Estat actual del coneixement de l'estructura del material genètic

The answers to the question, what is a gene? have been changing since this term was coined by Johannsen. The classical gene was an abstract entity; it was the unit of function, mutation and recombination in the genetic material. This concept was the result of the application of the Mendelian method of genetic analysis until the forties of the present century. From Benzer's dissection of the rll region of the T4 phage there arose an operative concept of gene based on the cis-trans effect. However, with the development of Molecular Genetics, it soon became possible to visualize the gene as a precise material unit, a DNA segment with information for the synthesis of functional molecules, whether protein or RNA. The techniques of recombinant DNA, that give us an exact knowledge of DNA sequences, have shown a greater complexity in genetic information than was previously expected. Overlapping genes, segments of DNA carrying information in both chains, split genes and the possibility of different m-RNA being processed from these genes, make it impossible et present to give a precise definition of gene which corresponds to a real material entity having a general validity. At this moment «gene» is only an operative term useful in genetic analysis. This is a paradox, now that we are on the verge of creating new genes. Also it is becoming apparent that the genome is organized in other units besides the genes, in which the information carried by the genes is integrated. Thus, the new definition of the gene must take into account not only the elements of which the gene is composed but also its relationships to the larger units of which it is an integral part