USA Surgery Today

Advances in the molecular genetics laboratory have led to DNA diagnostic techniques for many single-gene conditions. These methods can be direct or indirect. Direct methods evaluate the DNA for specific mutations. To use a direct method, the gene must have been identified and an efficient technique for identification of the mutation must be available. For example, sickle cell disease is due to a single nucleotide substitution at the sixth codon of the ОІ-globin chain. The mutation changes codon 6 from guanine-adenine-guanine, which codes for glutamic acid, to guanine-thymine-guanine, which codes for valine. Early on, this single nucleotide change was shown to be detectable using a RFLP. The restriction enzyme cuts the normal DNA sequence, but the mutation changes the cut site so it is no longer recognized by the enzyme, allowing diagnosis to be made by examining the size of the resulting DNA fragments (24:Mevacor).