Every human cell contains more than 50,000 genes that control birth, development, health, disease and, finally, death.
The genes are packaged into 46 segments called chromosomes--23 contributed by each parent. The chromosomes, located in each cell's nucleus, contain the molecule deoxyribonucleic acid, or DNA, the basic hereditary unit.
A gene is the portion of the DNA molecule that carries information coding for a specific protein. Proteins function as building blocks of cells and enzymes that manufacture all the molecules in the cells.
In gene therapy, a normal gene (isolated in a laboratory through recombinant DNA techniques) is inserted into the cells of a patient whose abnormal genes have caused a disease.
It is one of the enduring mysteries of life why cells "differentiate"--that is, why and how genes cause some cells to become part of a kidney and others part of an ear.
