Deoxyribonucleic Acid (DNA)

An organic substance occurring in chromosomes in the nuclei of cells, which encodes and carries genetic information, and is the fundamental element of heredity.

As the transmitter of inherited characteristics, deoxyribonucleic acid (DNA) replicates itself exactly and determines the structure of new organisms, which it does by governing the structure of their proteins. The Swiss researcher Friedrich Miescher first discovered DNA in 1869 when he extracted a substance (which 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 two British scientists, Francis Crick, a biochemist, and Maurice Wilkins, a biophysicist. The thousands of genes that make up each chromosome are composed of 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

A scanning electron micrograph (SEM) of three neurons. The long thin structures connecting the cells are dendrites. (Secchi-Lecague/Roussel-UCLAF/CNRI/Science Photo Library. National Audubon Society Collection/Photo Researchers, Inc. Reproduced with permission.)

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.

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 subunits.) 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.

Scan of DNA strands. (Photo by Howard Sochurek. Stock Market. Reproduced by permission.)

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 own genetic material. The term "recombinant DNA" also refers to other laboratory techniques, such as splitting DNA with microbial enzymes called endonucleases, splicing fragments of DNA, and even synthesizing it chemically. Although controversial, gene cloning is an important scientific accomplishment which has enabled researchers to gain new understanding of the structure of genes through the ability to produce an unlimited number of gene copies gathered from a variety of organisms, including human ones.