40
J . B. GURDON
The interpretation of these experiments is not entirely straightforward since the apparent regulatory role of histones may be incidental
to a structural function, and the trypsinated thymus nuclei appear
microscopically quite different from normal nuclei. These experiments
do show however, that the amount of RNA synthesized by chromatin
or isolated nuclei can be affected by a kind of protein which is always
present in chromosomes in about the same quantity as the DNA. They
are therefore at least consistent with the view that histones, together
with substances from outside the cell, may play some part in controlling
the reversible activity of genes.
V. Summary and Prospects
Nuclear transfer experiments have contributed to the problem of cell
differentiation in the following ways.
The genetic material of somatic cells does not usually change during
cell differentiation except for the occurrence of occasional spontaneous
mutations. Nor does the genetic material of differentiating cells generally
undergo any stable changes which restrict gene activity and which
persist in nuclei transplanted to egg cytoplasm. These conclusions are
drawn from the facts that (i) normal adult frogs can be obtained by
transplanting nuclei from differentiated intestine cells to enucleated
eggs of Xenopus, and (ii) nearly normal tail-bud tadpoles with many
different cell types can be obtained by transplanting nuclei from
adenocarcinoma cells to enucleated eggs of Rana. These conclusions are
consistent with the results of other experiments since chromosome
changes which occur regularly in development take place in only a few
species of animals, and stable gene changes, such as result from transposable elements and paramutation in maize, do not appear to be
subject to the kind of control required for cell differentiation. It is
deduced that cell differentiation must therefore result from the reversible activation and repression of gene activity and that this is
controlled by the nature of the chromosomal environment. Direct
support for this conclusion comes not only from the normal development and type of RNA synthesis supported by transplanted nuclei, but
also from the cytological evidence of gene activity and from the study
of induced enzyme synthesis.
The following conclusions relevant to the control of cell differentiation
can also be drawn from nuclear transfer experiments. A protein which
may be involved in the cytoplasmic control of gene activity has been
revealed by nuclear transfer experiments in Amoeba. The response of
the cell to the information carried in its DNA probably involves the
release of substances from the nucleus to the cytoplasm. Morphogenetic
J . B. GURDON
The interpretation of these experiments is not entirely straightforward since the apparent regulatory role of histones may be incidental
to a structural function, and the trypsinated thymus nuclei appear
microscopically quite different from normal nuclei. These experiments
do show however, that the amount of RNA synthesized by chromatin
or isolated nuclei can be affected by a kind of protein which is always
present in chromosomes in about the same quantity as the DNA. They
are therefore at least consistent with the view that histones, together
with substances from outside the cell, may play some part in controlling
the reversible activity of genes.
V. Summary and Prospects
Nuclear transfer experiments have contributed to the problem of cell
differentiation in the following ways.
The genetic material of somatic cells does not usually change during
cell differentiation except for the occurrence of occasional spontaneous
mutations. Nor does the genetic material of differentiating cells generally
undergo any stable changes which restrict gene activity and which
persist in nuclei transplanted to egg cytoplasm. These conclusions are
drawn from the facts that (i) normal adult frogs can be obtained by
transplanting nuclei from differentiated intestine cells to enucleated
eggs of Xenopus, and (ii) nearly normal tail-bud tadpoles with many
different cell types can be obtained by transplanting nuclei from
adenocarcinoma cells to enucleated eggs of Rana. These conclusions are
consistent with the results of other experiments since chromosome
changes which occur regularly in development take place in only a few
species of animals, and stable gene changes, such as result from transposable elements and paramutation in maize, do not appear to be
subject to the kind of control required for cell differentiation. It is
deduced that cell differentiation must therefore result from the reversible activation and repression of gene activity and that this is
controlled by the nature of the chromosomal environment. Direct
support for this conclusion comes not only from the normal development and type of RNA synthesis supported by transplanted nuclei, but
also from the cytological evidence of gene activity and from the study
of induced enzyme synthesis.
The following conclusions relevant to the control of cell differentiation
can also be drawn from nuclear transfer experiments. A protein which
may be involved in the cytoplasmic control of gene activity has been
revealed by nuclear transfer experiments in Amoeba. The response of
the cell to the information carried in its DNA probably involves the
release of substances from the nucleus to the cytoplasm. Morphogenetic