EARLY DEVELOPMENT OF THE SEA URCHIN
119
guanine. On the other hand, determinations of acid-soluble nucleotides
in early development (Hultin, 1957) demonstrate that the AMP fraction
is fairly small and does not change appreciably whereas the ATP
fraction rapidly decreases. Aiello, Rossi and Scarano (personal communication), using enzymatic methods, have estimated the concentration of
ATP, ADP and AMP in unfertilized eggs and developmental stages. A
decrease of the ATP concentration and an increase of ADP has been
observed 30 minutes after fertilization while the AMP concentration
remains constant. Between the 2-cell stage and the morula the
concentration of ATP drops to about half of its initial value but there is
no proportional increase in ADP. The value of AMP is almost doubled.
Hence, the decrease of ATP does not seem to be balanced by the
combined increase of ADP + AMP, as is also shown by the value of the
total concentration of nucleotides.
These data indicate that a considerable portion of the DNA purines
are derived from an unknown endogenous source.
In recent experiments by Nemer (1962) unfertilized eggs and embryos
of Paracentrotus
at different stages of development were exposed to
14
C-ribonucleosides (uridine and cytidine) deoxynucleosides (deoxyuridine and thymidine) and deoxynucleotides (thymydylic acid and
deoxyuridylic acid) and examined for the incorporation of
14
C into the
DNA, RNA and acid-soluble fraction.
Preliminary experiments showed that incorporation of
14
C into the
total acid-insoluble fraction (DNA and RNA) by the unfertilized eggs is
very small, even after 5 hours of incubation. One hour after fertilization
incorporation is twenty times greater and an additional increase of
three to five times is observed in 24-hour embryos. Most of the uptake of
exogenous compounds occurs in the acid-soluble fraction. In addition, the
embryos have a greater ability to add ribonucleosides than to add deoxynucleosides and deoxynucleotides to their endogenous pool, but the
deoxynucleotides are more quickly incorporated into the total nucleic
acid fraction. In other words 'the half-lives of the pools supplied by exogenous ribonucleosides are four to six times longer than those of
the deoxy compounds'.
When the incorporation of cytidine and uridine in DNA and RNA
was compared it was found that in early cleavage the incorporation of
cytidine into DNA is eight times greater than into RNA, while that of
uridine is only twice as great. Furthermore in the early stages cytidine is
preferentially utilized for the synthesis of the pyrimidines of DNA, but
in the 24-hour embryos the incorporation of cytidine and uridine becomes
similar.
One hour after fertilization the incorporation of thymidine and
deoxyuridine was found almost exclusively in DNA. In batches of
119
guanine. On the other hand, determinations of acid-soluble nucleotides
in early development (Hultin, 1957) demonstrate that the AMP fraction
is fairly small and does not change appreciably whereas the ATP
fraction rapidly decreases. Aiello, Rossi and Scarano (personal communication), using enzymatic methods, have estimated the concentration of
ATP, ADP and AMP in unfertilized eggs and developmental stages. A
decrease of the ATP concentration and an increase of ADP has been
observed 30 minutes after fertilization while the AMP concentration
remains constant. Between the 2-cell stage and the morula the
concentration of ATP drops to about half of its initial value but there is
no proportional increase in ADP. The value of AMP is almost doubled.
Hence, the decrease of ATP does not seem to be balanced by the
combined increase of ADP + AMP, as is also shown by the value of the
total concentration of nucleotides.
These data indicate that a considerable portion of the DNA purines
are derived from an unknown endogenous source.
In recent experiments by Nemer (1962) unfertilized eggs and embryos
of Paracentrotus
at different stages of development were exposed to
14
C-ribonucleosides (uridine and cytidine) deoxynucleosides (deoxyuridine and thymidine) and deoxynucleotides (thymydylic acid and
deoxyuridylic acid) and examined for the incorporation of
14
C into the
DNA, RNA and acid-soluble fraction.
Preliminary experiments showed that incorporation of
14
C into the
total acid-insoluble fraction (DNA and RNA) by the unfertilized eggs is
very small, even after 5 hours of incubation. One hour after fertilization
incorporation is twenty times greater and an additional increase of
three to five times is observed in 24-hour embryos. Most of the uptake of
exogenous compounds occurs in the acid-soluble fraction. In addition, the
embryos have a greater ability to add ribonucleosides than to add deoxynucleosides and deoxynucleotides to their endogenous pool, but the
deoxynucleotides are more quickly incorporated into the total nucleic
acid fraction. In other words 'the half-lives of the pools supplied by exogenous ribonucleosides are four to six times longer than those of
the deoxy compounds'.
When the incorporation of cytidine and uridine in DNA and RNA
was compared it was found that in early cleavage the incorporation of
cytidine into DNA is eight times greater than into RNA, while that of
uridine is only twice as great. Furthermore in the early stages cytidine is
preferentially utilized for the synthesis of the pyrimidines of DNA, but
in the 24-hour embryos the incorporation of cytidine and uridine becomes
similar.
One hour after fertilization the incorporation of thymidine and
deoxyuridine was found almost exclusively in DNA. In batches of