Complementary DNA libraries and neurological disease.

T he DNA in our cell nuclei contains all the information necessary to direct our biological functions. The gene is 1 unit of such information that usually specifies the expression of 1 protein product. A detailed knowledge of each gene will enable us to understand disease at the molecular level. To study each gene we must purify and study it in isolation. This process is usually described as gene cloning. However, this is a difficult task since most of the human genome is junk DNA that does not encode genes. Messenger RNA (mRNA), on the other hand, is representative of only that DNA that has been transcribed. More to the point, within each tissue or cell type, only mRNA that is important for the function of the selected cells will be present. Indeed, different cell types, developmental stages, and many disease states arise because of differential gene expression. The problem is that mRNA itself cannot be easily manipulated to clone genes. On the other hand, DNA can be digested with restriction enzymes and propagated in a bacterial vector. Therefore, to isolate specific genes, DNA (which can be easily manipulated) is made from the mRNA by a process called reverse transcription. The resulting DNA is called complementary DNA (cDNA). When the mRNA from any specific cell type is used to generate cDNA, that collection of cDNA corresponds to the genes that are expressed in those cells at that time. This is especially valuable for the cloning of genes that are expressed specifically in a particular cell type or expressed differentially between normal and disease cells.