ttest and the evolution of living organisms that best fi t any given environ-
mental niche.
As brilliant as this overall theme is in explaining the great diversity that
one sees in the living world, Darwin was at a loss to explain the underlying
cellular and molecular processes that are the basis of these modifi cations.
For the answer to these questions, we had to wait for the discoveries of the
next century, with the establishment of nucleic acids as the molecular carrier
of genetic information and the elucidation of the processes that permitted
the transference of this information into proteins. It is through this knowl-
edge that we have learned the many methods nature has developed to pro-
duce changes to this information pathway, and that these changes are the
basis for the genetic diversity seen at every level of life.
The model of DNA that was fi rst published in
Nature
by Watson and
Crick in 1953, based on the X-ray crystallography of Wilkins and Franklin,
was the seminal work in establishing this information pathway. This elegant
model, having two nucleotide strands held together at its axis by hydrogen
bonding of specifi c nitrogen bases arranged like steps in a spiral staircase
of life, immediately showed how nucleic acids stored information (Figs. 7.1
and 7.2). This information could be copied and distributed through cell divi-
sion. The model also provided a key to how this information could be read
and translated into a vast diversity of protein structure. Later work soon
unraveled the three-letter genetic code that was held within DNA, copied
by messenger RNA (mRNA) and translated into the primary polypeptide
sequence.
As the twentieth century progressed, advances in our understanding of the
nature of genes brought surprising discoveries hidden in these information-
coding nucleotide sequences. The genes of higher organisms represented
only a small fraction of the total cellular DNA, about 1% of the 3 billion
base pairs of the human genome and the genes themselves were arrayed with
informational (exon) and non-information (intron) segments. Eventually
towards the end of the century, reliable methods were developed to cut
