258
JEAN BRÄCHET
substances must be produced by the two nuclei in order to form the caps
of intermediate type which are observed.
This short account of the important work done by Hämmerling and
his school, despite its incompleteness, will be sufficient for an understanding of the biochemical experiments on Acetabularia which will be
discussed later in this review. The important point is that, in view of the
above mentioned species-specificity, the formation of the cap must be
genetically controlled. Nevertheless, removal of the nucleus, and, thus, of
the genes and DNA, does not inhibit the production of this genecontrolled structure. These experiments with Acetabularia
clearly
demonstrate that intermediaries must exist between DNA and the
genetically determined structure. We shall see, later on, that there is
good reason to believe that these intermediaries are ribonucleic acids.
2. DNA Content
Unfortunately very little is known about the DNA content of
Acetabularia. There is no doubt that the nuclei present in the cysts, in
the gametes, and in the zygotes give a positive Feulgen reaction. But, as
soon as the nucleus of the zygote begins to grow, the Feulgen reaction
becomes negative, even if the algae are strongly centrifuged with the
hope of bringing together, in a single mass, the dispersed chromatin.
The Feulgen reaction again becomes positive in the daughter nuclei
originating from the breakdown of the large, single nucleus in the fullgrown algae (Schulze, 1939; Vanderhaeghe, 1956).
DNA synthesis can be followed by autoradiography by studying the
incorporation of labelled thymidine into DNA : this method when used
by us has always, in the case of growing algae, yielded entirely negative
results. Incorporation of
3
£f-thymidine into the chloroplasts, after long
treatments with the precursor, can however be observed but the
radioactivity completely disappears after a treatment with ribonuclease,
while it remains unaffected by deoxyribonuclease. It is likely that
thymidine is metabolized by the alga and that a derivative of the
precursor can be utilized for the synthesis of chloroplastic RNA
(Brächet, 1959c; de Vitry, unpublished).
Finally, various methods for the chemical estimation of DNA in
Acetabularia
have been attempted in our laboratory. All of them,
including a very sensitive isotope dilution method, have so far failed to
demonstrate the presence of measurable amounts of DNA in the algae
at the unicellular stage.
We can conclude from these negative results that the DNA content of
the huge nucleus must be very small, possibly the same as that of the
zygotic nucleus : if endopolyploidy occurs during the growth of the alga,
it must be a slow process or be limited to a short period when the
JEAN BRÄCHET
substances must be produced by the two nuclei in order to form the caps
of intermediate type which are observed.
This short account of the important work done by Hämmerling and
his school, despite its incompleteness, will be sufficient for an understanding of the biochemical experiments on Acetabularia which will be
discussed later in this review. The important point is that, in view of the
above mentioned species-specificity, the formation of the cap must be
genetically controlled. Nevertheless, removal of the nucleus, and, thus, of
the genes and DNA, does not inhibit the production of this genecontrolled structure. These experiments with Acetabularia
clearly
demonstrate that intermediaries must exist between DNA and the
genetically determined structure. We shall see, later on, that there is
good reason to believe that these intermediaries are ribonucleic acids.
2. DNA Content
Unfortunately very little is known about the DNA content of
Acetabularia. There is no doubt that the nuclei present in the cysts, in
the gametes, and in the zygotes give a positive Feulgen reaction. But, as
soon as the nucleus of the zygote begins to grow, the Feulgen reaction
becomes negative, even if the algae are strongly centrifuged with the
hope of bringing together, in a single mass, the dispersed chromatin.
The Feulgen reaction again becomes positive in the daughter nuclei
originating from the breakdown of the large, single nucleus in the fullgrown algae (Schulze, 1939; Vanderhaeghe, 1956).
DNA synthesis can be followed by autoradiography by studying the
incorporation of labelled thymidine into DNA : this method when used
by us has always, in the case of growing algae, yielded entirely negative
results. Incorporation of
3
£f-thymidine into the chloroplasts, after long
treatments with the precursor, can however be observed but the
radioactivity completely disappears after a treatment with ribonuclease,
while it remains unaffected by deoxyribonuclease. It is likely that
thymidine is metabolized by the alga and that a derivative of the
precursor can be utilized for the synthesis of chloroplastic RNA
(Brächet, 1959c; de Vitry, unpublished).
Finally, various methods for the chemical estimation of DNA in
Acetabularia
have been attempted in our laboratory. All of them,
including a very sensitive isotope dilution method, have so far failed to
demonstrate the presence of measurable amounts of DNA in the algae
at the unicellular stage.
We can conclude from these negative results that the DNA content of
the huge nucleus must be very small, possibly the same as that of the
zygotic nucleus : if endopolyploidy occurs during the growth of the alga,
it must be a slow process or be limited to a short period when the