The origin of these chromosome abnormalities is probably to be understood as an incompatibility between the very slow rate of division of differentiating cells—only one division every two days or more—and the rapid rate of division in an egg, which starts to divide (and causes any injected nucleus to try to divide) about an hour after injection. Unless an injected nucleus can complete the replication of its chromosomes within this brief period, they will be torn apart and broken at division. This concept is supported by the observation, made at Oxford in collaboration with my colleagues C. F. Graham and K. Arms, that many transplanted nuclei continue to synthesize the genetic material DNA right up to the time of the first nuclear division, whereas sperm and egg nuclei always complete this synthesis well before division. Presumably molecules associated with the DNA of specialized cells prevent the chromosomes of such cells from undergoing replication as rap idly as those of sperm nuclei, thereby leading to the chromosome abnormalities commonly observed in nuclear-trans plant embryos.
Having concluded that the specialization of cells involves the differential activity of genes present in all cells, rather than the selective elimination of unwanted genes, we can now consider how genes are activated or repressed during early embryonic development. Nuclear transplantation has been used to demonstrate that the signals to which genes or chromosomes respond are normal constituents of cell cytoplasm. This information has come from experiments in which the nucleus of a cell carrying out one kind of activity is combined with the enucleated cytoplasm of a cell whose nucleus would normally be active in quite another way. One of two results is to be expected: either the transplanted nucleus should continue its previous activity or it should change function so as to conform to that of the host cell to whose cytoplasm it has been exposed. For the purposes of these experiments changes in nuclear activity have to be recognized by the appearance of direct gene products and not by the much less direct criterion of the normality of nuclear-transplant embryo development. Many of these experiments have been carried out in collaboration with Donald D. Brown of the Carnegie Institution of Washing ton or with another of my Oxford colleagues, H. R. Woodland.
The first experiments were designed to find out if the different functions performed by any one gene—the synthesis of DNA, the synthesis of RNA and chromosome condensation in preparation for cell division—are determined by cytoplasmic constituents. Three kinds of host cell were used: unfertilized but activated eggs whose nucleus would normally synthesize DNA but no RNA; growing oocytes in which the nucleus synthesized RNA but not DNA, and oocytes maturing into eggs, in which situation the nucleus consists of condensed chromosomes arranged in the "spindle" of cell division, and synthesizes neither RNA nor DNA. Two kinds of test nuclei were used: nuclei from adult brain tissue, which synthesize RNA but almost never synthesize DNA or divide, and nuclei from embryonic tissue at the mid blastula stage of development; mid-blastula nuclei do not synthesize RNA but synthesize DNA and divide about every 20 minutes. For technical reasons it was desirable to inject each host cell with many nuclei, even though this can prevent the subsequent division of the injected cell. The results were clear: In all respects tested the transplanted nuclei changed their function within one or two hours so as to conform to the function characteristic of the normal host-cell nucleus. Mid-blastula nuclei injected into growing oocytes stopped synthesizing DNA and dividing and entered a continuous phase of RNA synthesis that lasted for as long as the injected oocytes survived in culture (about three days). Adult brain nuclei injected into eggs stopped RNA synthesis and began DNA synthesis. When the same nuclei were injected into maturing oocytes, they synthesized neither RNA nor DNA but were rapidly converted into groups of chromosomes on spindles.