EARLY DEVELOPMENT OF THE SEA URCHIN
123
It is also pertinent to mention the finding by Scarano and Orengo
(1957) that extracts of unfertilized eggs and embryos are able to
phosphorylate the dTMP to energy-rich derivatives.
2. RNA
The data quoted in the previous section give evidence that RNA is
not very active metabolically during the first period of development.
They also show that at the blastula stage, the increased nucleotide
requirements of the embryo are not only due to DNA production, but
are also to provide for a new synthesis of RNA. It is interesting to note
that in this period of development, simple nucleotide metabolites such
as ammonia, carbon dioxide, formiate and glycine are utilized for the
synthesis of new RNA purines and pyrimidines.
16
N-ammonium chloride administered to eggs and embryos of
Paracentrotus (Hultin, 1953a) begins to be incorporated into the acidsoluble fraction to a great extent during blastula formation and a peak is
reached at the early gastrula stage. The label is present both in hypoxanthine and urydylic acid. Incorporation into the RNA begins to increase
at the blastula stage and remains high during gastrulation. In the RNA
the isotope is distributed between the rings and amino groups of
pyrimidines and purines. This result indicates that in the sea urchin
embryo the utilization of ammonium nitrogen for the synthesis of
purines and pyrimidines proceeds in the same way as in mammals and
birds (Barnes and Schoenheimer, 1943). Furthermore the results show
that hypoxanthine and urydylic acid are the closest precursors for the
synthesis of the new RNA and it is likely that the purine ring of
hypoxanthine is utilized for the synthesis of the purine nucleotides of
the RNA.
Very little is known of the precursors of the RNA pyrimidines.
The relationship between RNA metabolism and the respiratory curve
has been studied by means of
14
C-carbonate (Hultin, 1953d). It is known
that carbon dioxide is utilized in the formation of purines (Sonne et al.,
1946). The experiments demonstrated that the incorporation of
14
C from
carbonate into free hypoxanthine and into guanine and adenine isolated
from RNA was more intense during the period of exponential respiratory
increase, whereas a decrease was observed at the gastrula stage which
may have been due to the isotope dilution caused by the intensified
production of respiratory carbon dioxide.
When
14
C-acetate was used as a precursor, the rate of the incorporation of
14
C into free hypoxanthine and into RNA purines, as expected,
increased considerably during gastrulation. In addition, during gastrulation, the acetate-114
C was also utilized for the formation of fatty acids
123
It is also pertinent to mention the finding by Scarano and Orengo
(1957) that extracts of unfertilized eggs and embryos are able to
phosphorylate the dTMP to energy-rich derivatives.
2. RNA
The data quoted in the previous section give evidence that RNA is
not very active metabolically during the first period of development.
They also show that at the blastula stage, the increased nucleotide
requirements of the embryo are not only due to DNA production, but
are also to provide for a new synthesis of RNA. It is interesting to note
that in this period of development, simple nucleotide metabolites such
as ammonia, carbon dioxide, formiate and glycine are utilized for the
synthesis of new RNA purines and pyrimidines.
16
N-ammonium chloride administered to eggs and embryos of
Paracentrotus (Hultin, 1953a) begins to be incorporated into the acidsoluble fraction to a great extent during blastula formation and a peak is
reached at the early gastrula stage. The label is present both in hypoxanthine and urydylic acid. Incorporation into the RNA begins to increase
at the blastula stage and remains high during gastrulation. In the RNA
the isotope is distributed between the rings and amino groups of
pyrimidines and purines. This result indicates that in the sea urchin
embryo the utilization of ammonium nitrogen for the synthesis of
purines and pyrimidines proceeds in the same way as in mammals and
birds (Barnes and Schoenheimer, 1943). Furthermore the results show
that hypoxanthine and urydylic acid are the closest precursors for the
synthesis of the new RNA and it is likely that the purine ring of
hypoxanthine is utilized for the synthesis of the purine nucleotides of
the RNA.
Very little is known of the precursors of the RNA pyrimidines.
The relationship between RNA metabolism and the respiratory curve
has been studied by means of
14
C-carbonate (Hultin, 1953d). It is known
that carbon dioxide is utilized in the formation of purines (Sonne et al.,
1946). The experiments demonstrated that the incorporation of
14
C from
carbonate into free hypoxanthine and into guanine and adenine isolated
from RNA was more intense during the period of exponential respiratory
increase, whereas a decrease was observed at the gastrula stage which
may have been due to the isotope dilution caused by the intensified
production of respiratory carbon dioxide.
When
14
C-acetate was used as a precursor, the rate of the incorporation of
14
C into free hypoxanthine and into RNA purines, as expected,
increased considerably during gastrulation. In addition, during gastrulation, the acetate-114
C was also utilized for the formation of fatty acids