Oligonucleotides
Related Terms
Adenosin, cancer, cytosine, deoxyribonucleic acid (DNA), genetics, genetic biosensors, genomics, guanine, infection, inherited disorders, microarray, molecular genetic testing, nucleic acid, nucleotide, oligonucleotides, probe, ribonucleic acid (RNA), sequencing, thymine.
Background
Biosensors are molecules that can attach to specific molecules, such as proteins and genes. Genetic biosensors, or oligonucleotides, are small segments of DNA (deoxyribonucleic acid) or RNA (ribonucleic acid). When these oligonucleotides attach to their target, they release a signal that can be measured. In this way, biosensors can be used to detect certain genes or parts of genes associated with disease, such as viral infections and cancer.
Genes are found inside the cells of all organisms and are located in a compartment within the cell called the nucleus. An individual's genes are present in a large molecule called DNA (deoxyribonucleic acid), which looks like a twisted ladder. This unique shape is called a double helix. The sides of the double helix are made of alternating sugar and phosphate molecules. The "rungs" of the "ladder" are made of smaller molecules called nucleic acids, or nitrogen bases. There are four different types of these smaller molecules in DNA: adenine, thymine, cytosine, and guanine. The four nitrogen bases pair with each other in a specific manner to create the double helix structure. All genes are made up of different combinations of these four molecules, which are arranged in different lengths.
The sequence of these molecules provides the "code," or instructions, for each of the genes involved in the development, growth, and function of all the cells in the body. Each gene provides the instructions for making a specific protein, which may be involved in the structuring of cells and organs or in complex metabolic activities, such as food digestion, drug interactions, and wound healing.
Biosensors can be used to detect specific genes or parts of genes. This is useful in the diagnosis of some types of infections, where the presence of viral or bacterial DNA indicates an active infection. Biosensors can also be used to detect mutations, or defects, in genes that are associated with a certain disease. This occurs in some types of inherited disorders, such as cystic fibrosis, and in cancer.
Methods
Biosensors can be used to distinguish genetic material from infectious organisms, such as viruses, fungi, and bacteria. They can also be used to detect genetic mutations that are associated with a specific disease, such as cancer or inherited disorders or to locate specific segments of DNA or RNA for research.
Biosensor testing is used to detect segments of genetic material in an individual's blood or tissue. To create a biosensor, a probe is created. A probe is a segment of DNA called an oligonucleotide, which is designed to recognize and bind to the genetic material being tested.
Nucleotide sequencing: In order to create an oligonucleotide that binds to DNA or RNA, the specific sequence of the bases must be determined. Nucleotide sequencing is a laboratory technique used to determine this sequence. To sequence DNA or RNA, the segment is mixed with an enzyme that creates copies of the genetic material, a primer or a segment of DNA that tells the enzyme where to begin copying, and the four nitrogen bases required to synthesize the DNA or RNA molecule. The primer attaches to the molecule of interest, and the enzyme creates copies with the nucleotide bases. The nucleotide bases have specific signals on them so that they can be detected and the sequence can be determined.
Synthesis: Oligonucleotides are created from single nitrogen bases. The oligonucleotide is synthesized in an automated machine that adds the necessary nitrogen bases in the appropriate amounts and at the appropriate times. One end of the oligonucleotide is attached to a glass bead, and nitrogen bases are added to the other end through chemical reactions until the oligonucleotide is complete. Then the extra nitrogen bases are washed off and the oliognucleotide is left attached to the glass bead. A chemical, usually ammonia, is added, which causes the oligonucleotide to be released from the bead, making it ready to be used.
Once an oligonucleotide probe has been sequenced, a signal molecule is added to it. This signal may be a radioactive molecule, a protein that creates a chemical reaction resulting in the production of a specific color, or a molecule that produces fluorescent light. These signals are detected by machines that detect color changes or fluorescent light.
The probe is then attached to a surface, such as a glass slide, and the DNA or RNA is added. The segment of genetic material that is present in the patient's DNA will attach to the probe. After it attaches, the signal molecule on the probe can be detected. The method of detection depends on the type of signal used. For instance, a fluorescent signal would be measured with a sensor that detects fluorescent light. This would signal the presence or absence of the genetic material and indicate how much is present.
DNA microarray is a method of studying multiple genes at a single time. In a microarray, hundreds or thousands of probes are attached to a solid support system, and the patient's DNA is added. Each probe identifies a different genetic segment. Microarrays are used to detect changes in dozens, hundreds, or thousands of genes and can therefore provide more information about a person's specific genetic makeup. This information may be used to detect genetic variations that make an individual more susceptible to certain diseases, such as heart disease or diabetes. Because microarrays are a relatively new technology, they are primarily used in research.
Research
Most infections are now diagnosed by identifying the bacteria or viral particles microscopically or by growing them in the laboratory over a period of days or weeks. The use of biosensors to detect the presence of infectious agents would allow for more rapid and accurate diagnosis. For example, genetic biosensors for the detection of the bacteria methicillin-resistant Staphylococcus aureus (MRSA) are currently in development. MRSA is a bacterium that can cause serious infections and is resistant to many common antibiotics. It is important to identify MRSA in a patient so that effective antibiotics are used.
All cancers have genetic mutations or defects. Some of these mutations are small, affecting only one or two molecules within the gene, while others can be large and may involve the loss of large segments of genes. Biosensors can be used to detect these defects even when they are present in very small amounts. Researchers are attempting to create diagnostic tests that would allow cancer to be detected from very small amounts of tissue and very early in the course of the disease, before the cancer has grown large enough to create significant symptoms. Biosensor detection of gene variants has been used to detect a mutation in a gene called BRCA1 that occurs in people with some types of breast cancer.
DNA biosensors can be used to detect genetic mutations associated with inherited disorders, which are passed from parents to children. Examples of inherited disorders include cystic fibrosis and muscular dystrophy. New tests are being developed to detect a number of different inherited genetic disorders, including the blood disorder thalassemia. Thalassemia is a disorder that causes red blood cells to produce less hemoglobin than normal. Hemoglobin is the molecule within red blood cells that carries oxygen from the lungs to the body. In patients with some types of thalassemia, the red blood cells are not capable of carrying enough oxygen, and they must be treated with repeated blood transfusions. Biosensors could be used to detect the gene mutation that causes thalassemia, making it possible to diagnose patients at an earlier age, before they develop symptoms.
DNA microarrays are being used to determine genetic patterns associated with certain diseases. Microarrays are capable of evaluating hundreds or thousands of genes at a time. Researchers can use this tool to detect combinations of genes that put humans at risk for diseases such as cancer and heart disease.
Implications
Biosensors can detect very small amounts of genetic material. Thus, they may be used to diagnose infections in the very earliest stages and to monitor patients with chronic infections. They can also diagnose infections much faster than current techniques. In the detection of cancer, biosensors may be used to detect genetic mutations associated with cancer before the cancer produces symptoms, allowing for earlier diagnosis and treatment.
Limitations
Biosensors are a relatively new technology and are therefore not available in many laboratories. In smaller labs, the cost of biosensors and the technical skills required may make this technology unavailable.
In order for a biosensor to be created, the sequence of the DNA segment of interest must be determined. For instance, to create a biosensor that detects infection with a specific type of bacteria, a piece of genetic material that is unique to the bacteria must first be identified. Thus, biosensor tests cannot be developed without a significant amount of preliminary research.
Biosensors must be created that attach specifically to the genetic molecule of interest and not to other pieces of DNA or RNA. It can be difficult to design biosensor probes that are highly specific and that do not cross-react with other genetic molecules.
Future research
Future areas of research include the application of biosensors to the diagnosis of additional types of inherited diseases and cancer. Biosensors may be developed that are cheaper, easier to use, faster, and more accurate.
Some areas of current and future research include tests to diagnose infections such as Staphylococcus aureus, Escherichia coli, and hepatitis B virus.
Author information
This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).
Bibliography
Erdem A. Nanomaterial-based electrochemical DNA sensing strategies. Talanta. 2007 Dec 15;74(3):318-25.
Linsey WC, Woodruff ES, Weed D, et al. Development of a rapid diagnostic assay for methicillin-resistant Staphylococcus aureus and methicillin-resistant coagulase-negative Staphylococcus. Diagn Microbiol Infect Dis. 2008 Jul;61(3):273-9.
Natural Standard: The Authority on Integrative Medicine. .
Oak Ridge National Laboratory. Biosensors and other Medical and Environmental Probes. .
Odenthal KJ, Gooding JJ. An introduction to electrochemical DNA biosensors. Analyst. 2007 Jul;132(7):603-10.
Sassolas A, Leca-Bouvier BD, Blum LJ. DNA biosensors and microarrays. Chem Rev. 2008 Jan;108(1):109-39.
Shi H, Xia T, Nel AE, Yeh JI. Part II: coordinated biosensors--development of enhanced nanobiosensors for biological and medical applications. Nanomed. 2007 Oct;2(5):599-614.
Song S, Xu H, Fan C. Potential diagnostic applications of biosensors: current and future directions. Int J Nanomedicine. 2006;1(4):433-40.
Soper SA, Brown K, Ellington A et al. Point-of-care biosensor systems for cancer diagnostics/prognostics. Biosens Bioelectron. 2006 Apr 15;21(10):1932-42.
Zhao W, Wang L, Tan W. Fluorescent nanoparticle for bacteria and DNA detection. Adv Exp Med Biol. 2007;620:129-35.