Chapter 4 • Lesson 26

Chapter 4 • Lesson 26

DNA Technology

Objectives: 3.3,1,3.3.2, 3,3,3

Key Words

DNA fingerprinting • gel electrophoresis • biotechnology recombinant DMA • plasmid • cloning • stem cell

genetic engineering

Getting the Idea

Recall that DNA carries the genetic information that determines an organism's traits. The sections of DNA that code for specific traits are called genes. Although the DNA of all organisms is composed of the same four nucleotides, the arrangement of these units varies. As a result, different organisms inherit different genes. Information about an organism's DNA is useful in many ways—in scientific studies, crime solving, agriculture, medicine, and many other fields.

DNA Fingerprinting

No two people's fingerprints are the same. Similarly, with the exception of identical twins, no two people have the same DNA. (Even twins may develop small differences in their DNA during their lives.) The same is true of other animals. This knowledge has led to a technology known as DNA fingerprinting. DNA fingerprinting compares images of DNA molecules to determine relationships among individuals.

DNA fingerprints are developed with gel electrophoresis. Gel electrophoresis is a method of separating DNA fragments by passing an electric current through a gel. First, restriction enzymes are used to cut a DNA molecule into smaller pieces. The fragments are then placed in a gel, and the gel is placed between positive and negative electrodes. Objects with the same electrical charge repel each other. DNA has a negative charge, so the DNA fragments move away from the negative electrode. Because opposite electrical charges attract each other, the fragments are attracted to the positive electrode.

Gel electrophoresis sorts the DNA fragments according to size because they move through the gel at different rates. Smaller fragments move more quickly and farther through the gel than larger fragments do. As the DNA fragments become sorted by size, they appear as lines of different lengths on the gel. The resulting pattern of lines is called a DNA fingerprint.

DNA fingerprinting technology has several uses. For example, DMA fingerprints like the ones shown in the diagram below can be used to determine how closely related various organisms are to one another. The more similar the patterns in two DMA fingerprints, the closer the relationship of the organisms. Biologists sometimes use this technique to identify related species and to identify and catalog endangered species. DMA fingerprinting can also be used to identify a child's mother or father.

Forensics is the use of scientific evidence to solve crimes and answer legal questions. Police departments and criminal lawyers use forensic data such as DNA fingerprinting. For example, technicians may collect blood, skin, or other samples from a crime scene and make DNA fingerprints from those samples. Investigators can then compare the DNA fingerprints with the DNA fingerprint of someone suspected of the crime. (Note that this method requires a sample from the suspect. It cannot match a blood sample to a name, drawing, or verbal description.) If the DNA fingerprints match, the person is presumed to have been at the crime scene. If they do not match, the person is presumed not to have been there. It is important for the technicians to work carefully. Although any two people will always have differences in their DNA, a comparison of only part of the DNA may not pick up differences between close relatives.

The Human Genome Project

All the hereditary information of an organism makes up its genome. Scientists determine an organism's genome partly by identifying the sequence of the bases in the organism's DNA. However, to understand the genome, scientists must also map the positions of an organism's genes on its chromosomes and identify the traits each gene codes for.

In 1990, scientists began work on the Human Genome Project. One goal of the project was to identify and map the locations of all of the approximately 20,000-25,000 genes in human DNA. Scientists also hoped to determine the sequence of the 3 billion chemical base pairs that make up human DNA.

The Human Genome Project was completed in 2003. It involved the efforts of many scientists working in different locations. Because humans are complex organisms that show a great diversity of traits, the scientists did not study DNA from only one individual. Instead, they examined chromosomes donated by many people and combined the data to reveal an average human genome. All the data from the project were incorporated into a database that is available to scientists worldwide. Since the project's completion, scientists have sequenced the genomes of many other species of organisms. These data have also been entered into databases that make them widely available.

Scientists are using data from the Human Genome Project and similar sequencing work in many ways. Medical researchers can use the data to determine whether people carry the genes for certain diseases and to develop treatments for abnormalities that lead to diseases. In some cases, scientists have found multiple mutations, sometimes to different genes, that can cause the same disease. These results have made the development of tests and treatments for some diseases more complicated than had been hoped.

The results of the Human Genome Project have led to the possibility of using gene therapy to treat genetic disorders. Gene therapy involves replacing a defective or missing gene in a person's genome. One disease for which scientists are working on gene therapy is cystic fibrosis (CF). Recall from the last lesson that CF is an inherited disease that usually affects the lungs. At present, doctors can treat some of the symptoms but are not able to cure the disease. Scientists have experimented with several methods of replacing the defective genes in people with CF. So far, none of these experiments has been successful.

Several problems are limiting the use of gene therapy. The first is the need to find the exact gene or genes that cause a specific disease. If a genetic disease has more than one cause, a gene therapy might help only some of the people with the disease. Once a missing or harmful gene is identified, researchers need to find a way to deliver a healthy replacement gene to the affected cells. The gene must then insert itself in the cell's genome in a place where it will be expressed and make protein. Recall that not all the genes in a cell are activated. Also, it is not sufficient to insert the gene once. The replacement gene must be inserted in many cells of the body because inherited diseases affect many cells.

Gene therapy might also harm some people. One danger is that gene therapy often uses weakened viruses to transport genes. The viruses could make some patients sicker. Gene therapy could also be harmful if a gene inserted itself incorrectly. For example, a healthy replacement gene inserted in the middle of another gene could stop that gene from functioning.

Genetic Engineering and Recombinant DNA

Biotechnology is the manipulation of living organisms or their parts to produce useful products. The main uses of biotechnology are to improve human health and food production. People have selectively bred plants and animals to produce offspring with desired traits for thousands of years. Today, scientists also use modern genetics to introduce new characteristics into organisms or populations.

Genetic engineering is one technique of biotechnology that is being used to alter the traits of organisms. Genetic engineering is the transfer of genes or pieces of DNA from one organism to another. An organism whose genes have been altered by the insertion of DNA from another organism is called a transgenic organism, or genetically modified organism (GMO).

Genetic engineers have altered the genes of bacteria and yeasts to produce medicines used to treat human diseases. They have modified lab animals to make it easier to study cancer and other diseases. They have improved some crops to make them more nourishing. They have modified other plants to be more resistant to damage from frost, insect pests, diseases, and herbicides.

When DNA from one organism is inserted into the DNA of another organism, the new DNA that results is called recombinant DNA. Recombinant DNA is often formed by transferring DNA from a complex organism into a simpler organism. For example, scientists might place a gene that has been removed from human DNA into a bacterium. Recall that some of the DNA in a bacterial cell is in the form of a ring called a plasmid. To form recombinant DNA, biotechnologists remove the plasmid from a bacterium, use restriction enzymes to cut the plasmid open, and then insert a human gene into the plasmid. Once the gene is inserted, the plasmid returns to its circular shape. The biotechnologists then put the modified plasmid, containing the human gene, back inside the bacterium.

Bacteria reproduce asexually through a type of cell division known as binary fission. Binary fission is similar to cell division by mitosis, except that no nucleus is involved. Before reproducing, a bacterium makes a copy of its DNA, including the plasmid. If the plasmid contains recombinant DNA, the offspring will also contain the recombinant DNA. The drawing below shows the production of recombinant DNA in a common bacterium called E. coli. This technique has been used to form bacteria that produce human insulin. Many people with diabetes must receive insulin because their bodies cannot produce it.

Scientists have used recombinant DMA to modify bacteria for industrial and environmental uses. For example, genetically altered bacteria are used to help clean up oil spills. The bacteria eat the oil. Other genetically engineered bacteria are used to process minerals and make chemicals. Although the bacteria are useful, some people have concerns about introducing these new organisms into the environment.

Recombinant DMA has also been used to change the traits of animals and plants. For example, goats have been modified to produce a protein that is used to treat people whose blood clots too easily. Some genetically modified sheep produce a protein that is used to treat the lung disease emphysema. Bt corn is an example of a genetically engineered plant. This corn has a transferred gene (from a bacterium called Bacillus thuringiensis) that enables it to make a pesticide that protects the plants from a pest called the corn borer. Scientists have also modified cotton plants to reduce damage by worms that attack cotton buds. The genetically altered plants are resistant to the worms. This resistance increases the cotton yield and enables farmers to use fewer pesticides. Modifying the plants is often safer for the environment than growing unmodified cotton and using pesticides because the new trait affects only the worms that attack the plants. Other organisms are not harmed.

Cloning

Cloning is the artificial production of a DMA fragment, cell, or organism that is genetically identical to the original DMA, cell, or organism. In some cases, scientists can use a single cell from an adult organism to grow an entirely new individual that is genetically identical to the donor. Cloning is not difficult when it involves making a DMA fragment because DNA normally replicates to form an exact copy of itself. It is also fairly simple to clone a single cell or a single-celled organism because each normally produces a daughter cell that is genetically identical to the parent cell. However, cloning in animals is a complex and difficult process.

Some researchers hope to use cloning to increase the populations of endangered species. Cloning might also be used together with recombinant DNA to produce medically or commercially valuable substances. For example, recombinant DNA could be used to make goats whose milk contains valuable proteins. The goats could then be cloned to produce a new population of goats that would produce milk containing those proteins.

Stem Cells

Recall from Lesson 6 that during development, an organism's cells differentiate to form many types of cells, such as blood, muscle, and nerve cells. When such cells divide, they produce more of the same kind of cell. By contrast, some cells, called stem cells, are not specialized. These cells have the ability to develop into a wide variety of cells.

In recent years, researchers have been investigating ways to use stem cells to replace cells that have been damaged by injury or disease and that cannot regenerate. The goal is for the stem cells to develop into new, healthy body cells. Stem cell transplants might be used to help or cure people with diseases or injuries for which there is currently no good treatment. Possible applications include treatment of spinal cord injuries, which cause paralysis; heart disease; and Alzheimer's disease, which affects thinking and memory.

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Concerns regarding Genetic Engineering

There is debate about the ethics of using genetic engineering to alter future generations of humans, even to cure inherited disease. Some people fear that the same technologies that enable doctors to eliminate obviously harmful traits could be used to selectively change other characteristics, such as height, intelligence, and factors related to appearance, such as hair and eye color.

People also disagree about research and medical treatments using stem cells. There are two main sources of stem cells: embryonic tissue and adult tissue. Some people have ethical objections to using cells from embryos. Others argue that it would be unethical to just throw away unwanted embryos because the cells might be used to treat serious diseases. It is not yet clear how useful adult stem cells can be in the treatment of many medical conditions, although there have been some promising developments. Scientists continue to work with both embryonic and adult stem cells to learn more about how organisms develop and to treat diseases and injuries.

Many people have concerns about the uses of genetically modified organisms. For example, some genetically modified crops are used as food for humans. Scientists do not yet know the long-term effects of eating genetically engineered foods. One possible risk is allergic reactions. If a useful gene is taken from a plant people are allergic to, they might also be allergic to the resulting bioengineered food.

A less obvious risk of the use of genetically engineered crops is that such crops could reduce genetic variation. A population with more variation is better able to resist threats such as disease and drought. Until recently, farmers in different places have grown many varieties of corn, wheat, and other food crops. In some places, a dozen or more varieties of one type of crop may be grown in a small area. However, people often use genetic engineering to create a single crop variety with the desired trait. If all the farmers in an area buy and plant only those seeds, they may produce a larger wheat crop or more nourishing rice. However, they risk losing the entire harvest to a disease that would not kill some other varieties of wheat or rice.

Focus on Inquiry

Much research involving stem cells has been devoted to using stem cells to replace damaged organs. This research often involves manipulating stem cells so they divide repeatedly and then differentiate to form a specific type of tissue or organ

The skin is the largest organ of the human body. One role of the skin is to provide a protective barrier between an organism and its environment. The barrier plays a role in the immune system by keeping potentially harmful organisms out of the body. The skin also helps maintain homeostasis by sweating, which controls body temperature and eliminates excess salts and water.

Because of its many roles in the body, it is important to keep the skin healthy. When skin is damaged in some ways, such as by small cuts, the body can often repair the damage on its own. However, some types of skin damage, including large burns caused by heat or chemicals, cannot be easily repaired and need extensive medical treatment.

One active area of stem cell research is growing new skin for patients with severe burns or other injuries. Do research in books, in journals, or on the Internet to find out more about the use of stem cells to grow new skin. As you conduct you research, keep in mind that some sources are more reliable than others. For example, when using the Internet, try to locate information on Web sites that end in .gov or .edu. These endings indicate that the resource has been posted by a government agency or an educational facility such as a college or university. Some resources that end in .org (indicating an organization) may also be reputable if the organization is well recognized for its expertise in a particular area. Other reputable sources of information include articles by scientists working in the field, especially if the articles appear in a peer-reviewed science journal. Keep in mind that some articles, even when written by scientists, may be biased because the writers want the work to succeed, or because they want funding for their research. Think about whether the claims you read are reasonable.

Why would growing replacement skin from stem cells be valuable?

Doctors and scientists conduct many kinds of research. Not all of the techniques that are developed in the lab become part of ordinary medical practice. What factors might slow or stop the introduction of stem cell technology?