NUCLEIC ACIDS AND SULPHYDRYL GROUPS
253
another substance—oxidized a-lipoic acid. As found by our co-worker
J. Quertier (unpublished), it stimulates, at the gastrula or neurula stage,
the synthesis of RNA and that of DNA at the expense of thymidine ; but
it inhibits DNA synthesis at the expense of the nôonucleosides (uridine
and cytidine), probably because it inhibits the reduction of ribose into
deoxyribose (Bertani et al., 1961).
4. Basic Proteins During Cleavage and Gastrulation
Since DNA is normally associated with basic proteins (usually
histones) in chromatin and chromosomes, the distribution of histones in
developing amphibian eggs has particular interest. The question has
been studied, very recently only, by Bloch (1962a, b) and by Horn
(1962). Both used cytochemical as well as biochemical methods, and
their main conclusions, which are in good agreement, can be summarized
as follows : the protamines present in the sperm nucleus disappear soon
after fertilization and are replaced, during cleavage, by a particular type
of histone, called by Bloch (1962) 'cleavage histones'. The staining
properties of the nuclei change at gastrulation, cleavage histones being
replaced by true, adult histones. Both Bloch (1962a, b) and Horn (1962)
agree upon the existence of histones in the cytoplasm of amphibian eggs ;
they are localized in the yolk platelets, especially in the large platelets of
the vegetal pole. According to Horn (1962), histones are absent in the
nuclei of cleavage stages and appear in the nuclei, probably by migration
from the cytoplasm, at the gastrula stage. It is suggested by Horn that
histones might act as gene inhibitors, and that the nuclei are incapable
of RNA synthesis until histones have migrated into them from the yolk.
More will be said about this hypothesis in the final discussion of this
chapter. However, two remarks should be made : first, it is difficult to
prove, with the methods used, that the nuclear histones of the gastrula
derive only from a migration from pre-existing cytoplasmic histones to
the nuclei. One cannot rule out the possibility of independent histone
synthesis in gastrula nuclei until it is proved, by autoradiographic
methods, that these nuclei are not the site of a strong incorporation of
basic amino-acids, such as arginine or lysine. Such experiments have not
yet been performed. Secondly, the conclusion of Horn (1962), that the
yolk platelets contain true histones, might be premature. In fact, it was
reported by Nace, at a recent symposium (1962), that the basic protein
present in the yolk platelets is not a histone, but, oddly enough,
lysozyme.
5. Lethal Hybrids
The importance of gastrulation is conspicuous in many lethal
combinations between different frog species, the development of which
253
another substance—oxidized a-lipoic acid. As found by our co-worker
J. Quertier (unpublished), it stimulates, at the gastrula or neurula stage,
the synthesis of RNA and that of DNA at the expense of thymidine ; but
it inhibits DNA synthesis at the expense of the nôonucleosides (uridine
and cytidine), probably because it inhibits the reduction of ribose into
deoxyribose (Bertani et al., 1961).
4. Basic Proteins During Cleavage and Gastrulation
Since DNA is normally associated with basic proteins (usually
histones) in chromatin and chromosomes, the distribution of histones in
developing amphibian eggs has particular interest. The question has
been studied, very recently only, by Bloch (1962a, b) and by Horn
(1962). Both used cytochemical as well as biochemical methods, and
their main conclusions, which are in good agreement, can be summarized
as follows : the protamines present in the sperm nucleus disappear soon
after fertilization and are replaced, during cleavage, by a particular type
of histone, called by Bloch (1962) 'cleavage histones'. The staining
properties of the nuclei change at gastrulation, cleavage histones being
replaced by true, adult histones. Both Bloch (1962a, b) and Horn (1962)
agree upon the existence of histones in the cytoplasm of amphibian eggs ;
they are localized in the yolk platelets, especially in the large platelets of
the vegetal pole. According to Horn (1962), histones are absent in the
nuclei of cleavage stages and appear in the nuclei, probably by migration
from the cytoplasm, at the gastrula stage. It is suggested by Horn that
histones might act as gene inhibitors, and that the nuclei are incapable
of RNA synthesis until histones have migrated into them from the yolk.
More will be said about this hypothesis in the final discussion of this
chapter. However, two remarks should be made : first, it is difficult to
prove, with the methods used, that the nuclear histones of the gastrula
derive only from a migration from pre-existing cytoplasmic histones to
the nuclei. One cannot rule out the possibility of independent histone
synthesis in gastrula nuclei until it is proved, by autoradiographic
methods, that these nuclei are not the site of a strong incorporation of
basic amino-acids, such as arginine or lysine. Such experiments have not
yet been performed. Secondly, the conclusion of Horn (1962), that the
yolk platelets contain true histones, might be premature. In fact, it was
reported by Nace, at a recent symposium (1962), that the basic protein
present in the yolk platelets is not a histone, but, oddly enough,
lysozyme.
5. Lethal Hybrids
The importance of gastrulation is conspicuous in many lethal
combinations between different frog species, the development of which