Molecular & Cell Biology For Dummies
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Recombinant DNA technology can be controversial. People, including scientists, worry about the ethical, legal, and environmental consequences of altering the DNA code of organisms:
  • Genetically modified organisms (GMOs) that contain genes from a different organism are currently used in agriculture, but some people are concerned about the following potential impacts on wild organisms and on small farms:
    • Genetically modified plants may interbreed with wild species, transferring genes for pesticide resistance to weeds.
    • Crop plants that are engineered to make toxins intended to kill agricultural pests can also impact populations of other insects.
    • Small farmers may not be able to afford genetically modified crop plants, putting them at a disadvantage to larger corporate farms.
  • Genetic testing of fetuses allows the early detection of genetic disease, but some people worry that genetic testing will be taken to extremes, leading to a society where only “perfect” people are allowed to survive.
  • Genetic testing of adults allows people to learn whether they have inherited diseases that run in their family, but some people worry that one day insurance companies will use genetic profiles of people to make decisions about who to insure.
  • Parents of children with life-threatening diseases that can be treated with bone marrow transplants are using genetic testing to conceive children that can provide stem cells for their sick siblings. The umbilical cord is an excellent source of these stem cells, so the new babies aren’t harmed, but people worry that this may lead to an extreme future scenario where babies are born to serve as bone marrow or organ donors for existing people.
  • Human hormones like insulin and human growth hormone are produced by bacteria through recombinant DNA technology and used to treat diseases like diabetes and pituitary dwarfism. However, some people seek hormones like human growth hormone for cosmetic reasons (for example, so that their children can be a little taller). People question whether it’s ethical for parents to make these choices for their children and whether too much emphasis is being placed on certain physical traits in society.

Making useful proteins through genetic engineering

Scientists use the bacterium E. coli as a little cellular factory to produce human proteins for treatment of diseases. To get E. coli to produce human proteins, cDNA copies of human genes are put into plasmid vectors and then the vectors are introduced into E. coli.

Genetic engineering. Genetic engineering.

The bacterium transcribes and translates the human gene, producing a human protein that is identical to the protein made by healthy human cells. Several human proteins are currently produced by this method, including the following:

  • Human insulin for treatment of diabetes
  • Human growth hormone for treatment of pituitary dwarfism
  • Tumor necrosis factor, taxol, and interleukin-2 for treatment of cancer
  • Epidermal growth factor for treatment of burns and ulcers

Searching for disease genes

Some people carry the potential for future disease in their genes. Genetic screening allows people to discover whether they’re carrying recessive alleles for genetic diseases, allowing them to choose whether or not to have children. Also, diseases that show up later in life, such as Alzheimer’s and Huntington’s disease, can be detected early, to seek the earliest possible treatment.

In order to screen for a particular genetic disease, scientists must first discover the gene that causes the disease and study the normal and disease-causing sequences. Scientists have identified the genes for several genetic diseases, including cystic fibrosis, sickle-cell anemia, Huntington’s disease, an inherited form of Alzheimer’s, and an inherited form of breast cancer.

Once the gene for a genetic disease has been identified, doctors can screen people to determine whether they have normal or disease-causing alleles.

In order to screen a person for a particular gene, scientists amplify the genes linked to the disease using PCR. Then, scientists screen the genes for disease alleles:
  • Scientists can copy and sequence a specific gene. If you have risk for a genetic disease, perhaps because people in your family suffer from the disease, scientists can use PCR to make amplify your copies of the gene associated with that disease. They use DNA sequencing to read the code of your genes, then compare your code to known codes for normal and disease-causing alleles of the gene. You might find out that you don’t have any disease-causing alleles, or that you’re a carrier who has one disease and one normal allele, or that you have two copies of the disease-causing form.
  • Scientists can sequence your genome. If a specific gene isn’t identified as causing a problem, a doctor may order genome sequencing. A sample of all of your DNA will be cut into pieces, then sequenced using next-generation sequencing methods. The code from your DNA will be compared to reference human genomes to look for variations in your code that might be associated with disease.

Building a “better” plant with genetic engineering

Many important crop plants contain recombinant genes. These transgenic plants, which are a type of genetically modified organism (GMO), provide labor-saving advantages to farmers who can afford them:
  • Transgenic plants that contain genes for herbicide resistance require less physical weed control. Farmers can spray crop plants that are resistant to a particular herbicide with that herbicide to control weeds. Weed plants will be killed, but the modified crop plants will not.
  • Transgenic plants that contain genes for insect toxins will be less damaged by grazing insects. The crop plants use the introduced gene to produce insect toxins that kill insects that graze on the plants.
Scientists often use the bacterium Agrobacterium tumefaciens to modify plant genomes. In nature, this soil bacterium slips a piece of its DNA into plant cells, resulting in crown gall disease. Scientists studying this disease discovered that Agrobacterium tumefaciens contains a small circle of DNA they named the Ti plasmid (Ti for tumor-inducing), which contains the genes necessary for the bacterium to transfer a section of its DNA into plant cells. When this bacterium receives the right signals, it takes a piece of DNA from the Ti plasmid and sends it into plant cells where it integrates into the plant genome. In the case of crown gall disease, the bacterial DNA causes production of plant hormones that produce a tumor-like growth (see the following figure). In the case of genetic engineering, scientists replace the disease-causing genes with the genes they want to introduce into the plant.

Transgenic plants. Transgenic plants.

Another potential benefit of transgenic plants is that certain crop plants may be altered to become more nutritious. For example, scientists are currently working on developing a strain of golden rice that may help combat Vitamin A deficiency in people around the world. Vitamin A deficiency can cause blindness and increase susceptibility to infectious diseases. Golden rice is being engineered to contain the genes necessary for the rice plants to produce beta-carotene. When people eat golden rice, their bodies will use beta-carotene to make Vitamin A. Rice is a staple food for half of the world’s people, so golden rice has great potential for fighting Vitamin A deficiency!

Fixing a broken gene with gene therapy

The ultimate cure for a genetic disease would be if scientists could replace the defective genes. As soon as recombinant DNA technology became available, scientists started wondering if they could use this technology to create cures for genetic diseases. After all, if scientists can transfer genes successfully into bacteria and plants, perhaps they can also transfer them into people that have defective disease-causing alleles (see the following figure). By introducing a copy of the normal allele into affected cells, the cells could be made to function normally, eliminating the effects of the disease.

The introduction of a gene in order to cure a genetic disease is called gene therapy.

Gene therapy for humans is being studied, and clinical trials have occurred for some diseases, but this type of treatment is far from being perfected. Many barriers to successful human gene therapy still need to be overcome:
  • Scientists must discover safe vectors that can transfer genes into human cells. One possible vector is viruses that naturally attack human cells and introduce their DNA. Viral DNA is removed and replaced with therapeutic genes that contain the normal allele sequence. The viruses are allowed to infect human cells, thus introducing the therapeutic genes. Following are several safety issues associated with the use of viruses as vectors in gene therapy:
    • Viruses that have been altered may recombine with existing viruses to recreate a disease-causing strain.
    • Viruses that have been altered so that they can’t directly cause disease may still cause a severe allergic reaction that is potentially life threatening.
    • Viruses that introduce genes into human cells may interrupt the function of normal genes.
Gene therapy in humans. Gene therapy in humans.
  • Scientists must develop methods for introducing therapeutic genes into populations of target cells. Humans are multicellular and have complex tissues. Genetic diseases can affect entire populations of cells. If gene therapy is to cure these diseases, the therapeutic genes must be introduced into all of the affected cells.
  • Stem cells that produce target populations of cells need to be identified. If therapeutic genes are introduced into cells that have a limited lifespan in the body, then gene therapy will need to be repeated at regular intervals to maintain populations of healthy cells. On the other hand, if stem cells could be repaired with normal alleles, then they would continuously produce new populations of healthy cells, and the cure would be permanent.
Because of the challenges of successfully treating people with genes delivered with vectors, many scientists are turning their attention to the newer technology of genome editing.

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