EARLY DEVELOPMENT OF THE SEA URCHIN
117
Karnofsky (1961) in the sand-dollar and Reverberi et al. (1960) in the
ascidians have shown an incorporation of thymidine in the pronuclei
even before their fusion. Although this does not necessarily imply a
synthesis of DNA it is at least an indication of an active turnover. The
amount of DNA increases rapidly during early development (Brächet,
1937, 1960a).
Throughout the period of active cleavage (i.e., from fertilization
to the early blastula) DNA synthesis is, if not the most important
phenomenon, certainly the most conspicuous one. Indeed there is no
evidence of any other synthesis of comparable importance during this
period.
Since the sea urchin egg is a closed system, the building stones for
DNA synthesis must be in the egg itself.
The original suggestion (Brächet, 1933) of a conversion of RNA into
DNA during development, does not now appear very likely. In fact the
work of Schmidt et al. (1948) (in which a new technique of separation
and estimation of DNA and RNA was used) as well as the experiments
of Villee et al. (1949) using
3 2
P showed that : (a) between the 3rd and 72nd
hour of development there is an increase in DNA whereas the RNA
remains essentially constant; (b) experiments with
3 2
P demonstrated
that the specific activity of the DNA was higher than that of the RNA ;
(c) synthesis of DNA is independent of RNA. It must be noted, however,
that
3 2
P is not too favourable a precursor for such studies in view of the
considerable risk of contamination of the nucleic acid fractions with
non-nucleic acid phosphorus. Nevertheless, these investigations certainly
indicate (a) that if RNA contributes at all to the DNA synthesis it is
very unlikely to be a mass conversion, and (b) that DNA synthesis
follows an independent pathway.
There are indications that the sea urchin egg does contain a small
DNA reserve, which could supply DNA for 16 to 32 nuclei (HoffJorgensen, 1954; Elson et al., 1954; Agrell and Persson, 1956; Whiteley
and Baltzer, 1958; Kavanau, 1958). Although the chemical nature of
such a DNA reserve is not known, it seems likely to consist of low
molecular compounds such as deoxynucleosides, deoxynucleotides or
small polymers of deoxynucleotides. This suggestion is also supported by
observations of the ability of the developing embryos to use exogenous
precursors of this type (see below).
Experiments designed to test whether the carbon skeleton of the
nucleic acid is synthesized from small precursor molecules were made by
Abrams (1951). They showed that blastulae of Arbacia
punctulata
utilized acetate-114
C, glycine-l13
C and glycine-l14
C for the synthesis
of nucleic acid. Incorporation into total DNA was found to be ten times
higher than in RNA. As compared with incorporation into proteins, the
117
Karnofsky (1961) in the sand-dollar and Reverberi et al. (1960) in the
ascidians have shown an incorporation of thymidine in the pronuclei
even before their fusion. Although this does not necessarily imply a
synthesis of DNA it is at least an indication of an active turnover. The
amount of DNA increases rapidly during early development (Brächet,
1937, 1960a).
Throughout the period of active cleavage (i.e., from fertilization
to the early blastula) DNA synthesis is, if not the most important
phenomenon, certainly the most conspicuous one. Indeed there is no
evidence of any other synthesis of comparable importance during this
period.
Since the sea urchin egg is a closed system, the building stones for
DNA synthesis must be in the egg itself.
The original suggestion (Brächet, 1933) of a conversion of RNA into
DNA during development, does not now appear very likely. In fact the
work of Schmidt et al. (1948) (in which a new technique of separation
and estimation of DNA and RNA was used) as well as the experiments
of Villee et al. (1949) using
3 2
P showed that : (a) between the 3rd and 72nd
hour of development there is an increase in DNA whereas the RNA
remains essentially constant; (b) experiments with
3 2
P demonstrated
that the specific activity of the DNA was higher than that of the RNA ;
(c) synthesis of DNA is independent of RNA. It must be noted, however,
that
3 2
P is not too favourable a precursor for such studies in view of the
considerable risk of contamination of the nucleic acid fractions with
non-nucleic acid phosphorus. Nevertheless, these investigations certainly
indicate (a) that if RNA contributes at all to the DNA synthesis it is
very unlikely to be a mass conversion, and (b) that DNA synthesis
follows an independent pathway.
There are indications that the sea urchin egg does contain a small
DNA reserve, which could supply DNA for 16 to 32 nuclei (HoffJorgensen, 1954; Elson et al., 1954; Agrell and Persson, 1956; Whiteley
and Baltzer, 1958; Kavanau, 1958). Although the chemical nature of
such a DNA reserve is not known, it seems likely to consist of low
molecular compounds such as deoxynucleosides, deoxynucleotides or
small polymers of deoxynucleotides. This suggestion is also supported by
observations of the ability of the developing embryos to use exogenous
precursors of this type (see below).
Experiments designed to test whether the carbon skeleton of the
nucleic acid is synthesized from small precursor molecules were made by
Abrams (1951). They showed that blastulae of Arbacia
punctulata
utilized acetate-114
C, glycine-l13
C and glycine-l14
C for the synthesis
of nucleic acid. Incorporation into total DNA was found to be ten times
higher than in RNA. As compared with incorporation into proteins, the