ABSTRACT
Good evidence exists that differences in embryonic cell composition largely originate from the activation of different genes in different cells and that translation of the resulting RNA into protein is usually rapid. But if the nuclei are from the closely related Xenopus tropicalis, the eggs never develop beyond an early embryonic stage. Gene activation in embryonic cells has a feature that shows that the mechanism is more complex: although most changes in cell composition during development are temporary, they remain stable in the absence of the signal that induced them, unlike changes induced in the composition of bacterial cells. Since genes direct the composition of cells, differences in composition might be expected to arise between embryonic cells because they have been allotted different genes. Hence, when embryonic cells become committed to a certain fate, local self-sustaining changes in chromosome structure may be induced, which allow the later access of proteins to genes whose transcription is needed to fulfil that fate.
