An organic substance that encodes and carries genetic information and is the fundamental element of heredity.
The thousands of genes that make up each chromosome are composed of deoxyribonucleic acid (DNA), which consists of a five-carbon sugar (deoxyribose), phosphate, and four types of nitrogen-containing molecules (adenine, guanine, cytosine, and thymine). The sugar and phosphate combine to form the outer edges of a double helix, while the nitrogen-containing molecules appear in bonded pairs like rungs of a ladder connecting the outer edges. They are matched in an arrangement that always pairs adenine in one chain with thymine in the other, and guanine in one chain with cytosine in the other. A single DNA molecule may contain several thousand pairs. As the transmitter of inherited characteristics, DNA replicates itself exactly and determines the structure of new organisms, which it does by governing the structure of their proteins.
The specific order and arrangement of these bonded pairs of molecules constitute the genetic code of the organism in which they exist by determining, through the production of ribonucleic acid (RNA), the type of protein produced by each gene, as it is these proteins that govern the structure and activities of all cells in an organism. Thus, DNA acts as coded message, providing a blueprint for the characteristics of all organisms, including human beings. When a cell divides to form new life, its DNA is "copied" by a separation of the two strands of the double helix, after which complementary strands are synthesized around each existing one. The end result is the formation of two new double helices, each identical to the original. All cells of a higher organism contain that organism's entire DNA pattern. However, only a small percentage of all the DNA messages are active in any cell at a given time, enabling different cells to "specialize."
Many viruses are also composed of DNA, which, in some cases, has a single-strand form rather than the two strands forming the edges of a double helix. Each particle of a virus contains only one DNA molecule, ranging in length from 5,000 to over 200,000 subunits. (The total length of DNA in a human cell is estimated at five billion isubunits.) Radiation, thermal variations, or the presence of certain chemicals can cause changes, or "mistakes," in an organism's DNA pattern, resulting in a genetic mutation. In the course of evolution, such mutations provided the hereditary blueprints for the emergence of new species.
The Swiss researcher Frederick Miescher first discovered DNA in 1869. He extracted a substance (that he called nuclein) containing nitrogen and phosphorus from cell nuclei. The question of whether nucleic acids or proteins, or both, carried the information that make the genes of every organism unique was not answered, however, until the molecular structure of DNA was determined in 1953. This pioneering work was accomplished by an American biochemist, James D. Watson, and three British scientists—Francis Crick, a biochemist, Maurice Wilkins, a biophysicist, and Rosalind Frank, a microchemist.
Since the 1970s, scientists have furthered their understanding of the molecular structure of genes through experiments with recombinant DNA. As its name suggests, this technique combines fragments of DNA from two different species, allowing an experimenter to purify, or clone, a gene from one species by inserting it into the DNA of another, which replicates it together with its
PRACTICAL APPLICATIONS OF DNA RESEARCH
In the late 1990s, news reports focused on applications of DNA research. For example, medical researchers focused on finding ways to use DNA replication to repair damaged tissue, such as spinal cord, heart muscle, and brain, that is unable to repair itself or regenerate after injury. Other researchers sought ways to stop the uncontrolled growth of cancer cells by understanding the DNA replication process.
In 1997, a 7-month-old lamb named Dolly was introduced to the world by researcher Ian Wilmut of the Roslin Institute in Scotland. Dolly's life began as a speck of DNA from another female sheep, of which she is now a perfect genetic copy. This was the first successful cloning of a mammal from an adult of its species. Debate over ethical and moral issues of cloning will continue for years to come.
DNA analysis was also reportedly gaining increased application. Forensic laboratories used by police departments to analyze crime evidence were using complex DNA analysis on blood samples taken from crime scenes. The DNA from the blood evidence was analyzed and compared to blood samples taken from suspects.
In commerce, some manufacturers were considering implanting DNA into clothing products to fight against the production of illegal copies. DNA is used as a unique identification "tag" in fabric. The DNA molecule is attached to an inorganic base, and can be detected using a special handheld scanner. Counterfeiters would find it difficult to detect and copy the DNA marker in a clothing item, such as a pair of jeans.
Gribbin, John. In Search of the Double Helix. New York: McGraw-Hill, 1985.