Answers to Open-Ended Questions
Hoefnagels Essentials 2/e
Chapter 11
Mastering Concepts
11.1
1. What is DNA technology?
DNA technology is the manipulation of genes for a practical purpose.
2. In what fields is DNA technology useful?
DNA technology is useful in studies of evolutionary biology, agriculture, criminal justice,
medicine, and many other fields.
11.2
1. What are some uses for transgenic organisms?
Transgenic organisms have many uses; this answer lists a few examples. Transgenic can
produce drugs like insulin; transgenic plants may degrade petroleum and other toxic
wastes, or they may be resistant to herbicides; transgenic animals may produce human
proteins in milk.
2. What are the steps in producing a transgenic organism?
The steps depend on whether the DNA recipient is a bacterium, plant, or animal. In all
cases, researchers first identify and isolate the target DNA to be transferred into the
recipient. To produce a transgenic bacterium, researchers insert the target DNA into a
plasmid and induce bacterial cells to take up the recombinant plasmid. The bacterium
then expresses the target DNA. To produce a transgenic plant, one strategy is to insert a
plasmid containing target DNA into Agrobacterium, which injects the recombinant DNA
into plant cells. New transgenic plants can be grown from the modified cells. To produce
a transgenic animal, the target DNA may be packaged into a virus that infects a gamete or
zygote, which subsequently develops into an animal with the target DNA in every cell.
3. How do researchers determine a sequence of DNA?
The researchers divide the DNA sample among four test tubes, each containing the
unknown sequence, primers, normal nucleotides (A, C, T, and G), replication enzymes,
and a small amount of one type of fluorescently labeled terminator nucleotide.
Replication results in different-sized fragments of complimentary copies of the unknown
DNA. The fragments are sorted by size, using capillary gel electrophoresis. A laser and
detector read the order of the fluorescently labeled terminator nucleotides in the gel, and
a computer determines the DNA sequence.
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4. What is the function of the 98.5% of the human genome that does not encode protein?
Some of it encodes tRNA and rRNA, and some contains regions that regulate gene
expression. Some of it encodes pseudogenes. Much of it, however, has no known
function.
5. How does PCR work, and why is it useful?
PCR uses DNA replication enzymes to make many copies of a tiny sample of DNA.
Afterwards, sufficient DNA is available for sequencing and other forms of analysis. PCR
is useful in forensics, disease diagnosis, and many other fields.
6. How are short tandem repeats used in DNA profiling?
Short tandem repeats are a series of just a few repeating nucleotides that occur in
noncoding portions of DNA. Each individual varies in the number of repeats on each
chromosome, so by combining analysis of several STR sites between individuals, a DNA
profile can be produced.
7. Why do investigators sometimes analyze mitochondrial DNA instead of nuclear DNA?
Nuclear DNA is not always available, especially in badly degraded samples. Although
mitochondrial DNA is much shorter than nuclear DNA, it exists in many more copies
(since each cell contains many mitochondria). Mitochondrial DNA is inherited only from
the mother, so it cannot be used to distinguish between siblings, but it is useful for
tracking inheritance from mother to child over multiple generations.
11.3
1. Describe the differences between embryonic, adult, and induced pluripotent stem cells.
Embryonic stem cells are totipotent and ultimately give rise to every cell type in the
body. Adult stem cells are pluripotent, with the ability to become many (but not all) cell
types. Induced pluripotent stem cells are adult stem cells that have been manipulated in
such a way that they behave like embryonic stem cells.
2. What are the potential medical benefits of stem cells?
Stem cells help illustrate how animals develop and grow; they may be useful in treating
diseases that involve tissue loss or damage; and they are useful in observing the
development of a disease from its start. Finally, stem cells may also be useful in testing
for side effects of pharmaceutical drugs.
3. Summarize the steps scientists use to clone an adult mammal.
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McGraw-Hill Education.
Cells from the animal to be cloned are grown in culture, and then the nucleus of one of
those cells is removed. That nucleus is then fused with a denucleated cell from an egg
donor. The fused cell is allowed to divide into an embryo and is then implanted into the
surrogate, where it is allowed to develop to birth.
4. Why is the technique used to clone mammals called somatic cell nuclear transfer?
The nucleus of a somatic cell is transferred into an egg that has had its own nucleus
removed.
11.4
1. Explain how and why a researcher might use a DNA probe.
DNA probes are used to find a particular DNA sequence, often a gene associated with a
particular illness. The complimentary strand of the gene of interest is synthesized and
marked with a radioactive isotope or fluorescent tag. The marked strand will bond with
the gene sequence, if it is present, and the tag will reveal its location.
2. Compare and contrast preimplantation genetic diagnosis and genetic testing.
Preimplantation genetic diagnosis uses a DNA probe to screen a cell removed from an
embryo for a disease-causing allele. If the cell contains the allele, then the embryo is not
implanted into the uterus; otherwise, the embryo is implanted. Genetic testing also uses a
DNA probe to detect a specific allele, but the test is conducted on cells taken from a child
or adult. Genetic testing allows for a definitive diagnosis, which helps patients and
physicians develop a treatment plan.
3. What is gene therapy?
In gene therapy, a healthy gene can be inserted into cells to compensate for the missing
function of a faulty gene.
4. What are some examples of ethical questions raised by the medical use of DNA
technology?
The methods are expensive and would be cost prohibitive for lower-income individuals.
DNA technology makes it easier to know if a fetus has a particular disorder, which could
lead to an increase in pregnancy terminations. In general, DNA technology also leads to
arguments about “Where do we stop?” For example, if technology allows us to insert
functional alleles as therapy for a disease, what prevents us from inserting “better”
versions of other alleles?
Write It Out
1. What techniques might researchers use to produce transgenic bacteria that make
human growth hormone (a drug used to treat extremely short stature)?
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McGraw-Hill Education.
They would begin by identifying the gene that encodes human growth hormone. They
would then use restriction enzymes to cut the gene out of human DNA; they would use
the same restriction enzyme to cut a plasmid. After splicing the gene into the plasmid,
they would induce bacterial cells to take up the plasmid. The bacterial cells would then
produce the protein.
2. Transgenic crops often require fewer herbicides and insecticides than conventional
crops. In that respect, they could be considered environmentally friendly. Use the Internet
to research the question of why some environmental groups oppose transgenic
technology.
Environmental groups may oppose transgenic technology because they believe that
interfering with another species is unethical; that transgenic organisms may escape into
the ecosystem, with unknown consequences; and that transgenic crops are too expensive
for ordinary farmers to afford. (Other arguments are possible as well.)
3. Describe why sorting DNA fragments by size helps determine the sequence of
nucleotides in the segment of DNA.
During DNA sequencing, researchers use fluorescently labeled terminator nucleotides
that cause the strand to stop replicating when inserted. For example, imagine a segment
of DNA that is 30 nucleotides long. When many copies of this DNA sequence replicate in
a vial containing normal and terminator nucleotides, one strand will stop replicating after
only one nucleotide is inserted, another will stop replicating after two nucleotides are
inserted, and so on. The last nucleotide always has a fluorescent label. Arranging the
partially replicated DNA fragments by size and then analyzing the fluorescently labeled
nucleotides therefore reveals each nucleotide in the sequence.
4. Explain how the ingredients in a PCR reaction tube replicate DNA.
In a PCR reaction tube, heat separates the two strands of target DNA. The temperature is
lowered, then short laboratory-made primers attach to the separated target strands by
complementary base pairing. DNA polymerase adds nucleotides to the primers and
builds sequences complementary to the target sequence. The newly synthesized strands
then act as templates in the next round of replication, which can be initiated immediately
by raising the temperature to separate the strands once more.
5. In a 2013 investigation, researchers discovered that meat packaged as “100% beef” in
Europe actually contained traces of horse meat and pork. What DNA technology
techniques might the researchers have used to uncover the truth about the origin of the
meat?
Researcher may have used DNA profiling to determine the origin of the meats. They
would have extracted DNA from a sample of meat, applied PCR and DNA sequencing
techniques, and compared their results to known sequences in cow, horse, and pig DNA.
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6. Why are entire genomes not used for DNA profiling?
Analyzing an entire genome requires a substantial investment of time and money, and
most of the sequence would be identical between any two people. DNA profiling using
STRs is much more efficient because it focuses on a limited number of sites that are
known to be highly variable within the population.
7. What would be the advantage of sequencing an entire chromosome from a DNA
sample collected at a crime scene? What would be the disadvantage or limitations of this
approach?
Sequencing an entire chromosome from DNA collected at a crime scene and the same
chromosome from all possible suspects could provide definitive evidence of guilt or
innocence. However, sequencing an entire chromosome is much more demanding than
analyzing the 13 short tandem repeat loci that are typically used in DNA profiling. In
addition, the chance that a DNA profile matches an individual who is not the culprit is
one in 250 trillion, so the additional information provided by a whole-chromosome
analysis is not necessary.
8. Mature neurons in the brain do not replicate. Why are stem cells an intriguing solution
to patients who suffer from traumatic brain injury?
Embryonic stem cells and induced pluripotent stem cells divide indefinitely and can be
coaxed to divide into any cell type. These cells can therefore help restore brain tissue lost
in an injury.
9. Unneeded genes in an adult animal cell are permanently inactivated, making it
impossible for most specialized cells to turn into any other cell type. How does this
arrangement save energy inside a cell? Why does the ability to clone an adult mammal
depend on techniques for reactivating these “dormant” genes?
Permanently inactivating unneeded genes reduces the chance that they will be expressed;
protein production costs energy, so preventing unneeded gene expression saves energy.
These genes must be reactivated in cloning because many are required for the proper
development of an embryo.
10. Scientists are interested in cloning an extinct animal called the gastric brooding frog.
This strange frog swallows its eggs and broods its young within its stomach. So far,
scientists have successfully used cloning to make an embryo of the frog, but they have
yet to raise one to maturity. What steps might the scientists have used to clone this extinct
species? Why was it important for scientists to determine before the experiments that the
great barred frog is a close relative to the gastric brooding frog?
Scientists could use somatic cell nuclear transfer to clone this frog. DNA extracted from a
preserved gastric brooding frog could be inserted into a denucleated egg cell of a closely
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McGraw-Hill Education.
related frog species, such as a great barred frog. The embryo would then have been
allowed to develop. It is important that scientists use a related species to receive the
donor egg so that the conditions for early development are nearly the same as a gastric
brooding frog embryo would have faced.
11. This chapter’s Why We Care box describes some potential applications of gene
doping. What are some examples of ethical issues that gene doping presents? What might
the prospect of gene doping mean for the future of sports?
Gene doping may give athletes with money and a high risk tolerance an unfair advantage
over those who choose not to use the technique. Because gene doping is hard to detect, it
may lead to an “arms race” in which elite athletes must “enhance” themselves in more
and more ways, simply to remain competitive.
12. Describe gene therapy and explain the ethical issues that gene therapy presents.
In gene therapy, a healthy gene is placed into a cell to make up for the function of a faulty
gene. Ethical issues include high cost and the prospect of fixing genes in germ cells,
which would mean that gene “repairs” could be passed on to the next generation. The
potential consequences of germline gene therapy are unknown.
13. If a cell’s genome is analogous to a cookbook and a gene is analogous to a recipe,
what is an analogy for preimplantation genetic diagnosis? For gene testing? For gene
therapy?
Preimplantation genetic diagnosis would be like peeking into a cookbook before it is
published to see if it contains a particular recipe. Gene testing would be the same, only
the cookbook would already be published. Gene therapy would be like adding a new
recipe that corrects errors in an existing recipe.
Pull It Together
1. How does PCR relate to DNA profiling and preimplantation genetic diagnosis?
PCR is used to amplify small amounts of DNA, producing sufficient genetic material for
analysis. Tiny amounts of DNA may be left at a crime scene, and PGD relies on the
detection of a gene from the DNA in just one cell. PCR is necessary for both techniques
to work.
2. Add the terms restriction enzyme, plasmid, virus, DNA polymerase, and short tandem
repeat to this concept map.
“Restriction enzyme” leads with the word “cuts” to “DNA.” “Plasmid” connects with the
phrase “may carry DNA into” to “Transgenic organism.” “Virus” connects with the
phrase “may carry DNA in” to “Gene therapy.” “DNA polymerase” connects with the
Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of
McGraw-Hill Education.
phrase “copies DNA in” to “PCR” and to “DNA sequencing.” “Short tandem repeat”
connects with the phrase “is used in” to “DNA profiling.”
3. How is a patient who receives gene therapy similar to and different from a transgenic
organism?
A patient who receives gene therapy receives human DNA; a transgenic organism
typically receives DNA from a species other than its own. In addition, only a limited
subset of cells are modified in gene therapy; in transgenic organisms, every cell has
modified DNA.
Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of
McGraw-Hill Education.