118
ALBERTO MONROY AND RACHELE MAGGIO
incorporation of glycine into DNA was appreciably higher. However,
about 70 to 80 per cent of the DNA purines was not synthesized from
glycine. This result could not be attributed to the dilution of glycine by
the metabolic glycine pool of the embryo since varying the glycine concentration in the outer medium had almost no effect on the extent of
incorporation in the purines. Therefore the majority of the DNA
purines seems to arise from some other endogenous precursor.
Further studies were made by Scarano and Kalckar (1953) who
investigated the incorporation of adenine-814
C and glycine-114
C into
the purines of developing sea urchin eggs. In the experiments with
glycine, this was added to a suspension of Arbacia lixula eggs soon after
fertilization and the eggs were collected at 64-cell stage. It was found that
the isotope concentration in the DNA purines was twelve times higher
than in the corresponding part of RNA. In both nucleic acids, the specific
activity of adenine and guanine were approximately the same. In
shorter incubation experiments with Paracentrotus lividus and Sphaerechinus granulans with
14
C-adenine, it was found that the acid-soluble
nucleotide fraction became labelled before the nucleic acid and the
specific activity of the former was always higher than that of the latter.
This result may indicate that the acid-soluble nucleotide fraction
contains nucleic acid precursors. From a preliminary analysis it seems
that the adenylic compound with the higher specific activity is the
5'-adenylic acid.
At the swimming blastula stage,
14
C-adenine is extensively incorporated into nucleic acid. The incorporation into DNA is 8-4 times
higher than into RNA and the same is true for guanine, but in any case
the specific activity of adenine was about ten times higher than that of
guanine in both nucleic acids. This indicates that at the blastula stage,
adenine is utilized less for guanine synthesis than in later stages. At the
pluteus stage, utilization of adenine for guanine synthesis was observed
to be greater in both nucleic acids. At the same time the specific activity
of RNA increased considerably. This suggests that synthesis of RNA
begins afresh in the later stages of development.
The results reported by Villee et ah, Abrams, and Scarano and Kalckar
give evidence of a net synthesis of DNA from the early development of
the sea urchin embryo onwards. They further show that DNA cannot
arise from the RNA pre-existing in the unfertilized egg. It appears then
that during the first hours of development, DNA purines are not
synthesized only from glycine and adenine, and that glycine is utilized
for nucleic acid synthesis before adenine. Furthermore, glycine is utilized
for the synthesis of both purines by the same metabolic pathway. The
enzymatic conversion of adenine into guanine seems to be slower during
early cleavage, probably because adenine is not the only precursor of
ALBERTO MONROY AND RACHELE MAGGIO
incorporation of glycine into DNA was appreciably higher. However,
about 70 to 80 per cent of the DNA purines was not synthesized from
glycine. This result could not be attributed to the dilution of glycine by
the metabolic glycine pool of the embryo since varying the glycine concentration in the outer medium had almost no effect on the extent of
incorporation in the purines. Therefore the majority of the DNA
purines seems to arise from some other endogenous precursor.
Further studies were made by Scarano and Kalckar (1953) who
investigated the incorporation of adenine-814
C and glycine-114
C into
the purines of developing sea urchin eggs. In the experiments with
glycine, this was added to a suspension of Arbacia lixula eggs soon after
fertilization and the eggs were collected at 64-cell stage. It was found that
the isotope concentration in the DNA purines was twelve times higher
than in the corresponding part of RNA. In both nucleic acids, the specific
activity of adenine and guanine were approximately the same. In
shorter incubation experiments with Paracentrotus lividus and Sphaerechinus granulans with
14
C-adenine, it was found that the acid-soluble
nucleotide fraction became labelled before the nucleic acid and the
specific activity of the former was always higher than that of the latter.
This result may indicate that the acid-soluble nucleotide fraction
contains nucleic acid precursors. From a preliminary analysis it seems
that the adenylic compound with the higher specific activity is the
5'-adenylic acid.
At the swimming blastula stage,
14
C-adenine is extensively incorporated into nucleic acid. The incorporation into DNA is 8-4 times
higher than into RNA and the same is true for guanine, but in any case
the specific activity of adenine was about ten times higher than that of
guanine in both nucleic acids. This indicates that at the blastula stage,
adenine is utilized less for guanine synthesis than in later stages. At the
pluteus stage, utilization of adenine for guanine synthesis was observed
to be greater in both nucleic acids. At the same time the specific activity
of RNA increased considerably. This suggests that synthesis of RNA
begins afresh in the later stages of development.
The results reported by Villee et ah, Abrams, and Scarano and Kalckar
give evidence of a net synthesis of DNA from the early development of
the sea urchin embryo onwards. They further show that DNA cannot
arise from the RNA pre-existing in the unfertilized egg. It appears then
that during the first hours of development, DNA purines are not
synthesized only from glycine and adenine, and that glycine is utilized
for nucleic acid synthesis before adenine. Furthermore, glycine is utilized
for the synthesis of both purines by the same metabolic pathway. The
enzymatic conversion of adenine into guanine seems to be slower during
early cleavage, probably because adenine is not the only precursor of