EARLY DEVELOPMENT OF THE SEA URCHIN
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deoxyribonucleic acid synthesis to the 8-blastomere stage. The 5-iododeoxyuridine is incorporated in the total acid-insoluble material during
the entire cleavage period but its utilization is greatly reduced in the
presence of thymine. Evidently when high concentrations of 5-iododeoxyuridine are used, the resulting DNA is likely to be abnormal and
the result is the arrest of development at the blastula stage. It is
important to note that RNA synthesis is also reduced.
These experiments thus demonstrate that in the sea urchin egg DNA
synthesis (1) begins shortly after fertilization and (2) takes place at the
expense of reserve-precursors pre-existing in the unfertilized egg. It
further appears that such a reserve may suffice until the 3rd cleavage
The experiments also indicate that in the sea urchin egg the preferential
route for DNA synthesis is through cytidine and methyl-cytidine.
Further information has been obtained by the study of the enzymes
involved in DNA metabolism.
A few years ago, Scarano (Scarano, 1958 a-f, 1960; Scarano and
Maggio, 1959a, b; Scarano et al., 1960) discovered that homogenates or
extracts of acetone powders of unfertilized eggs and embryos of
Paracentrotus lividus and Sphaerechinus granulans, were able to convert
the 5'-deoxycytidylic acid (dCMP) to 5'-deoxyuridylic acid (dUMP) and
the 5-methyl-deoxycytidylic acid (MedCMP) into 5-thymidylic acid
(dTMP).
When first described, these hitherto unknown deamination reactions
were unexpected since the dUMP is not a DNA component and the
formation of dTMP from MedCMP was rather puzzling.
The work of Scarano and his group provides evidence that one single
enzyme catalyzes the hydrolytic deamination of dCMP to dUMP, of
MedCMP to dTMP and of CH 2 OHdCMP to CH 2 OHdUMP. This fact
points towards the presence of one enzyme with 6-aminopyrimidine
deoxyribonucleotides deaminase activity. The enzyme shows higher
affinity for MedCMP than for dCMP.
The behaviour of the dCMP and MedCMP deaminase was studied
during the development of the sea urchin embryos (Scarano and Maggio,
1959b).
The amount of enzyme per embryo did not change until the 64-cell
stage whereas a striking drop occurred from the late blastula onwards
(Fig. 13). In the pluteus stage, deaminase is about one-fifth of that
found in unfertilized eggs. It is important to note that in some experiments in which development was arrested at the gastrula stage, no such
decrease of the enzyme was observed (Scarano and Maggio, 1959b). The
technique used in these experiments rules out the possibility of the
enzyme being displaced from a soluble to an insoluble fraction during
embryonic development.
121
deoxyribonucleic acid synthesis to the 8-blastomere stage. The 5-iododeoxyuridine is incorporated in the total acid-insoluble material during
the entire cleavage period but its utilization is greatly reduced in the
presence of thymine. Evidently when high concentrations of 5-iododeoxyuridine are used, the resulting DNA is likely to be abnormal and
the result is the arrest of development at the blastula stage. It is
important to note that RNA synthesis is also reduced.
These experiments thus demonstrate that in the sea urchin egg DNA
synthesis (1) begins shortly after fertilization and (2) takes place at the
expense of reserve-precursors pre-existing in the unfertilized egg. It
further appears that such a reserve may suffice until the 3rd cleavage
The experiments also indicate that in the sea urchin egg the preferential
route for DNA synthesis is through cytidine and methyl-cytidine.
Further information has been obtained by the study of the enzymes
involved in DNA metabolism.
A few years ago, Scarano (Scarano, 1958 a-f, 1960; Scarano and
Maggio, 1959a, b; Scarano et al., 1960) discovered that homogenates or
extracts of acetone powders of unfertilized eggs and embryos of
Paracentrotus lividus and Sphaerechinus granulans, were able to convert
the 5'-deoxycytidylic acid (dCMP) to 5'-deoxyuridylic acid (dUMP) and
the 5-methyl-deoxycytidylic acid (MedCMP) into 5-thymidylic acid
(dTMP).
When first described, these hitherto unknown deamination reactions
were unexpected since the dUMP is not a DNA component and the
formation of dTMP from MedCMP was rather puzzling.
The work of Scarano and his group provides evidence that one single
enzyme catalyzes the hydrolytic deamination of dCMP to dUMP, of
MedCMP to dTMP and of CH 2 OHdCMP to CH 2 OHdUMP. This fact
points towards the presence of one enzyme with 6-aminopyrimidine
deoxyribonucleotides deaminase activity. The enzyme shows higher
affinity for MedCMP than for dCMP.
The behaviour of the dCMP and MedCMP deaminase was studied
during the development of the sea urchin embryos (Scarano and Maggio,
1959b).
The amount of enzyme per embryo did not change until the 64-cell
stage whereas a striking drop occurred from the late blastula onwards
(Fig. 13). In the pluteus stage, deaminase is about one-fifth of that
found in unfertilized eggs. It is important to note that in some experiments in which development was arrested at the gastrula stage, no such
decrease of the enzyme was observed (Scarano and Maggio, 1959b). The
technique used in these experiments rules out the possibility of the
enzyme being displaced from a soluble to an insoluble fraction during
embryonic development.