NUCLEIC ACIDS AND SULPHYDRYL GROUPS
293
are in keeping with present knowledge; future experiments will show
whether, or to what extent, they are justified.
There is no doubt that the nuclei, which are duplicating at full speed
during cleavage, have little importance in the very first phases of
development. At that time, they are occupied with the reduplication of
DNA and associated proteins, and do not produce appreciable amounts
of RNA (Brächet, 1954). That DNA and RNA synthesis are mutually
exclusive (when DNA 'makes' DNA, it cannot 'make' RNA and
vice versa) seems to be a general fact (Prescott and Kimball, 1961;
Goldstein and Brown, 1961).
It is clear that, in amphibian eggs, considerable metabolic change
occurs at gastrulation; in particular, carbohydrate metabolism is
enhanced, while synthesis of nuclear RNA now commences, RNAcontaining nucleoli become visible, and uridine is no longer utilized
solely for DNA synthesis, but can be used for nuclear and cytoplasmic
RNA synthesis. Experiments with pyrimidine and folic acid analogs
show that new metabolic pathways become available for nucleic acids.
Possibly, thymidilate kinase is synthesized or becomes active at that
stage.
Many biological experiments indicate that at gastrulation, genes become 'activated' (see Brächet, 1960, for a more complete presentation).
What exactly is meant by
c
gene activation' in developing embryos
remains obscure. But, in the future, it may prove useful to consider the
hypotheses of Jacob and Monod (1961), which have been based on
studies of the simpler biological system of the bacteria. In addition to
structural genes, which determine the molecular organization of proteins,
they postulate regulator and operator genes which control the rate of
protein synthesis, through the intermediary of cytoplasmic repressors.
The latter can be inactivated (enzymatic induction) or activated
(repression) by specific metabolites. Regulation would directly control
synthesis by the gene of the messenger RNA. How the hypothesis might
explain the regulation of cell differentiation has been discussed by
Jacob and Monod (1961) in a very interesting paper.
Thus 'activation' of the genes, at the gastrula stage, might be controlled by the regulator and operator genes, which would commence
their functions at that stage. A less sophisticated view has been
suggested by Craft et al (1954), Leslie (1961), Bloch (1962a, b) and
Horn (1962); namely that histones act as gene regulators; nuclear
histones could suppress the template function of nuclear (messenger)
RNA and ribosomal histones might protect (or block?) template RNA.
Whether or not operator and regulator genes play a role in development, one thing seems to be clear: DNA does not become functional
before gastrulation, at which time extensive synthesis of nuclear
293
are in keeping with present knowledge; future experiments will show
whether, or to what extent, they are justified.
There is no doubt that the nuclei, which are duplicating at full speed
during cleavage, have little importance in the very first phases of
development. At that time, they are occupied with the reduplication of
DNA and associated proteins, and do not produce appreciable amounts
of RNA (Brächet, 1954). That DNA and RNA synthesis are mutually
exclusive (when DNA 'makes' DNA, it cannot 'make' RNA and
vice versa) seems to be a general fact (Prescott and Kimball, 1961;
Goldstein and Brown, 1961).
It is clear that, in amphibian eggs, considerable metabolic change
occurs at gastrulation; in particular, carbohydrate metabolism is
enhanced, while synthesis of nuclear RNA now commences, RNAcontaining nucleoli become visible, and uridine is no longer utilized
solely for DNA synthesis, but can be used for nuclear and cytoplasmic
RNA synthesis. Experiments with pyrimidine and folic acid analogs
show that new metabolic pathways become available for nucleic acids.
Possibly, thymidilate kinase is synthesized or becomes active at that
stage.
Many biological experiments indicate that at gastrulation, genes become 'activated' (see Brächet, 1960, for a more complete presentation).
What exactly is meant by
c
gene activation' in developing embryos
remains obscure. But, in the future, it may prove useful to consider the
hypotheses of Jacob and Monod (1961), which have been based on
studies of the simpler biological system of the bacteria. In addition to
structural genes, which determine the molecular organization of proteins,
they postulate regulator and operator genes which control the rate of
protein synthesis, through the intermediary of cytoplasmic repressors.
The latter can be inactivated (enzymatic induction) or activated
(repression) by specific metabolites. Regulation would directly control
synthesis by the gene of the messenger RNA. How the hypothesis might
explain the regulation of cell differentiation has been discussed by
Jacob and Monod (1961) in a very interesting paper.
Thus 'activation' of the genes, at the gastrula stage, might be controlled by the regulator and operator genes, which would commence
their functions at that stage. A less sophisticated view has been
suggested by Craft et al (1954), Leslie (1961), Bloch (1962a, b) and
Horn (1962); namely that histones act as gene regulators; nuclear
histones could suppress the template function of nuclear (messenger)
RNA and ribosomal histones might protect (or block?) template RNA.
Whether or not operator and regulator genes play a role in development, one thing seems to be clear: DNA does not become functional
before gastrulation, at which time extensive synthesis of nuclear