Cytochemical nucleic acid research during the twentieth century.

Nucleic acids have the interest of scientists in many basic and applied biomedical disciplines. During this century, investigations have revealed, that nucleic acids are ‘the bearers of life and heredity’. They contain programs which regulate the many activities of the cells of which organisms are comprised, in the form of a series of discrete units of specific sequences of nucleotide bases. These specific sequences are called ‘genes’ (from pangenes - de Vries, 1889). The genes are arranged linearly on the chromosomes. Prior to each cell division the complete set of genes of the mother cell is duplicated, so that during the division the two daughter cells each receive a complete set. In unicellular organisms, all genes are active to control the cell activities from cell division to interactions with the environment. In the cells of multicellular creatures, only those genes necessary to develop and maintain the functions of those organs are active, although - again - all genes are present in each of the cells.