Gene activity in early development / Eric H. Davidson.

  • Eric H. Davidson
Date:
[1968]
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene activity in early development / Eric H. Davidson. Source: Wellcome Collection.

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    58 I. GENE ACTIVITY IN EARLY EMBRYOGENESIS animal cell single-copy DNA. Furthermore, it can be demonstrated that hybridization of homologous RNA with bacterial DNA proceeds until the maximum theoretical saturation ratio of RNA to DNA in the hybrids is attained. Thus saturation of bacterial DNA with metabolically labeled RNA at close to a 0.5 RNA/DNA ratio has been reported for at least two species of bacteria [(113, 114): see Section IV for detailed discussion of these studies]. Since only one strand of the DNA is functional in RNA synthesis, 50% saturation of the DNA indicates that 100% of the genomic information is present in the hybridizing RNA populations. It is empirically clear, there¬ fore, that hybridization can proceed to absolute saturation in the annealing systems used, even where the DNA is of the single-copy class. To summarize this digression: The RNA-DNA hybridization procedure is sufficiently specific to distinguish sharply the RNA populations of genera as closely related as Xenopus and Rana, just as it can distinguish the spectra of informational RNA's present in different tissues of the same organism. The technique cannot, however, be expected to distinguish among nearly identical polynucleotides. This limitation could conceivably affect the absolute accuracy of determinations by the excess RNA saturation method of the percentage of the genome which is active in a given circumstance, since the RNA could hybridize with genes of an internally replicate gene family other than those genes actually responsible for its synthesis. Redundancy in the hybridizing DNA fraction could thus result in a limited overestimation of the proportions of the genome actually active. However, except in saturating experiments in which higher concentrations and longer annealing times than usual are em¬ ployed, the proportion of the genome active may be considerably underesti¬ mated because of failure to monitor single-copy DNA hybridization. The length of annealing time and nucleic acid concentration required in order to obtain hybridization within the single-copy portion of the genome will depend, if other parameters are held equal, on the genome size of the organ¬ ism. Thus failure to discriminate at the intragene family level and partial failure to monitor hybridization of the single-copy portion of the genome both affect the interpretation of experimental data regarding the apparent percentage of the genome active. Neither source of difficulty, however, need directly affect comparisons of the relative amount of the genome active at successive stages of differentiation in the same tissues. Though it is primarily the activity of the internally redundant gene families which is monitored in the hybridization studies we are now to consider, it is clear that sharp changes in the pattern of activity among these families occur in differentia¬ tion. Analysis of the spectrum of activity in the redundant portion of the genome is therefore of fundamental importance in the study of differen¬ tiation. These arguments lead to the conclusion that the data of Table IV pro¬ vide a useful index of the relative diversity of gene transcription as develop-
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