NUCLEIC ACIDS AND SULPHYDRYL GROUPS
259
zygote starts growing (Puiseux-Dao, 1963). Finally, the present results
do not support the idea that the chloroplasts in Acetabularia
contain
large amounts of DNA.
However, these conclusions, based on negative evidence, should still
be considered as tentative: new methods are now being tried in our
laboratory in order to solve the important questions raised by the
apparent absence of DNA in Acetabularia
at the unicellular stage. In
particular, it is important to know whether the fully grown algae
contain, in their large nucleus, the same amount of DNA as the Feulgen
positive zygotic nucleus. Is there a DNA breakdown or, as we rather
believe, a mere dilution of DNA when the nuclear volume increases?
Is there no synthesis at all of DNA, thus no multiplication of the genetic
material, when the algae grow tremendously in size? Are the chloroplasts
completely devoid of DNA? This last question is of special importance in
view of the well-known genetic continuity of the chloroplasts. For the
time being, there is no certain answer to these questions, because of the
enormous technical difficulties—as already mentioned in the case of
amphibian eggs—involved in estimating, in a specific way, very small
amounts of DNA in the presence of large quantities of foreign material.
III. RNA Distribution and Synthesis in Amphibian Eggs
and Acetabularia
A. Amphibian Egg Development
Since an attempt is being made in the present review to present
recent material rather than findings which are now classical, as little as
possible will be said about our earlier work on the role of RNA in the
development of fertilized amphibian eggs (see Brächet, 1960, 1961, for
recent reviews).
1. Distribution of RNA in Normal Amphibian
Eggs
Unfertilized, fertilized and cleaving eggs show a distinct polarity
gradient, the RNA concentration decreasing from the animal to the
vegetal pole. At gastrulation, net RNA synthesis begins. Newly
synthesized RNA is found first in the nuclei, then in the cytoplasm. The
synthesis is more active on the dorsal than on the ventral side and this
results in the formation of a secondary, dorso-ventral gradient which
superimposes itself on the primary polarity gradient. As a result of
further RNA synthesis and morphogenetic movements, well-defined
anterio-posterior (cephalocaudal) and dorso-ventral gradients can be
seen in the neurula and tail-bud stages. At later stages, the RNA
content of every organ increases just before its differentiation begins.
Differentiation itself results in a decrease in the RNA content of the
259
zygote starts growing (Puiseux-Dao, 1963). Finally, the present results
do not support the idea that the chloroplasts in Acetabularia
contain
large amounts of DNA.
However, these conclusions, based on negative evidence, should still
be considered as tentative: new methods are now being tried in our
laboratory in order to solve the important questions raised by the
apparent absence of DNA in Acetabularia
at the unicellular stage. In
particular, it is important to know whether the fully grown algae
contain, in their large nucleus, the same amount of DNA as the Feulgen
positive zygotic nucleus. Is there a DNA breakdown or, as we rather
believe, a mere dilution of DNA when the nuclear volume increases?
Is there no synthesis at all of DNA, thus no multiplication of the genetic
material, when the algae grow tremendously in size? Are the chloroplasts
completely devoid of DNA? This last question is of special importance in
view of the well-known genetic continuity of the chloroplasts. For the
time being, there is no certain answer to these questions, because of the
enormous technical difficulties—as already mentioned in the case of
amphibian eggs—involved in estimating, in a specific way, very small
amounts of DNA in the presence of large quantities of foreign material.
III. RNA Distribution and Synthesis in Amphibian Eggs
and Acetabularia
A. Amphibian Egg Development
Since an attempt is being made in the present review to present
recent material rather than findings which are now classical, as little as
possible will be said about our earlier work on the role of RNA in the
development of fertilized amphibian eggs (see Brächet, 1960, 1961, for
recent reviews).
1. Distribution of RNA in Normal Amphibian
Eggs
Unfertilized, fertilized and cleaving eggs show a distinct polarity
gradient, the RNA concentration decreasing from the animal to the
vegetal pole. At gastrulation, net RNA synthesis begins. Newly
synthesized RNA is found first in the nuclei, then in the cytoplasm. The
synthesis is more active on the dorsal than on the ventral side and this
results in the formation of a secondary, dorso-ventral gradient which
superimposes itself on the primary polarity gradient. As a result of
further RNA synthesis and morphogenetic movements, well-defined
anterio-posterior (cephalocaudal) and dorso-ventral gradients can be
seen in the neurula and tail-bud stages. At later stages, the RNA
content of every organ increases just before its differentiation begins.
Differentiation itself results in a decrease in the RNA content of the