Chapter 20
DNA Technology
Introduction
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Sequencing of the human genome was completed by 2007
DNA sequencing has depended on advances in technology, starting with making
recombinant DNA
In recombinant DNA, nucleotide sequences from two different sources, often two
species, are combined in vitro into the same DNA molecule
DNA technology has launched a revolution in biotechnology, the manipulation of
organisms or their components to make useful products.
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Microbes to make wine and cheese and the selective breeding of livestock,
are examples of biotechnology.
Now biotechnology based on manipulations of DNA enables scientists to
modify genes and move them between organisms like, bacteria, plants, and
animals.
DNA technology is now applied in areas ranging from agriculture to criminal law, but
its most important achievements are in basic research.
Uses:
Food crops:
delayed ripening/spoilage
disease and insect resistance
salt resistant gene put in rice in India
rice with beta-carotene for vitamin A (prevents blindness)- a staple in the diets of many
in SE Asia
Cleaning up toxic wastes: take genes from bacteria that can clean toxic wastes and put
in another organism
Production of medications: insulin, growth hormone, vaccines
Concept 20.1 DNA Cloning
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To study a particular gene, scientists needed to develop methods to isolate only the
small, well-defined, portion of a chromosome containing the gene.
Techniques for gene cloning enable scientists to prepare multiple identical copies
of gene-sized pieces of DNA.
Most methods for cloning pieces of DNA share certain general features.
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For example, a foreign gene is inserted into a bacterial plasmid (cloning
vector) and this recombinant DNA molecule is returned to a bacterial cell.
Every time this cell reproduces, the recombinant plasmid is replicated as well
and passed on to its descendents.
Under suitable conditions, the bacterial clone will make the protein encoded by
the foreign gene.
One basic cloning technique begins with the insertion of a foreign gene into a
bacterial plasmid.
The potential uses of cloned genes fall into two general categories.
First, the goal may be to produce a protein product.
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For example, bacteria carrying the gene for human growth hormone can
produce large quantities of the hormone for treating stunted growth.
Alternatively, the goal may be to prepare many copies of the gene itself.
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This may enable scientists to determine the gene’s nucleotide sequence or
provide an organism with a new metabolic capability by transferring a gene
from another organism.
PCR
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DNA cloning is the best method for preparing large quantities of a particular gene or
other DNA sequence.
When the source of DNA is scanty or impure, the polymerase chain reaction
(PCR) is quicker and more selective.
This technique can quickly amplify any piece of DNA without using cells.
The DNA is incubated in a test tube with special DNA polymerase, a supply of
nucleotides, and short pieces of single-stranded DNA as a primer.
PCR can make billions of copies of a targeted DNA segment in a few hours.
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In PCR, a three-step cycle: heating, cooling, and replication, brings about a chain
reaction that produces an exponentially growing population of DNA molecules.
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This is faster than cloning via recombinant bacteria.
The key to easy PCR automation was the discovery of an unusual DNA
polymerase, isolated from bacteria living in hot springs, which can withstand
the heat needed to separate the DNA strands at the start of each cycle.
Devised in 1985, PCR has had a major impact on biological research and
technology.
PCR has amplified DNA from a variety of sources:
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fragments of ancient DNA from a 40,000-year-old frozen wooly mammoth,
DNA from tiny amount of blood or semen found at the scenes of violent crimes,
DNA from single embryonic cells for rapid prenatal diagnosis of genetic
disorders,
DNA of viral genes from cells infected with difficult-to-detect viruses such as
HIV.
Concept 20.2 DNA Technology Applied to Studying Genes
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DNA cloning allows researchers to
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Several techniques are used to analyze the DNA of genes
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– Compare genes and alleles between individuals
– Locate gene expression in a body
– Determine the role of a gene in an organism
One indirect method of rapidly analyzing and comparing genomes is gel
electrophoresis
This technique uses a gel as a molecular sieve to separate nucleic acids or proteins
by size
A current is applied that causes charged molecules to move through the gel
Molecules are sorted into “bands” by their size
Concept 20.3 Cloning
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Organismal cloning produces one or more organisms genetically identical to the
“parent” that donated the single cell
In nuclear transplantation, the nucleus of an unfertilized egg cell or zygote is
replaced with the nucleus of a differentiated cell
Experiments with frog embryos have shown that a transplanted nucleus can often
support normal development of the egg
However, the older the donor nucleus, the lower the percentage of normally
developing tadpoles
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In 1997, Scottish researchers announced the birth of Dolly, a lamb cloned from an
adult sheep by nuclear transplantation from a differentiated mammary cell
Dolly’s premature death in 2003, as well as her arthritis, led to speculation that her
cells were not as healthy as those of a normal sheep, possibly reflecting incomplete
reprogramming of the original transplanted nucleus
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Since 1997, cloning has been demonstrated in many mammals, including mice,
cats, cows, horses, mules, pigs, and dogs
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CC (for Carbon Copy) was the first cat cloned; however, CC differed somewhat from
her female “parent
Therapeutic cloning: repairing damaged or diseased organs
 Stem cells: unspecialized cell that can reproduce and can, under certain conditions,
differentiate into specialized cells
-Embryonic: can become anything
-Adult: can generate some cells; ex. Bone marrow
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Totipotent cell: can give rise to all the specialized cell types of the organism
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Must be able to obtain a stem cell and get it to differentiate
Concept 20.4 DNA Tech Applications
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Techniques for gene manipulation hold great potential for treating disease by gene
therapy.
– This alters an afflicted individual’s genes.
– Adisorder.
normal allele is inserted into somatic cells of a tissue affected by a genetic
– For gene therapy of somatic cells to be permanent, the cells that receive the
normal allele must be ones that multiply throughout the patient’s life.
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Gene therapy raises some difficult ethical and social questions.
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critics suggest that tampering with human genes, even for those with
life-threatening diseases, is wrong.
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argue that this will lead to the practice of eugenics, a deliberate effort to
control the genetic makeup of human populations.
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The most difficult ethical question is whether we should treat human embryo cells to
correct the defects of future generations.
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laboratory mice, transferring foreign genes into egg cells is now a routine
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technical problems relating to similar genetic engineering in humans are
solved, we will have to face the question of whether it is advisable, under any
circumstances, to alter the genomes of human embryos.
– Should we interfere with evolution in this way?
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From a biological perspective, the elimination of unwanted alleles from the gene
pool could backfire.
– Genetic
variation is a necessary ingredient for the survival of a species as
environmental conditions change with time.
– Genes
that are damaging under some conditions could be advantageous
under other conditions, for example the sickle-cell allele.
Final Thoughts
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As with all new technologies, developments in DNA technology have ethical
overtones.
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Who should have the right to examine someone else’s genes?
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How should that information be used?
Should a person’s genome be a factor in suitability for a job or eligibility for life
insurance?
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Some jobs check your credit, why not your genes?
The power of DNA technology and genetic engineering demands that we proceed
with humility, caution, and respect.