NUCLEIC ACIDS AND SULPHYDRYL GROUPS
251
It thus appears that the unfertilized egg contains a store of cytoplasmic
DNA which can be transformed into chromosomal DNA during cleavage.
For several reasons this conclusion is not completely convincing. First,
we have found (Baltus and Brächet, 1962) that the major part (65%) of
this 'reserve' DNA is bound to the yolk platelets, whose utilization
apparently does not begin until a much later stage of embryogenesis.
Secondly, as we shall soon see, the eggs are capable of using simple
precursors for DNA synthesis even during cleavage. Finally, according
to Kuriki and Okazaki (1959), DNA synthesis can already be detected
during cleavage in at least one amphibian species, Bufo. The eggs of
Bufo have a large pool of soluble deoxyribonucleoside di- and triphosphates and it is believed by the Japanese authors that the precursors
for DNA synthesis during cleavage are the constituents of this pool
rather than cytoplasmic DNA.
Whatever the meaning of the DNA store might be—and we believe
that it is merely a reserve which can be utilized with the other yolk
constituents at relatively late stages of embryogenesis—it is very
doubtful that any genetic meaning should be attached to it. This is
already clear in the case of insect (Gryllus) eggs, which also contain a
large excess of DNA ; but this DNA, as shown recently by Durand (1961),
has an abnormal base composition, entirely different from that of
chromosomal DNA in the same species. It would certainly be interesting
to extend the work of Durand (1961) to the amphibian eggs and to try
to isolate the cytoplasmic DNA in order to study its base composition
and its physical properties (molecular weight, single or double strandedness, etc.).
2. Incorporation of Labelled Precursors into DNA During Cleavage
There is no doubt that amphibian eggs can already utilize simple
precursors for DNA synthesis during early cleavage. This was clearly
shown by B. C. Moore (1959) and by Tencer (1961a) who used a specific
precursor, ^-thymidine.
An observation by our co-workers, Bieliavsky and Tencer (1960), is of
special interest : they found that, during cleavage, uridine and cytidine
are selectively incorporated into DNA; at later stages (gastrulation),
they become precursors for both DNA and RNA (nuclear and cytoplasmic). These results show that ribonucleosides can be used as DNA
precursors, as was suggested many years ago by the author (1933, 1937).
During cleavage, DNA synthesis is thus possible at the expense of both
deoxyribonucleosides and ribonucleosides ; we shall now see that these
two metabolic pathways can be dissociated, to a certain extent, by the
addition of various substances.
251
It thus appears that the unfertilized egg contains a store of cytoplasmic
DNA which can be transformed into chromosomal DNA during cleavage.
For several reasons this conclusion is not completely convincing. First,
we have found (Baltus and Brächet, 1962) that the major part (65%) of
this 'reserve' DNA is bound to the yolk platelets, whose utilization
apparently does not begin until a much later stage of embryogenesis.
Secondly, as we shall soon see, the eggs are capable of using simple
precursors for DNA synthesis even during cleavage. Finally, according
to Kuriki and Okazaki (1959), DNA synthesis can already be detected
during cleavage in at least one amphibian species, Bufo. The eggs of
Bufo have a large pool of soluble deoxyribonucleoside di- and triphosphates and it is believed by the Japanese authors that the precursors
for DNA synthesis during cleavage are the constituents of this pool
rather than cytoplasmic DNA.
Whatever the meaning of the DNA store might be—and we believe
that it is merely a reserve which can be utilized with the other yolk
constituents at relatively late stages of embryogenesis—it is very
doubtful that any genetic meaning should be attached to it. This is
already clear in the case of insect (Gryllus) eggs, which also contain a
large excess of DNA ; but this DNA, as shown recently by Durand (1961),
has an abnormal base composition, entirely different from that of
chromosomal DNA in the same species. It would certainly be interesting
to extend the work of Durand (1961) to the amphibian eggs and to try
to isolate the cytoplasmic DNA in order to study its base composition
and its physical properties (molecular weight, single or double strandedness, etc.).
2. Incorporation of Labelled Precursors into DNA During Cleavage
There is no doubt that amphibian eggs can already utilize simple
precursors for DNA synthesis during early cleavage. This was clearly
shown by B. C. Moore (1959) and by Tencer (1961a) who used a specific
precursor, ^-thymidine.
An observation by our co-workers, Bieliavsky and Tencer (1960), is of
special interest : they found that, during cleavage, uridine and cytidine
are selectively incorporated into DNA; at later stages (gastrulation),
they become precursors for both DNA and RNA (nuclear and cytoplasmic). These results show that ribonucleosides can be used as DNA
precursors, as was suggested many years ago by the author (1933, 1937).
During cleavage, DNA synthesis is thus possible at the expense of both
deoxyribonucleosides and ribonucleosides ; we shall now see that these
two metabolic pathways can be dissociated, to a certain extent, by the
addition of various substances.