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Genetic
Technologies
New applications, new ethical issues
DNA
Fingerprinting
DNA Fingerprinting
Used in a wide range of areas, from
forensics to medicine to taxonomy, to
analyze DNA.
Researchers pick out areas of interest in
DNA, and often use “junk” DNA because it
tends to have more mutations than genes,
so has greater differences from person to
person.
DNA fingerprinting can also be used to
analyze genes to determine a person’s
genotype for a known genetic disorder.
Researchers have identified short tandem
repeats (STRs) that vary widely between
individuals. DNA fingerprinting usually
focuses on these STRs.
The samples are then loaded into a gel (usually
agarose or polyacrylamide)
Different fragments have different molecular sizes, so move at
different rates through the gel. Because they are polar molecules,
they move in response to the electrical field across the gel.
At this point, the gel could be stained, photographed, and
discarded. For finer analysis, such as locating specific
segments of interest the gel may be placed on a nylon fiber
pad, and the DNA fragments driven into the pad using a
downward-directed electrical current.
Labeled DNA probes designed to bind to segments of interest are
loaded onto the pad. These may be tagged with radioactive
phosphorous or fluorescent dye.
Probes stick to the segments of interest, such as known STRs or
specific forms of a disease-causing allele.
(Why do some of these people have
one band, while others have two?)
The resulting DNA fingerprint
can then be analyzed by the
experts. The segment above,
for example, shows DNA from
the same region for 13
different people. It could be
used to determine who was at
a crime scene, or identify a
child’s real parents.
Microarrays
Microarrays
DNA fingerprinting is useful
for testing a few genes or loci
at a time.
Microarrays, however, can
analyze thousands of genes,
proteins, or other molecules
all at once.
Microarrays are used to
determine which genes in a
cell are being expressed, and
to analyze gene-gene
interactions.
Microarrays
Microarrays are arrays
of tens of thousands
of artificial DNA
probes arranged on a
small glass plate.
Each probe can be
designed to bind to a
particular segment of
interest on a particular
gene or DNA locus.
2. mRNA is
extracted from
the subject.
(Why mRNA?)
3. mRNA is
used to
synthesize
cDNA, which is
tagged with
fluorescent
dyes.
1. The
microarray is
designed with
probes that will
bind to specific
loci.
4. The cDNA is
6. A machine scans the
applied to the 5. The microarray is
microarray and washed to remove microarray with red and green
laser light, making the probes
allowed to
cDNA that did not
fluoresce, and photographs
hybridize with
stick.
the results.
the probes.
The results are analyzed to
determine which genes are being
expressed in specific cells, and
which are turned off.
The differing brightness of the
fluorescence also tells researchers
how much mRNA is being
generated by each of the genes
that is active.
Researchers might, for example,
compare gene expression in
normal cells and cancerous cells to
see which genes are active in
cancer cells that are not in normal
cells, and which genes are
supressed in cancer cells.
Gene Therapy
Ashanti deSilva was
one of the first
patients to undergo
gene therapy.
Ashi was born with
ADA deficiency, a
genetic condition in
which she is missing
an enzyme critical for
the immune system.
Because this is a
single-gene trait, it
was a good candidate
for gene therapy.
The enzyme was missing from Ashi’s white blood
cells. Doctors inserted a good copy of the ADA gene
into a virus known to parasitize white blood cells.
The virus successfully inserted the gene into the
cells, where it began producing the enzyme.
However, white blood
cells only live a few
months. Ashi had to
return several times a
year for a new
treatment.
The goal for ADA
therapy is to treat the
bone marrow stem
cells that give rise to
all blood cells.
In spite of having to
return for frequent
treatments, Ashi today
lives a healthy,
productive life.
Without gene therapy,
she would have no
immune system and
might have died of
what would be a minor
illness for anyone else.
Genetically
Modified
Organisms
Recombinant DNA
If gene therapy can “fix” genes in people,
why not insert helpful genes into
organisms?
Recombinant DNA technology allows
researchers to take a gene from one
organism and insert it into another. This has
been done most successfully with plants to
give them resistance to disease, pests, or
herbicides.
Recombinant DNA is also used in “biopharming,” in which genes for medically
therapeutic proteins are inserted into plants
or into milk-producing animals. The
proteins can then be purified from the plant
tissue or milk for use in medical treatments.
Bio-pharming may also produce fruits that
produce proteins found in specific
vaccines, making edible vaccines that could
be grown in third-world countries.
However, altering the genes of organisms,
especially those used for foods, remains
highly controversial.
Creating a transgenic organism begins with locating the
desired gene and creating a trans-gene. The DNA segment
includes the desired gene and may include some “marker”
gene that will be expressed in the phenotype, showing the
gene has been incorporated.
Plant cells can be grown in Petri dishes, and the cells treated
with the gene.
The trans gene may be inserted by a so-called “Gene Gun”
that shoots small gold pellets, coated with the genes, into
the cells.
Virus vectors may be used, since they already have the
machinery to insert genes into a cell.
Bacteria that attack plants are also used.
Here, a gene is prepared for insertion into a DNA plasmid from a
bacteria, which will be used to insert the gene into a plant cell.
The enzyme ligase seals the ends of the trans gene into
the bacterial plasmid.
Plasmids are applied to a culture of bacteria that are known to infect
plant cells.
This particular bacteria attacks by inserting plasmids into the plant
host cell. Now it inserts the plasmid containing the trans gene.
If all goes well, some of the cells will incorporate the trans gene into
their own DNA, where it will be expressed.
The transgenic cells are treated with plant hormones to grow new
plants, and the plants are tested for the expression of the gene.
Cloning
Twins out of time
Lots of myths exist about cloning. Clones
are not:
Mindless zombies slaves raised for organ harvest
later.
Instant identical copies of yourself with all your
memories.
Clones made by nuclear transfer are
genetically identical to the cell donor. They
are the donor’s twin, delayed by time.
Natural Clones
Identical twins are natural clones, created
by the complete division of a fertilized egg.
Plants clone themselves when they produce
shoots, runners, or other structures that
take root and live independently.
Some simple animals clone themselves by
budding, a form of asexual reproduction.
Hello, Dolly
Dolly the sheep was the first mammal
produced by nuclear transfer cloning.
This process involves removing an
intactcell of an adult and inserting it into an
egg cell from which the nucleus has been
removed.
If the egg can be stimulated to divide, it will
grow into a normal embryo that can be
implanted into a host animal’s uterus.
Dolly began life as a single cell from one breed of sheep, a whitefaced Finn Dorset.
An egg of a Scottish Blackface ewe was harvested
and its nucleus removed.
The two cells were
stimulated with an
electrical pulse to
unite. This also
stimulated mitosis.
The egg cell carried
on with multiple cell
divisions as though it
had been fertilized.
The embryo was implanted into the uterus of a Blackface ewe. Some
months later, she gave birth to the white-faced lab, Dolly.
Cloning as a way to produce livestock is
impractical. It’s far more expensive than
nature’s way.
However, owners of expensive and valuable
animals, such a race horses, are interested
in the technique, which raises a whole new
set of ethical questions.
One problem: cells seem to know how old
they are. Animals born from cloned cells are
born with aged cells and don’t live as long.
Many other genetic technologies
exist, and new technologies will
arise in the future.
All genetic technologies raise
ethical concerns about the
organisms involved and their use.