ABSTRACT
The ambition of molecular biology is to interpret the essential properties of organisms in terms of molecular structures. The essence of genetic engineering is the cloning of particular genes from one organism and their functional incorporation into the genetic material of another. This is accomplished by means of two basic tools: conjugal plasmids and restriction enzymes. Conjugal plasmids are small, self-replicating circles of DNA present in the cytoplasm of many bacteria, especially the normal intestinal flora of animals and man. Restriction enzymes comprise a wide range of specific endonucleases produced by various bacteria, each of which recognizes short, randomly occurring DNA sequences, usually four to six pairs long, which are absent from the DNA of the bacteria producing them and their lysogenic phages. The genes responsible for nodule formation (nod) and symbiotic nitrogen fixation (fix) in most fast-growing Rhizobium strains reside, not in the bacterial chromosome, but in large indigenous plasmids which are lost at high temperature.
