Chapter 20x - HCC Learning Web

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Oct 23, 2013 (3 years and 9 months ago)

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Chapter 20

DNA Technology



Introduction



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.



M
icrobes 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/spoilag
e


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



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 scientist
s to prepare multiple identical copies
of gene
-
sized pieces of DNA.





Most methods for cloning pieces of DNA share certain general features.



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 protei n encoded by
the foreign gene.





One basi
c 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.



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.



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



DNA cloni ng 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 polym
erase, a supply of
nucleoti
des, 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.



This is faster than cloning via recombinant bacteria.



In PCR, a three
-
step cycle: heating, cooling, and replication, brings

about a chain
reaction that produces an exponentially growing population of DNA molecules.



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:



fragments of ancient DNA from a 40,000
-
year
-
old frozen wooly mammoth,



DNA from ti ny 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



DNA cloning allows researchers to



Compare genes and alleles between individuals



Locate gene expression in a body



Determine the role of a gene in an organism



Several techniques are used to analyze the D
NA of genes





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



Organismal cloning produces one or more organisms genetically identical to the
“parent” that donated the single cell





In nuclear transplantation, the nucleus of an unfe
rtilized 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 perce
ntage of normally
developing tadpoles






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 reflecti ng incomplete
reprogramming of the original transplanted nucleus





Since 1997, cloning has been demonstrated in many mammals, including mice,
cats, cows, hor
ses, mules, pigs, and dogs



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




Totipotent cell: can give rise to all the specialized cell types of the organism




Must be able to obtain a stem
cell and get it to differentiate



Concept 20.4 DNA Tech Applications



Techniques for gene manipulation hold great potential for treating disease by
gene
therapy
.



This alters an afflicted individual’s genes.



A normal allele is inserted into somatic cells of

a tissue affected by a genetic
disorder.



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.






Gene therapy raises some difficult ethical and social quest
ions.



Some critics suggest that tampering with human genes, even for those with
life
-
threatening diseases, is wrong.



They argue that this will lead to the practice of eugenics, a deliberate effort to
control the genetic makeup of human populations.





The
most difficult ethical question is whether we should treat human embryo cells to
correct the defects of future generations.



In laboratory mice, transferring foreign genes into egg cells is now a routine
procedure.



Once technical problems relati ng to simila
r genetic engi neering 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?




From a biological perspective, the e
limination 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
un
der other conditions, for example the sickle
-
cell allele.



Final Thoughts



As with all new technologies, developments in DNA technology have ethical
overtones.



Who should have the right to examine someone else’s genes?



How should that information be used?



Should a person’s genome be a factor i n suitability for a job or eligibility for life
insurance?



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 respe
ct.