Transcription from Nonrepeated DNA Base Sequences

The attractiveness of hybridization reactions utilizing unique DNA base sequences was quickly recognized by many workers. Usually the experiments use excess nonradioactive RNA in the presence of small amounts of radioactive nonrepeated DNA. Davidson and Hough (1971) 

The extent of reaction of total mouse embryo RNA with repeated sequences of the genome increases from about 2 to 5% during the preimplantation stages of development and then plateaus at about 4% during the rest of embryonic development (Fig. 2). In contrast, total embryonic RNA which is complementary to single copy DNA sequences shows a marked increase in the postimplantation period. Approximately 1% of the total single copy DNA sequences form a duplex in the preimplantation stages of development which is followed by an inerease in complexity to approximately 10% by the time of parturition (Fig. 2 Church and Brown, 1972). It must be emphasized that the 1% value for complexity for the preimplantation mouse embryo in Fig. 2 is a minimal estimate at best. 

It is not possible to obtain enough material at these early stages of mouse embryo development to aehieve fully saturating eonditions in the hybridization reaction. The estimates are based on the reaction of H-labeled RNA with C-labeled nonrepeated DNA in a formamide reaction mixture incubated at 30°C for several weeks (Church, 1970). The rabbit blastocyst, however, expands tremendously prior to implantation, and suflBcient RNA can be obtained to determine the diversity of transcription by the conventional procedures outlined above. Here a value of about 1.8% is obtained (Fig. 3 Schultz et al, 1973b). Since the rabbit genome is comprised of about 4.5 X 10 base pairs (Vendrely and Vendrely, 1949) and 70% of the nuclear DNA is nonrepeated, 1.8% 

The diversity of transcription of unique DNA base sequences does increase, after implantation, as organogenesis commences. Data obtained in the laboratories of the authors and by others to support this conclusion are summarized in Table II. In the mouse embryo, a marked increase in transcriptional complexity appears very shortly after implantation oc-ciurs (Fig. 2). Although every effort is made, it is difficult to obtain embryonic material totally free of contaminating extraembryonic membranes or decidual tissue at the seventh day of pregnancy in the mouse. Hence, the value reported for 7-day embryos in Fig. 2 may be an overestimate of embryonic transcriptional activity. The increase in complexity of transcription could be more gradual as development proceeds as is the case of the rabbit. The apparent differences between the mouse 

Transcriptional Activity of Nonrepeated DNA Base Sequences in Mammalian Development 

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