NUCLEIC ACIDS AND SULPHYDRYL GROUPS
255
is known that TPNH is required for the reduction of ribose (present in
uridine) to deoxyribose (Cohen et al., 1961), substrates such as citric acid
and oxaloacetate would increase the production of TPNH and thereby
shift the nucleic acid synthesis towards that of DNA rather than of
RNA. If TPNH production is subnormal in the lethal hybrids, an
assumption which is in agreement with the deficiency in their carbohydrate metabolism, the observation that uridine is a poor precursor for
DNA in the blocked hybrids would have a simple explanation.
Another observation deserves mention : we found that when cells of
lethal hybrids which have been dissociated with citrate are allowed to
reaggregate in the presence of Ca
2+ ions, only very small groups of cells
reaggregate; on cytochemical examination, they are always found to
have relatively normal nuclei. These observations clearly show that the
presence of an abnormal nucleus deeply modifies the properties of the
cell surface (Brächet et al., 1962).
A few words should be said about a hybrid combination in which
lethality appears at later stages than the end of cleavage or beginning of
gastrulation. In hybrids between toads, protein synthesis is apparently
greatly modified. Immunological studies by Ranzi and his co-workers
(1961) have revealed the interesting fact that more new antigens are
synthesized in viable combinations than in lethal ones.
Although Echinoderms are not within the scope of the present
review, it is worth mentioning that, according to Whiteley and Baltzer
(1958) and Baltzer and Chen (1960), DNA synthesis is normal for
15 to 20 hours in the lethal hybrid Paracentrotus Ç χ Arbacia
At that
time, the lethal block occurs; nevertheless, as in the amphibians, DNA
synthesis continues for many hours, indicating once more that the
arrest of DNA synthesis is not the direct cause of the block in
development.
Recently, in still unpublished experiments, A. Ficq and the author
studied, by autoradiographic methods, the incorporation of various
precursors in the same lethal combination Paracentrotus $ χ Arbacia <£.
The present results can be summarized in the following way : the intensity
of the incorporation, for all precursors (thymidine, uridine and leucine),
is intermediate, in the hybrids, between the two parental species. DNA
synthesis, at the expense of thymidine, continues in the blocked hybrids ;
but it is interesting that, in contrast to the general rule for normal cells,
the DNA which is synthesized by the lethal hybrid is metabolically
unstable. The hybrids are also capable of synthesizing nuclear RNA;
however, although in normal embryos nuclear RNA easily migrates
towards the cytoplasm, this migration is strongly inhibited in the lethal
hybrids. An attempt to explain these puzzling results will be made at the
end of the chapter (Discussion).
I
255
is known that TPNH is required for the reduction of ribose (present in
uridine) to deoxyribose (Cohen et al., 1961), substrates such as citric acid
and oxaloacetate would increase the production of TPNH and thereby
shift the nucleic acid synthesis towards that of DNA rather than of
RNA. If TPNH production is subnormal in the lethal hybrids, an
assumption which is in agreement with the deficiency in their carbohydrate metabolism, the observation that uridine is a poor precursor for
DNA in the blocked hybrids would have a simple explanation.
Another observation deserves mention : we found that when cells of
lethal hybrids which have been dissociated with citrate are allowed to
reaggregate in the presence of Ca
2+ ions, only very small groups of cells
reaggregate; on cytochemical examination, they are always found to
have relatively normal nuclei. These observations clearly show that the
presence of an abnormal nucleus deeply modifies the properties of the
cell surface (Brächet et al., 1962).
A few words should be said about a hybrid combination in which
lethality appears at later stages than the end of cleavage or beginning of
gastrulation. In hybrids between toads, protein synthesis is apparently
greatly modified. Immunological studies by Ranzi and his co-workers
(1961) have revealed the interesting fact that more new antigens are
synthesized in viable combinations than in lethal ones.
Although Echinoderms are not within the scope of the present
review, it is worth mentioning that, according to Whiteley and Baltzer
(1958) and Baltzer and Chen (1960), DNA synthesis is normal for
15 to 20 hours in the lethal hybrid Paracentrotus Ç χ Arbacia
At that
time, the lethal block occurs; nevertheless, as in the amphibians, DNA
synthesis continues for many hours, indicating once more that the
arrest of DNA synthesis is not the direct cause of the block in
development.
Recently, in still unpublished experiments, A. Ficq and the author
studied, by autoradiographic methods, the incorporation of various
precursors in the same lethal combination Paracentrotus $ χ Arbacia <£.
The present results can be summarized in the following way : the intensity
of the incorporation, for all precursors (thymidine, uridine and leucine),
is intermediate, in the hybrids, between the two parental species. DNA
synthesis, at the expense of thymidine, continues in the blocked hybrids ;
but it is interesting that, in contrast to the general rule for normal cells,
the DNA which is synthesized by the lethal hybrid is metabolically
unstable. The hybrids are also capable of synthesizing nuclear RNA;
however, although in normal embryos nuclear RNA easily migrates
towards the cytoplasm, this migration is strongly inhibited in the lethal
hybrids. An attempt to explain these puzzling results will be made at the
end of the chapter (Discussion).
I