Biology by Karl Irvin Baguio (the ebook reader TXT) 📕

that recognizes and binds to a complementary section of DNA on a larger DNA molecule. After the probe unites with its target, it emits a signal like that from a radioactive isotope to indicate that a reaction has occurred.

 

To work effectively, a sufficiently large amount of target DNA must be available. To increase the amount of available DNA, a process called the polymerase chain reaction (PCR) is used. In a highly automated machine, the target DNA is combined with enzymes, nucleotides, and a primer DNA. In exponential fashion, the enzymes synthesize copies of the target DNA, so that in a few hours, billions of molecules of DNA exist where only a few were before.

 

Using DNA probes and PCR, scientists are now able to detect the DNA associated with HIV and AIDS. This has yielded a direct test for HIV that is preferred over the HIV antibody test. Lyme disease and genetic diseases such as cystic fibrosis, muscular dystrophy, Huntington’s disease, and fragile X syndrome can be identified by DNA probes. (Cystic fibrosis is a respiratory disease in which mucus clogs the respiratory passageways and makes breathing difficult; muscular dystrophy is a disorder of the nervous system in which destruction of nerve fibers leads to erratic muscular activity; Huntington’s disease is a disease of the nervous system accompanied by erratic movements and nervous degeneration; and fragile X syndrome is a disease of the X chromosome accompanied by a form of mental deficiency.)

 

Segments of DNA called restriction fragment length polymorphisms (RFLPs) are the objectives of the tests with gene probes. RFLPs are apparently useless bits of DNA located near genes associated with the diseases. By locating RFLPs, the biotechnologist can locate the disease gene. DNA probes also detect microorganisms in the environment and identify viral and bacterial pathogens.

 

Gene Therapy

 

Gene therapy is a recombinant DNA process in which cells are taken from the patient, altered by adding genes, and replaced in the patient. The genes then provide the genetic codes for proteins the patient is lacking. Nonreproductive cells are used in gene therapy, so there is no carryover of inserted genes to the next generation.

 

In the early 1990s, gene therapy was used to correct a deficiency of the enzyme adenosine deaminase (ADA). Blood cells called lymphocytes were removed from the bone marrow of two children, then genes for ADA production were inserted into the cells using viruses as vectors. Finally, the cells were reinfused in the bodies of the children. Once established in the bodies, the gene-altered cells began synthesizing the enzyme ADA. Thus, the deficiency was removed and the disease resolved.

 

There are over 4,000 single-gene defects, and patients with these defects may be candidates for gene therapy. A multilayered review system now exists to ensure the safety of gene therapy proposals. Many aspects must be considered before approval is granted for gene therapy experiments.

 

DNA Fingerprinting

 

The use of DNA probes and the development of retrieval techniques have made it possible to match DNA molecules to one another for identification purposes. This process has been used in a forensic procedure called DNA fingerprinting.

 

DNA fingerprinting depends on the presence of RFLPs, the repeating base sequences that exist in the human genome. As the pattern of RFLPs is unique for each individual, it can be used as a molecular fingerprint.

 

To perform DNA fingerprinting, DNA is obtained from an individual’s blood cells, hair fibers, skin fragments, or other tissue. The DNA is then extracted from the cells and digested with enzymes. The resulting fragments are separated by a process called electrophoresis, in which electrical charges separate DNA fragments according to size. The separated DNA fragments are then detected with DNA probes and used to develop a fingerprint. A statistical evaluation enables the forensic pathologist to compare a suspect’s DNA with the DNA recovered at a crime scene and to state with a high degree of certainty (usually 99 percent) that the suspect was at the crime scene.

 

Searching for DNA

 

The ability to retrieve DNA from ancient materials and museum specimens has given archaeologists and anthropologists hope of a glimpse at ancient life. Biochemists have successfully obtained DNA from extinct animals and plants. Evolutionary biologists have used the DNA to draw lineage patterns from the data. This often gives a better understanding of relationships between species. DNA isolated from ancient humans has been used to trace the movements of populations, such as the Anglo-Saxons, as well as to determine whether males were favored over females in certain societies.

 

Studies have also been performed on human origins by using the DNA found in the mitochondria. All of an offspring’s mitochondrial DNA is derived from its mother. Because this DNA represents an unbroken line of genetic information, an analysis of mutation sites in the mitochondrial DNA can conceivably lead one back to the first human female.

 

DNA and Agriculture

 

Although plants are more difficult to work with than bacteria, gene insertions can be made into single plant cells. Then the cells can be cultivated to form a mature plant. The major method for inserting genes is through the plasmids of the bacterium called Agrobacterium tumefaciens. This bacterium invades plant cells, and its plasmids insert into plant chromosomes carrying the genes for tumor induction. Scientists remove the tumor-inducing genes and obtain a plasmid that unites with the plant cell without causing any harm.

 

Recombinant DNA and biotechnology have been used to increase the efficiency of plant growth by increasing the efficiency of the plant’s ability to fix nitrogen. Scientists have obtained the genes for nitrogen fixation from bacteria and have incorporated those genes into plant cells. The plant cells can then perform a process that normally takes place only in bacteria.

 

DNA technology has also been used to increase plant resistance to disease by reengineering the plant to produce viral proteins. Also, the genes for an insecticide obtained from a bacterium have been inserted into plants to allow the plants to resist caterpillars and other pests.

 

One of the first agricultural products of biotechnology was the rot-resistant tomato. This plant was altered by adding a gene that produces an antisense molecule. The antisense molecule inhibits the tomato from producing the enzyme that encourages rotting. Without this enzyme, the tomato can ripen longer on the vine.

 

Cloning refers to the ability to make a genetic replica of a cell (or even an entire organism, also called reproductive cloning). Interest in cloning is due primarily to its potential to create stem cells. A stem cell is a cell that has not yet differentiated and retains the ability to turn into many different tissues. Stem cells have enormous potential in medical applications because they can be used to replace diseased or damaged tissues and aid in organ repair or replacement.

 

Cloning and Stem Cells

 

In order for reproductive cloning to work, a differentiated cell must “dedifferentiate,” allowing it to access the genes for any given cell type. This cell can then give rise to all the specialized cell types of an organism. One source of undifferentiated stem cells is from early-stage embryos. Considering the ethical implications of embryonic stem (ES) cells, research has begun to focus on adult stem cells. Adult stem cells are found in a number of tissues, including dental pulp, bone marrow, and the brain. These cells, however, are not able to differentiate into all cell types and thus are not as beneficial as embryonic stem cells.

 

In 2007, researchers announced their successful reprogramming of fully differentiated adult cells, thus inducing an ES-cell state. These induced pluripotent stem (iPS) cells can do everything an ES cell can do without the ethical implications associated with embryonic cells.

 

Tools of Biotechnology

 

The basic process of recombinant DNA technology involves manipulating an organism’s DNA and thus altering the proteins being produced (see Chapter 10). During this synthesis, DNA provides the genetic code for the placement of amino acids in proteins. By intervening in this process, scientists can change the nature of the DNA, thereby changing the nature of the protein expressed by that DNA. By inserting genes into the genome of an organism, the scientist can induce the organism to produce a protein it does not normally produce.

 

The technology of recombinant DNA has been made possible in part by extensive research on microorganisms during the last half-century. One important microorganism in recombinant DNA research is Escherichia coli, commonly referred to as E. coli. The biochemistry and genetics of E. coli are well known, and its DNA has been isolated and made to accept new genes. The DNA can then be forced into fresh E. coli cells, and the bacteria will begin to produce the proteins specified by the foreign genes. Such altered bacteria are said to have been transformed.

 

Knowledge about viruses has also aided the development of DNA technology. Viruses are fragments of nucleic acid surrounded by a protein coat. Viruses attack cells and replicate within the cells, thereby destroying them. By attaching DNA to viruses, scientists use viruses to transport foreign DNA into cells and to connect it with the nucleic acid of the cells.

 

Another common method for inserting DNA into cells is to use plasmids, which are small loops of DNA in the cytoplasm of bacterial cells. Working with a plasmid is much easier than working with a chromosome, so plasmids are often the carriers, or vectors, of DNA. Plasmids can be isolated, recombined with foreign DNA, and then inserted into cells where they multiply as the cells multiply.

 

Interest in recombinant DNA and biotechnology heightened considerably during the 1960s and 1970s with the discovery of restriction enzymes. These enzymes catalyze the opening of a DNA molecule at a “restricted” point, regardless of the source of the DNA. Figure 11-1 shows that a human DNA molecule is opened at a certain site by the restriction enzyme EcoRI (upper left), and the desired DNA fragment is isolated (lower left). Plasmid DNA is treated with the same enzyme and opened. The DNA fragment is spliced into the plasmid to produce the recombinant DNA molecule.

 

Figure 11-1   Construction of a recombinant DNA molecule.

 

Certain restriction enzymes leave dangling ends of DNA molecules at the point where the DNA is opened. Foreign DNA can therefore be combined with the carrier DNA at this point. An enzyme called DNA ligase forges a permanent link between the dangling ends of the DNA molecules at the point of union.

 

Recombinant DNA technology is sophisticated and expensive. Genes must be isolated, vectors must be identified, and gene control must be maintained. Stability of the vector within a host cell is important, and the scientist must be certain that nonpathogenic bacteria are used. Cells from mammals can be used to synthesize proteins, but cultivating these cells is difficult. In addition, the proper gene signals must be identified, RNA molecules must be bound to ribosomes, and the presence of introns must be considered. Collecting the gene product and exporting it from the cell are other considerations.

 

The genes used in DNA technology are commonly obtained from host cells or organisms called gene libraries. A gene library is a collection of cells identified as harboring a specific gene. For example, E. coli cells can be stored with the genes for human insulin in their chromosomes.

 

Transgenic Animals

 

A transgenic animal is an animal in which one or more genes have been introduced into its nonreproductive cells. The first transgenic animal was produced in 1983 when genes for human growth hormone were introduced into mice.

 

Transgenic animals can be used to produce valuable products. For example, a transgenic pig has been produced with the ability to synthesize human hemoglobin for use as a blood substitute. Also, a transgenic cow has been bred with the ability to produce human lactoferrin, an iron-building milk protein and a potential antibacterial agent. A transgenic sheep can synthesize a protein that helps emphysema patients breathe more easily, and a transgenic goat has been developed to produce a protein needed by cystic fibrosis patients.

 

Human Genome

 

In 1990, researchers at Celera Genomics and at the National Human Genome Research Institute began an ambitious endeavor to sequence the entire human genome. In 2003, the project was completed, resulting