as the development of properties into specific adult traits.» [19] However, at the turn of the twentieth century a fundamental change was underway whereby the study of the heredity and variation of organisms began to separate off from the study of embryos and their development to form two separate branches of biology. [20] Henceforth, genetics and embryology went their separate ways, each developing their own specific terminology and spawning their own specialist journals and literature. To begin with, genetic research concentrated on investigating the transmission of traits to offspring but soon came to the conclusion that this process must depend on the existence of elements inside the cell. However, when the U.S. American embryologist Thomas Hunt Morgan [21] claimed in 1933 that «There is no consensus opinion amongst geneticists as to what the genes are — whether they are real or purely fictitious» [22] , 1 for the majority of his geneticist colleagues genes were already «real, material entities — the biological analogue of the molecules and atoms of physical science.» [23] In the early 1940s, geneticists established the chemical identity of genes and proved that these

molecules are constituted of DNA. Nearly ten years later, DNA was identified as the material carrying specific biological traits in bacteria. From this point, it was but a short step to an optical representation, which gave DNA a «face»: in 1953, James D. Watson and Frances Crick published their model of the molecular structure of DNA in the form of a double helix. The model proved that genes are the units of inheritance and this is encoded in sequences of base pairs of chromosomes arranged linearly along the strands of DNA. It became clear that this nucleic acid, that is, real molecules, carries the genetic information of an organism and not, as previously thought, proteins. Today the model of the double helix is found in every text book on genetics and functions as a socalled «black box.» [24] Prised out of the historic and social context of its development, in the following years the double helix became the most fundamental scientific fact of genetics and a symbol of «the stuff that life is made of» in popular culture.

However, the model of DNA’s molecular structure in the form of a double helix is not capable of explaining which chemical process is responsible for