Deoxyribonucleic Acid (DNA)

Genetic information flows from DNA to RNA to protein. Ultimately, the linear sequence of nucleotides in DNA directs the production of a protein molecule with a characteristic three dimensional structure essential to its proper function. Initially, information is transcribed from DNA to RNA. The information in the resulting messenger RNA is then translated from RNA into protein by small transfer RNA molecules.

In some exceptional cases the flow of genetic information from DNA to RNA is reversed. In retroviruses, such as the AIDS virus, RNA is the hereditary material. An enzyme known as reverse transcriptase makes a copy of DNA using the virus' RNA as a template. In still other viruses which use RNA as the hereditary material, DNA is not involved in the flow of information at all.

Most cells in the body contain the same DNA as that in the fertilized egg. (Some exceptions to this are the sex cells, which contain only half of the normal complement of DNA, as well as red blood cells which lose their nucleus when fully developed.) Some "housekeeping" genes are expressed in all cells because they are involved in the fundamental processes required for normal function. (A gene is said to be expressed when its product, the protein it codes for, is actively produced in a cell.) For example, since all cells require ribosomes, structures which function as protein assembly lines, the genes for ribosomal proteins and ribosomal RNA are expressed in all cells. Other genes are only expressed in certain cell types, such as genes for antibodies in certain cells of the immune system. Some are expressed only during certain times in development. How is it that some cells express certain genes while others do not, even though all contain the same DNA? A complete answer to this question is still in the works. However, the main way is by controlling the start of transcription. This is accomplished by the interaction of proteins called transcription factors with DNA sequences near the gene. By binding to these sequences transcription factors may turn a gene on or off.

Another way is to change the rate of messenger RNA synthesis. Sometimes the stability of the messenger RNA is altered. The protein product itself may be altered, as well as its transport or stability. Finally, gene expression can be altered by DNA rearrangements. Such programmed reshuffling of DNA is the means of generating the huge assortment of antibody proteins found in immune cells.