the genetic continuity of chromosomes do not depend on the degree of
quantitative changes of chromatin”.
The “mysterious” substance incorporated in the chromosomes has
become a subject of special attention. Around 1890 R. Altmann and A. Kossel
have proven that nuclein consists of two components — protein and an acid
rich in phosphorus which has later been known as nucleic. Let remember,
Miescher has suggested that the substance isolated by him is an acid in its
nature. He has established the phosphorus content in this substance to 9.5
per cent. It has taken a long time to understand that there are two kinds of
nucleic acids. The one isolated from the thymus of calves and the other —
from yeasts. This has created the incorrect idea in some authors that the one
is an animal acid and the other — a plant one.
By chemical analyses in 1900—1903 the research groups of A. Kossel
in Germany and P. A. Levene in the USA have clarified this
misunderstanding. Kossel has established that the thymic nucleic acid contains
adenine, cytosine, guanine and thymine while the yeast one contains uracil
instead of thymine. The presence of uracil in the yeast nucleic acid has
been established by A. Ascoli before him. Levene on his part has
established that the carbohydrate component of the yeast derived nucleic
acid is ribose and the one of the thymus — deoxyribose (see Fig. 2–48),
which has not been known up till then. The ribose containing nucleic acid
was given the name ribonucleic acid (RNA) and the deoxyribose
containing one — deoxyribonucleic acid (DNA).
A major contribution to the cytological visualization of DNA was made
by the German chemist R. Feulgen. In 1924 Feulgen and Rossenbeck have
proposed a histochemical method for a selective visualization of DNA in the
nuclei of cells of animal and plant origin. It was later applied to
microorganisms. This method became known as the Feulgen reaction. It
consists of two stages: a) hydrolysis with acids (mainly HCl) as a result
from which the bonds of the nitrogenous bases of DNA are broken and
aldehydes are formed; b) placement in Schiff’s reagent (fuchsine
sulphurous acid), dehydration in ethanol and xylene, and the presence of
DNA is recorded by the red-violet structures formed in the nucleus.
In spite of the numerous studies on the Feulgen reaction its specificity is
not clear to the present day. There is no convincing answer as to why
aldehyde groups are formed from DNA and not from RNA. The authors have
expressed an opinion that in hydrolysis the bonds between the purine bases
and the carbohydrate components of DNA are broken more easily than in
RNA. Some researchers, however, have shown the release of great
quantities of RNA in acid hydrolysis. Besides, in some samples the reaction
is positive, and in others it is negative i.e. there is no staining. The negative
answer cannot be accepted as a proof for the absence of DNA in the nuclei.
There are also other methods for cytological studies, which are based on
other reagents for fixing and staining. They are being successfully applied but