they are not to be discussed here, being out of the subject of this book.
Till the middle of the XX century the studies on nucleic acids have been
episodic and in small numbers. The attention was mainly directed to the
discovery of the structure and biological importance of proteins. It was already
known that protein molecules represent long amino-acid polymer chains that
are very variable in shape and size and are of high-molecular weight. Because
of these important biochemical characteristics of theirs, a kind of genetic
control over them by some proteins (especially the enzymes) was admitted.
This has led to the assumption that if not enzymes were the genes
themselves, then their structure could have been determined by proteins.
At this time nucleic acids were considered short and simply structured
polymer chains that could not be capable of preserving and transmitting
hereditary information. The majority of authors were supporters of the
tetranucleotide hypothesis, which has postulated that the four types of
nucleotides (including the nitrogenous bases — adenine, thymine, cytosine
and guanine) are present in the DNAmolecule in equimolar quantities of
identical recurring monomers, similar to these in glycogen. That is why it was
thought that DNA does not have the capacity to ensure the gene variety
needed for determination of the thousands of hereditary features displayed by
living organisms. The noted English crystallographer W. T. Astbury has stated
“that the symmetrically situated nucleotides in DNA are only outlining the
framework in which the amino acids of the protein genes would extend into
chains before they reproduce themselves”.
The formation of the concept of proteins as bearers of genetic
information resulted from an enormous number of studies carried out in that
period. Especially important are the X-ray diffraction analyses of proteins and
enzymes (hemoglobin, myoglobin, pepsin, myosin, etc.) made by
researchers such as J. Bernal, L. Pauling, M. Perutz, J. Kendrew, etc. In
his famous book “The Nature of the Chemical Bond” Pauling (1960) has
offered a summary of his studies on the structure of proteins and the αspiral model,
which will be discussed later in Section 2. 7 (Proteins).
An interesting biological phenomenon was discovered by the
English bacteriologist F. Griffith (1928). His experiment is a classic in the
history of genetics and a turning point in proving that DNA represents the
genetic material and therefore it will be spelled out in detail (Fig. 2–11).
Figure 2–11. Schematic representation of the Griffith’s experiments with pneumococci.