This document summarizes key points from a chapter about controlling gene expression and the applications of biotechnology. It discusses how transcription factors regulate genes and three types of mutations. It also covers genetic engineering techniques like restriction enzymes and PCR. Applications of biotechnology include producing nutrient-rich crops, insect-resistant plants, and using recombinant DNA to produce human insulin and growth hormone. The document notes both promises and concerns of genetic modification, and challenges of gene therapy. It also describes DNA fingerprinting for identifying individuals.
Diploma in Nursing Admission Test Question Solution 2023.pdf
Day 7 September 18th Chapter 5
1. Day 7 September 18th Chapter 5
Dr. Amy B Hollingsworth
The University of Akron
Fall 2014
2. Controlling Gene Expression
Transcription factors
• Proteins that bind to specific regulatory sites on
the DNA
“Positive control”
“Negative control”
4. Three types of mutations – spontaneous, radiation-induced, or chemically-induced
5. Why is it dangerous to be near
the core of a nuclear power
plant?
Radiation-induced mutations
6. Mutations are alterations in a single base
or changes in large segments of DNA that
include several genes or more.
They are rare, but when they do occur,
they may disrupt normal functioning of the
body (although many mutations are
neutral).
7. Extremely rarely, mutations may have a
beneficial effect.
They play an important role in evolution.
8. 5.10 Faulty genes, coding for faulty
enzymes, can lead to sickness.
How can people respond so differently to
alcohol?
A single difference in a single pair of bases
in their DNA.
9. From mutation to illness in
just four steps:
1. A mutated gene codes for a non-functioning
protein, usually an enzyme.
2. The non-functioning enzyme can’t
catalyze the reaction as it normally
would, bringing it to a halt.
10. From mutation to illness in just
four steps:
3. The molecule with which the enzyme
would have reacted accumulates, like a
blocked assembly line.
4. The accumulating chemical causes
sickness and/or death.
11. Insert section 5.11-5.13 opener photo
5.11–5.13
Biotechnology is
producing
improvements in
agriculture.
12. 5.11 What is biotechnology?
Genetic Engineering
Adding, deleting, or transplanting genes
from one organism to another, to alter the
organisms in useful ways
19. Take-home message 5.11
The methods rely on naturally occurring
restriction enzymes for cutting DNA, the
polymerase chain reaction for amplifying
small amounts of DNA, inserting the DNA
into bacterial or viral vectors, and cloning
and identifying the cells with the
transferred DNA of interest.
20. 5.12
Biotechnology
can improve food
nutrition and
make farming
more efficient
and eco-friendly.
Insert figure 5-30
21. How might a genetically
modified plant help 500
million malnourished people?
Nutrient-rich “golden rice”
23. Almost everyone in the United States
consumes genetically modified foods
regularly without knowing it.
What foods are responsible for this?
29. 5.13 Fears and risks: Are genetically
modified foods safe?
30. Fear #1: Organisms
that we want to kill
may become
invincible.
Fear #2: Organisms
that we don’t want
to kill may be killed
inadvertently.
Fear #3: Genetically
modified crops are
not tested or
regulated
adequately.
31. Fear #4: Eating
genetically modified
foods is dangerous.
Fear #5: Loss of
genetic diversity
among crop plants
is risky.
Fear #6: Hidden
costs may reduce
the financial
advantages of
genetically modified
crops.
33. 5.14 The treatment of diseases and
production of medicines are improved
with biotechnology
Prevent diseases
Cure diseases
Treating diseases
• The treatment of diabetes
44. Why has gene therapy had
such a poor record of success
in curing diseases?
45. Gene Therapy Difficulties
1. Difficulty getting the working gene into
the specific cells where it is needed.
2. Difficulty getting the working gene into
enough cells and at the right rate to
have a physiological effect.
46. Gene Therapy Difficulties
3. Difficulty arising from the transfer
organism getting into unintended cells.
4. Difficulty regulating gene expression.
Cells have a wide variety of mechanisms by which they can control when individual genes are expressed. One of the main ways expression is controlled is by transcription factors, proteins that bind to specific regulatory sites on the DNA. These sites are often located in front of genes. The regulation may be “positive control,” in which binding of the regulatory protein initiates or speeds up gene expression, or “negative control,” in which the protein slows or blocks gene expression.
Mutations generally fall into two types: point mutations and chromosomal aberrations (Figure 5-21 Point mutations and chromosomal aberrations). In point mutations, one base pair is changed, whereas in chromosomal aberrations, entire sections of a chromosome are altered.
Point mutations are mutations in which one nucleotide base pair in the DNA is replaced with another or in which a base pair is inserted or deleted. Insertions and deletions can be much more harmful than substitutions because they can alter the reading-frame for the rest of the gene. Remember that the amino acid sequence of a protein is determined by reading the bases on an mRNA molecule three at a time and attaching the specific amino acid that is specified by that sequence. If a single base is added or removed, the three-base groupings get thrown off and the sequence of amino acids stipulated will be all wrong. It’s almost like putting your hands on a computer keyboard, but offset by one key to the left or right, and then typing what should be a normal sentence. It comes out as gibberish.
Chromosomal aberrations are changes to the overall organization of the genes on a chromosome. Chromosomal aberrations are like the manipulation of large chunks of text within a paper. They can involve the complete deletion of an entire section of DNA, the moving of a gene from one part of a chromosome to another, or the duplication of a gene with the new copy inserted elsewhere on the chromosome. In any case, a gene’s expression—the production of the protein the gene’s sequence codes for—can be altered when it is moved, as can the expression of the genes around it.
Given the potentially hazardous health consequences of mutations, it is advisable to minimize their occurrence. Can this be done? Yes and no. There are three chief causes of mutation, and although one of them is beyond our control, the other two can be significantly reduced.
1. Spontaneous mutations. Some mutations arise by accident as long strands of DNA are duplicating themselves—at the rate of more than a thousand bases a minute in humans—when cells are dividing (more on this process in Chapter 6). Most errors are repaired by DNA repair enzymes but some still slip by and there’s not much we can do about them.
2. Radiation-induced mutations. Ionizing radiation is radiation with enough energy that it can disrupt atomic structure—even breaking apart chromosomes—by removing tightly bound electrons. Sources of ionizing radiation include X rays and the ultraviolet rays of the sun. When you lie in the sun, for example, its ultraviolet rays can induce a mutation in one of your skin cells that can transform the cell into a cancer cell. This is why long-term sun exposure can contribute to the development of skin cancer (Figure 5-22 Gambling with mutation-inducing activities?).
3. Chemical-induced mutations. Many chemicals, such as those found in cigarette smoke and in internal combustion engines, can also react with the atoms in DNA molecules and induce mutations.
Another source of dangerous radiation is found in the core of nuclear power plants, where radioactive atoms are used and produced in energy-generating reactions. While it is the high energy of the radioactivity that fuels the production of usable energy, this radioactivity is potentially harmful because it can pass through your body and disrupt your DNA, causing mutations. Radioactivity can also cause larger-scale mutations, breaking off entire sections of chromosomes. With the proper safety precautions, however, nuclear power plant workers can minimize their exposure to harmful radiation.
In the next section, we examine how even tiny changes in the base-pair sequence of DNA can lead to errors in protein production and profound health problems.
Isabella joins her friends in sipping wine during a dinner party. As the meal progresses, her companions become tipsy. Their conversations turn racy, their moods relaxed. They refill their glasses, reveling in a little buzz. Not so for Isabella. Before her first glass is empty, she experiences a “fast-flush” response: Her face turns crimson, her heart begins to race, and her head starts to pound. Worse still, she soon feels the need to vomit.
How can people respond so differently to alcohol? It comes down to a difference in a single pair of bases in their DNA, a difference that can dramatically influence a person’s behavior, digestion, respiration, and general ability to function. The single base pair change leads to the production of a non-functional enzyme, and the lack of a functional version of this enzyme leads to physical illness. Let’s look at the details.
Although the details differ from case to case, the overall picture is the same when it comes to many, if not most inherited diseases. The pathway from mutation to illness includes just four short steps:
1: A mutated gene codes for a non-functioning protein, commonly an enzyme.
2: The non-functioning enzyme can’t catalyze the reaction as it normally would, bringing it to a halt.
3: The molecule with which the enzyme would have reacted accumulates, like a blocked assembly line.
4: The accumulating chemical causes sickness and/or death.
The fact that most genetic diseases involve illnesses brought about by faulty enzymes suggests some strategies for treatment. These include administering medications containing the normal-functioning version of the enzyme. For instance, lactose-intolerant individuals can consume the enzyme lactase, which helps them to digest lactose. Alternatively, lactose-intolerant individuals can reduce their consumption of lactose-containing foods to keeps the chemical from accumulating, thus reducing the problems that come from its overabundance.
Section 5.4 Opener
A scientist works with transgenic plants in a greenhouse.
Understanding a phenomenon is nice, but controlling it takes the satisfaction to an entirely new level.
This may explain why there is so much excitement surrounding the field of biotechnology, in which technology is used to modify organisms, cells, and their molecules to achieve practical benefits.
The modern emphasis in biotechnology is on genetic engineering, the manipulation of organisms’ genetic material by adding, deleting, or transplanting genes from one organism to another.
How do you create a plant resistant to being eaten by insects? Or a colony of bacteria that can produce human insulin? Although there are many different uses of biotechnology, there is a surprisingly small number of recurring themes and tools used. Each of these applications, for example, utilizes a similar sequence of steps and applications (Figure 5-24 Five important tools and techniques of most biotechnology procedures):
1. Chop up the DNA from a donor organism that exhibits the trait of interest.
2. Amplify the small amount of DNA into more useful quantities.
3. Insert the different DNA pieces into bacterial cells or viruses.
4. Grow separate colonies of the bacteria or viruses, each containing a different inserted piece of donor DNA.
5. Identify the colonies that have received the DNA containing the trait of interest.
Now let’s explore each step in more detail (Figure 5-25 Restriction enzymes are used to isolate a gene of interest).
1. Chopping DNA from a donor organism
To begin, researchers select an organism that has a trait they want to make use of, perhaps in developing a product. The researchers might want to produce human growth hormone in large amounts. Their first step would be to obtain human DNA and cut the DNA into smaller pieces. Cutting DNA into small pieces is a process that is done by restriction enzymes.
Restriction enzymes have a single function: When they encounter DNA, they cut it into small pieces. These enzymes evolved to protect bacteria from attack by viruses.
Upon encountering DNA from an invading virus, a restriction enzyme recognizes and binds to a particular sequence of four to eight bases on the invader’s DNA and cuts there, thus making it impossible for the virus to reproduce within the bacterial cell. Since all DNA has the same structure, these enzymes can cut DNA from any source as long as the specific four-eight base sequence is present (bacteria protect their own DNA by modifying it, or it would be cut up as well). Dozens of different restriction enzymes exist, each of which cuts DNA at a different location.
2. Amplifying DNA pieces into more useful quantities
In many situations, only a small amount of DNA will be available for analysis or some other biotechnology use. The polymerase chain reaction (PCR) is a laboratory technique that allows a tiny piece of DNA (perhaps one that has been cut from a larger piece by restriction enzymes or one that has been recovered from a crime scene) to be duplicated repeatedly, producing virtually unlimited amounts of it.
The process involves heating up the DNA of interest, causing the two strands to separate. Then, in the presence of enzymes and plenty of free nucleotides, the DNA is cooled. As it cools, an enzyme matches free nucleotides with their complementary bases on each of the single strands, and covalently links these nucleotides together to form a new single strand across from each original. The results is two complete double-stranded copies of the DNA of interest. This process of heating and cooling can be repeated again and again until there are billions of identical copies of the target sequence.
3. Inserting foreign DNA into the target organism
In the human growth hormone example mentioned above, the researchers might want to transfer the human growth hormone gene into the bacterium E. coli, creating transgenic organisms.
To create a transgenic organism (that is, an organism with DNA inserted from a different species), researchers must physically deliver the DNA from a donor species into the recipient organism. This delivery often is accomplished using plasmids, circular pieces of DNA that can be incorporated into a bacterium’s genome (FIGURE 5-27).
Genes on the plasmid can then be expressed in the bacterial cell and are replicated whenever the cell divides, so both of the new cells contain the plasmid. In some cases, for the delivery process, genes are incorporated into viruses instead of plasmids. The viruses can then be used to infect organisms and transfer the genes of interest into those organisms.
4. Growing bacterial colonies that carry the DNA of interest: Cloning
Once a piece of foreign DNA has been transferred to a bacterial cell, every time the bacterium divides, it creates a clone, a genetically identical cell that contains that inserted DNA. The term cloning describes the production of genetically identical cells, organisms, or DNA molecules, a process that occurs each time a bacterium divides. With numerous rounds of cell division, it is possible to produce a huge number of clones, all of which transcribe and translate the gene of interest.
In a typical experiment, a large amount of DNA may be chopped up with restriction enzymes, incorporated into plasmids, and introduced into bacterial cells. The bacteria are then allowed to divide repeatedly, with each producing a clone of the foreign DNA fragment they carry. Together, all of the different bacterial cells containing all of the different fragments of the original DNA are called a clone library or a gene library (FIGURE 5-28).
5. Identifying bacterial colonies that have received the gene of interest
Chopping up human DNA and inserting the pieces (each of which carries different genes) into the genomes of bacteria, which divide repeatedly, leads to a large population of bacterial cells, with many carrying useful genetic information. But the information is in no particular order, much like a bookstore in which all the books are uncatalogued and in complete disarray. How can a researcher interested in working with bacterial cells that contain just the one human gene capable of producing human growth hormone identify and separate those bacteria from the other bacteria in the population?
Researchers have developed a way to make the bacteria of interest identify themselves (FIGURE 5-29). First, a chemical is added to the entire population of bacterial cells, separating the double-stranded DNA into single strands. Next, a short sequence of single-stranded DNA is washed over the bacteria. Called a DNA probe, this DNA contains part of the sequence of the gene of interest and has also been modified so that it is radioactive. Bacteria with the gene of interest bind to this probe and glow with radioactivity. These cells can then be separated out and grown in large numbers—for example, vats of E. coli that produce human growth hormone.
For thousands of years, humans have been practicing a relatively crude and slow form of genetic engineering—the manipulation of a species’ genome in ways that do not normally occur in nature. In its simplest form, genetic engineering is the careful selection of the plants or animals used as the breeders for a crop or animal population. Through this process, farmers and ranchers have produced meatier turkeys, seedless watermelons, and big, juicy corn kernels (FIGURE 5-30). But what used to take many generations of breeding can now be accomplished in a fraction of the time, using recombinant DNA technology, the combination of DNA from two or more sources into a product.
Almost 10% of the world’s population suffers from vitamin A deficiency, which causes blindness in a quarter-million children each year and a host of other illnesses in people of all ages. These nutritional problems are especially severe in places such as southern Asia and sub-Saharan Africa where rice is a staple of most diets. Addressing this global health issue, researchers have developed what may be the model for solving problems with biotechnology. It involves the creation of a new crop, called “golden rice.”
Mammals generally make vitamin A from beta-carotene, a substance found in abundance in most plants (it’s what makes carrots orange). Beta-carotene is also found in rice plants, but not in the edible part of the rice grains. Researchers set out to change this by inserting into the rice genome three genes that code for the enzymes used in the production of beta-carotene. It’s clear that the transplanted genes are working because the normally white rice takes on a golden color from the accumulated beta-carotene. The rice doesn’t yet supply a full-day’s requirement of vitamin A in one serving, but it does provide a significant amount. Since golden rice was first developed in 1999, new lines have been produced with more than 25 times as much vitamin A than the original strains had. Field tests of golden rice are still underway, so it is not yet being used widely; however, it is viewed as one of the most promising applications of biotechnology.
Figure 5-31 The potential to prevent blindness in 250,000 people each year.
Currently, more than 170 million acres worldwide are planted with genetically modified crops, most containing built-in insecticides and herbicide resistance, representing more than a fortyfold increase over the past 10 years. The financial benefits to farmers—at least in the short run—are so great that more and more of them are embracing the genetically modified crops.
The numbers are surprising: 86% of all corn grown in the United States is genetically modified. Ninety-three percent of all cotton grown is genetically modified. And 93% of all soybeans grown are genetically modified (FIGURE 5-32). Two factors explain much of the extensive adoption of genetically modified plants in U.S. agriculture. (1) Many plants have had insecticides engineered into them, which can reduce the amounts of insecticides used in agriculture. (2) Many plants also have herbicide-resistance genes engineered into them. Such herbicide-resistant plants, as
well as insect-resistant plants, can reduce the amount of plowing required around crops to remove weeds. As a consequence, then, the use of genetically modified plants can reduce both the costs of producing food and the loss of topsoil to erosion.
Insect pests have a field day on agricultural crops (Figure 5-33 Competing with us for food). Crops—whether cotton, potatoes, peas, or something else—that are planted at high densities and nurtured with ample water and fertilizer represent a huge potential food resource for insects. Every year about 40 million tons of corn are unmarketable as a consequence of insect damage. Increasingly, however, farmers have been enjoying greater success in their battles against insect pests, primarily through the use of transgenic crops.
Farmers owe much of this success to soil-dwelling bacteria of the species Bacillus thuringiensis. These bacteria produce spores containing crystals that are highly poisonous to insects but harmless to the crop plants and people. Within an hour of ingesting the crystals, the insect’s feeding is disrupted. The toxic crystals cause pores to develop throughout the insects’ digestive system, paralyzing their gut and making them unable to feed. Within a few days the insects die from a combination of tissue damage and starvation.
Beginning in 1961, the toxic “Bt” crystals were included in pesticides that were sprayed on plants. In 1995, however, recombinant DNA technology led to a huge improvement. The gene coding for the production of the Bt crystals was inserted directly into the DNA of many different crop plants, including corn, cotton, and potatoes, so that the plants themselves produced the crystals.
As a consequence, it is no longer necessary for farmers to apply huge amounts of Bt-containing pesticides (Figure 5-34 Help from bacteria in growing disease-resistant corn). Instead, the plants do that work themselves. The insects that try to eat the genetically modified plants ingest the toxin and soon die. There is no evidence that Bt crystals have any harmful effects on humans at all, even when they are exposed to very high levels.
Bacteria have come to the aid of farmers fighting pests in another way, as well. Consider a seemingly impossible challenge the farmers face when they attempt to kill weeds that harm crop plants by competing for light, water, and soil nutrients. Herbicides, chemicals that kill plants, can be applied to kill the weeds. These chemicals usually work by blocking the action of an enzyme that enables plants, fungi, and bacteria to build three critical amino acids. Without these amino acids, the organisms soon die. The problem is that because the herbicides affect all plants similarly, they are generally toxic to the crop plant as well as the weeds. This is where bacteria come in.
In the 1990s, researchers discovered bacteria that can resist the effect of the herbicide and build the three amino acids, even in the presence of the herbicide. The gene that gives the bacteria resistance to herbicides was identified and introduced into crop plants. Integration of this gene into the plants’ DNA gives them resistance to the herbicides and allows farmers to kill weeds with herbicides without harming the crop plants, greatly increasing yields (Figure 5-35 Crop duster).
Agriculture includes not just the cultivation of plants but also of animals. Currently, researchers have been developing transgenic salmon that grow significantly faster and much larger than normal salmon (Figure 5-36 Bigger salmon). The salmon carry a version of the growth hormone gene that functions year round, rather than primarily in the summer. It was isolated from a cold water fish species, called Arcticpout, and injected into the egg of a salmon. The super fish can be raised much more quickly and uses significantly less feed than normal salmon, reaching market size within 18 months rather than the usual 24 to 30 months. In taste tests, consumers cannot tell the difference between the transgenic and non-transgenic salmon.
Quite troubling is the fact that researchers expected growth rate increases of about 25% but found that the genetically modified fish grew about 500% faster. Additionally, if the larger, faster-growing fish escaped from their breeding nets back into their natural habitat—something that experts agree is inevitable—the fish might harm populations of other species because they can consume more of their own prey and because they may grow too large to be consumed by their natural predators. It is unclear what the outcome would be.
Chickens without feathers look ridiculous (Figure 5-37 “Naked” birds). But such a genetically modified breed was developed with a valuable purpose in mind: “Naked” birds are easier and less expensive to prepare for market, benefiting farmers by lowering their costs and consumers by lowering prices. Such chickens, however, turned out to be unusually vulnerable to mosquito attacks, parasites, and disease, and were ultra-sensitive to sunlight. They also have difficulty mating since the males are unable to flap their wings. Researchers currently are working to address these problems.
These chickens teach us an important lesson about genetically modified plants and animals. Although the new breed of featherless chickens was produced by traditional animal husbandry methods—the cross-breeding of two different types of chickens—as opposed to using recombinant DNA technology, the new breed ended up having not just the desired trait of no feathers, but it also had some unintended and undesirable traits.
EPO has been at the center of several “blood doping” scandals in professional cycling. This hormone increases the oxygen-carrying capacity of the blood, so some otherwise healthy athletes have used EPO to improve their athletic performance. It can be very dangerous, however. By increasing the number of red blood cells, the blood can become much thicker, and this can increase the risk of heart attack.
Would you want to know? Once this was just a hypothetical question: If you carried a gene that meant you were likely to develop a particular disease later in your life, would you want to know? Or another question: Would you want to know if a baby that you and your spouse were trying to conceive would be born with a genetic disease? Now, for better or for worse, these are becoming real questions that we all must address. And there is more at stake than simply peace of mind. As biotechnology develops the tools to identify some of the genetic time bombs that many of us carry, it also carries the danger that such information may become the basis for greater discrimination than we have ever known.
Intervening to prevent diseases using biotechnology focuses on answering questions at three different points in time:
1. Is a given set of parents likely to produce a baby with a genetic disease?
Many genetic diseases occur only if an individual inherits two copies of the disease-causing gene, one from each parent. This is true for Tay-Sachs disease, cystic fibrosis, and sickle-cell anemia, among others. Individuals with only a single copy of the disease-causing gene never fully manifest the disease but may pass the disease gene on to their children. Consequently, two healthy parents may produce a child with the disease. In these cases, it can be beneficial for the parents to be screened to determine whether they carry a disease-causing copy of the gene. Such screening, combined with genetic counseling and testing of embryos following fertilization, can reduce the incidence of the disease dramatically. This has been the case with Tay-Sachs disease, for example. Since screening begin in 1969, the incidence of Tay-Sachs disease has been reduced by more than 75% (Figure 5-41 Genetic screening can determine the presence of the Tay-Sachs gene).
2. Will a baby be born with a genetic disease?
Once fertilization has occurred, it is possible to test an embryo or developing fetus for numerous genetic problems. Prenatal genetic screening can detect disorders such as cystic fibrosis, sickle-cell anemia, Down syndrome, and others. The list of additional conditions that can be detected is growing quickly.
To screen the fetus, it is necessary to sample some of the fetal cells and/or the amniotic fluid, which carries many chemicals produced by the developing embryo. This is usually done via amniocentesis or chorionic villus sampling (CVS)—techniques that we explore in detail in Chapter 6. Once collected, the cells can be analyzed using a variety of means.
3. Is an individual likely to develop a genetic disease later in life?
DNA technology can also be used to detect disease-causing genes in individuals that are currently healthy but are at increased risk of developing an illness later. Early detection of many diseases, such as breast cancer, prostate cancer, and skin cancer, can greatly enhance the ability to treat the disease and can reduce the risk of more severe illness or death.
While biotechnology may reduce suffering and the incidence of diseases, these potential benefits come with significant potential costs, including ethical dilemmas. People who have a gene that puts them at increased risk of developing a particular disease, for example, might be discriminated against, even though they are not currently sick and may never suffer from the particular disease for which they are at heightened risk. Currently, only about half of the states in the United States prohibit genetic discrimination, and insurance companies have already cancelled or denied health and life insurance coverage upon discovering that an individual carries a gene that puts him or her at increased risk of disease. Additionally, parents who discover that their developing fetus will develop a painful, debilitating, or fatal disease soon after birth are confronted with the difficult question of how to proceed.
When it comes to curing a disease by using biotechnology, there is good news and bad news. The good news is that, in the 1990s, a handful of humans with a usually fatal genetic disease called severe combined immunodeficiency disease (SCID) were completely cured through the application of biotechnology. The bad news is that it has not been possible to apply these promising techniques to other diseases.
Let’s examine the case of SCID, which has served as a model for gene therapy. SCID is a condition in which a baby is born with an immune system unable to properly produce a type of white blood cell. This leaves the infant vulnerable to most infections and usually leads to death within the first year of life (FIGURE 5-42). In gene therapy for SCID, researchers removed from an affected baby’s bone marrow some stem cells, cells that have the ability to develop into any type of cell in the body. In bone marrow, they normally produce white blood cells, but in individuals with SCID, a malfunctioning gene disrupts normal white blood cell production.
Next, in a test tube, the bone marrow stem cells were infected with a transgenic virus carrying the functioning gene. Ideally, the virus inserted the good gene into the DNA of the stem cells, which were then injected back into the baby’s bone marrow. There, the cells could produce normal white blood cells, permanently curing the disease. Although this strategy worked to cure several cases of SCID, treatment has been suspended indefinitely following the recent deaths of two patients from illness related to their treatment.
Difficulties with gene therapy have been encountered in several different areas, usually related to the organism used to transfer the normal-functioning gene into the cells of a person with a genetic disease.
Cloning. Perhaps no scientific word more easily conjures horrifying images of the intersection of curiosity and scientific achievement. But is fear the appropriate emotion to feel about this burgeoning technology? Perhaps not.
For starters, let’s clarify what the word means. Cloning actually refers to a variety of different techniques. To be sure, cloning can refer to the creation of new individuals that have exactly the same genome as the donor individual—a process called “whole organism cloning.” That is, a clone is like an identical twin, except that it may differ in age by years or even decades. It is also possible to clone tissues (such as skin) and entire organs from an individual’s cells. And, as we saw in Section 5-11, it is possible to clone genes.
Cloning took center stage in the public imagination in 1997 when Ian Wilmut, a British scientist, and his colleagues first reported that they had cloned a sheep, which they named Dolly. Their research was based on ideas that went back to 1938 when Hans Spemann first proposed the experiment of removing the nucleus from an unfertilized egg and replacing it with the nucleus from the cell of a different individual. Although the process used by Wilmut and his research group was difficult and inefficient, it was surprisingly simple in concept (FIGURE 5-43). They removed a cell from the mammary gland of a grown sheep, put its nucleus into another sheep’s egg from which the nucleus had been removed, induced the egg to divide, and transplanted it into the uterus of a surrogate mother sheep. Out of 272 tries, they achieved just one success. But that was enough to show that the cloning of an adult animal was possible.
Shortly after news of Dolly’s birth, teams set about cloning a variety of other species including mice, cows, pigs, and cats (Figure 5-44 Genetically identical cloned animals). Not all of this work was driven by simple curiosity. For farmers, cloning could have real value. It can take a long time to produce animals with desirable traits from an agricultural perspective—such as increased milk production in cows. And with each successive generation of breeding it can be difficult to maintain these traits in the population. But through the process of transgenic techniques and whole-animal cloning, large numbers of the valuable animals with such traits can be produced and maintained.