Repetitive cistrons

While regulatory roles for repetitive transcripts have been suggested, at least some mRNA species originate from repeated DNA base sequences. Bishop et al. (1972) utilized the vast DNA excess hybridization procedure to demonstrate that the cistrons for the alpha and beta chains of hemoglobin are present at a reiteration frequency of about five each. A much higher reiteration frequency is reported for the cistrons coding for histone mRNA (Kedes and Birnstiel, 1971). However, the majority of polysomal mRNA molecules seem to be derived from nonrepeated sequences (Greenberg and Perry, 1971 Firtel etal., 1972). 

At various times during embryonic development there seem to be very distinct requirements for rapid synthesis of rRNA, 5 S RNA, and tRNA, and the rates of synthesis in such instances may be very much higher than the values reported by Quincey and Wilson (1969). However, gene products which are supposedly derived from repetitive DNA base sequences are reached by an operational definition. It is clear that heterogeneity of the cistrons for tRNA exists and rates of synthesis may affect only some, and not all, of these cistrons. Similarly, heterogeneity of the rRNA cistrons has been suggested (Moore and McCarthy, 1968) and sequence studies clearly show that more than one type of 5 S RNA exists in Xenopus (Ford and Southern, 1973). 

The scheme (Fig. 11a) thus provides a possible explanation of the synthesis of different virus proteins at different times and in different amounts (at different speeds), but it does require the existence of an as yet unknown mechanism of independent operation of individual cistrons in accordance with the program for virus development. In other words, it requires a programming mechanism, controlling protein synthesis by the polysome in the same way as a tune is produced from notes obtained by striking the keys of a piano. The scheme (Fig. 11b) resembles the simple repetition of a scale of successive sounds. In this case no notes, i.e.,programmed regulation of activity, are required it is assumed that control is by simple linear succession of the cistron-keys. 

Ihe occurrence of repetition of DNA segments carryii the same information many times over (conjectural of repeated genes or cistrons) was discovered by DNA/DNA molecular hybridization. When heated in solutions with low ionic strength, DNA dissociates (denatures), and its polynucleotides, which were joined... 

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