Deoxyribonucleic Acid (DNA)

Deoxyribonucleic acid (DNA), "the master molecule," is a natural polymer which encodes the genetic information required for the growth, development, and reproduction of an organism. Found in all cells, it consists of chains of units called nucleotides. Each nucleotide unit contains three components: the sugar deoxyribose, a phosphate group, and a nitrogen-containing amine or base with a ring-type structure. The base component can be any of four types: adenine, cytosine, guanine or thymine.

DNA molecules are very long and threadlike. They consist of two polymeric strands twisted about each other into a spiral shape known as a double helix, which resembles two intertwined circular staircases. DNA is found within the cell nucleus in the chromosomes, which are extremely condensed structures in which DNA is associated with proteins. Each species contains a characteristic number of chromosomes in their cells. In humans, The structure of a DNA molecule. Illustration by Argosy. The Gale Group. every cell contains 46 chromosomes (except for egg and sperm cells which contain only 23). The total genetic information in a cell is called its genome.

The fundamental units of heredity are genes. A gene is a segment of a DNA molecule that encodes the information necessary to make a specific protein. Proteins are the "workhorses" of the cell. These large, versatile molecules serve as structural components: they transport molecules in and out of cells, catalyze cellular reactions, and recognize and eliminate invaders. Imagine a community in which the trash collectors, goods distributors, manufacturers, and police are all on strike, and you get an idea of the importance of proteins in the life of a cell.

DNA not only encodes the "blueprints" for cellular proteins but also the instructions for when and where they will be made. For example, the oxygen carrier hemoglobin is made in red blood cells but not in nerve cells, though both contain the same total genetic content.

Thus, DNA also contains the information necessary for regulating how its genetic messages are used.

Human cells are thought to contain between 50,000 and 100,000 genes. Except in the case of identical twins, a comparison of the genes from different individuals always reveals a number of differences. Therefore, each person is genetically unique. This is the basis of DNA "fingerprinting," a forensic procedure used to match DNA collected from a crime scene with that of a suspect.

Through the sum of their effects, genes direct the function of all organs and systems in the body. Defects in the DNA of just one gene can cause a genetic disorder which results in disease because the protein encoded by the defective gene is abnormal. The abnormal hemoglobin produced by people afflicted with sickle cell anemia is an example. Defects in certain genes called oncogenes, which regulate growth and development, give rise to cancer. Only about 100 genes are thought to be oncogenes. Therefore, defects in DNA can affect the two kinds of genetic information it carries, messages directing the manufacture of proteins and information regulating the expression, or carrying out, of these messages.