NEL Molecular Genetics 677
DNA and Biotechnology 20.320.3
Carpenters require tools such as hammers, screwdrivers, and saws, and surgeons require
scalpels, forceps, and stitching needles. The tools of the molecular biologist are living
biological organisms or biological molecules. Using these tools, scientists can treat spe-
cific DNA sequences as modules and move them from one DNA molecule to another,
forming recombinant DNA. Research in exploring and using this type of biotechnology
has led to exciting new advances in biological, agricultural, and medical technology.
Biotechnology research has also found ways to introduce specific DNA sequences into
a living cell. For example, the gene that encodes insulin has been introduced into bac-
terial cells so that they become living factories producing this vital hormone. The intro-
duction and expression of foreign DNA in an organism is called genetic transformation.
In this section, you will explore some of the key tools used by molecular geneticists in
producing recombinant DNA and genetically transformed organisms.
DNA Sequencing
Before a DNA sequence can be used to make recombinant DNA or to transform an
organism, the scientist or technician must first identify and isolate a piece of DNA con-
taining that sequence. One of the tools used to do this is DNA sequencing. DNA
sequencing determines the exact sequence of base pairs for a particular DNA fragment
or molecule. In 1975, the first DNA sequencing techniques were simultaneously devel-
oped by Frederick Sanger and his colleagues and by Alan Maxim and Walter Gilbert.
Sanger’s technique relied on first replicating short segments of DNA that terminate due
to a chain-terminating nucleotide. Four separate reaction tubes are run, each with a
chain-terminating nucleotide incorporating a different base (i.e., A, T, G, and C). The var-
ious lengths of DNA segments are then separated by loading and running the contents
of the tubes on a sequencing gel (Figure 1). Because the end nucleotide of each segment
is chain-terminating, its base is already known. Consequently, the sequence can be read
directly from the gel in ascending order (shortest to longest segments). The sequence
of the strand is written along the edge of the gel diagram, starting from the bottom
where the shortest strands have travelled. This method is comparatively slow and can only
sequence short fragments of DNA.
DNA can also be sequenced in a test tube using isolated segments of DNA. This tech-
nique depends on a primer, DNA polymerase, and the four DNA nucleotides, each of
which is labelled with a specific dye. The complementary strand is built from these dye-
labelled nucleotides. The nucleotides in the synthesized strand can then be identified
by their colours, allowing the original strand sequence to be deduced according to
the rules of complementary base pairing.
recombinant DNAfragment
of DNA composed of sequences
originating from at least two
different sourcesgenetic transformation
introduction and expression of
foreign DNA in a living organismGGTC
A
T
TACGTAATCFigure 1
A sequencing gel is a matrix
containing many small spaces. The
DNA fragments are charged and
will move towards one pole of an
electric field. Smaller DNA
fragments move through the spaces
more quickly than larger fragments
and are found at the bottom of the
gel. The larger fragments will
remain towards the top of the gel.
The resulting ladder of fragments
can be read, giving the sequence of
the initial DNA fragment.WEBActivity
http://www.science.nelson.com GOSimulation—Electrophoresis
Electrophoresis is an important tool in molecular biology. In addition to nucleic acids, it is also used
to separate proteins from a mixture. Electrophoresis of nucleic acids and proteins depend on the
similar factors. In this Virtual Biology Lab, you will perform polyacrylamide gel electrophoresis
(PAGE) to identify proteins involved in the biochemistry of shell colour in an extinct organism.