An evolutionary progression leading toward replication is resolved into several phases; (a) the replication of RNA segments by self-priming and -templating, (b) the replication of single stranded molecules by elongation and controlled scission, (c) replication of complementary duplexes and (d) replication of DNA. The initial phase is suggested by evidence for the existence of tandem repeats in an early population of molecules presumed to be ancestral to today's structurl RNAs. Relics of these repeats are seen in the positioning of sequence matches between transfer and ribosomal RNAs. Conservation of the positions of the matches is indicated by persistence of a periodicity in their spacings along the molecules.

Many tRNAs of E. coli and yeast contain stretches whose base sequences are similar to those found in their respective rRNAs. The matches are too frequent and extensive to be attributed to coincidence. They are distributed without discernible pattern along and among the RNAs and between the two species. They occur in loops as well as in stems, among both conserved and non-conserved regions. Their distributions suggest that they reflect common ancestral origins rather than common functions, and that they represent true homologies.

Data from flow-cytometric analysis of DNA of Ehrlich ascites tumor cells were fitted using non-linear least squares curve fitting routines. Analysis of rates of synthesis from the derived S-period profiles revealed a pattern of changing rates of DNA synthesis during the S-period. Three main peaks are seen whose trough to through periods range from 0 to 16%, 16 to 65%, 65 to 100% of the DNA synthesized during S. The differences between the peak rates and rates in the intervening troughs are small, about 10% of the maximum, but these occur reproducibly. Some differences in the DNA distribution profiles, hence rate profiles, can be seen among samples taken at different times during the day. These are thought to reflect the effects of circadian rhythms, but they are not large enough to obscure the general pattern of rate shifts that occur during the S-period. Analyses of radioactivity of 3H-thymidine pulse labelled cells, sorted across the S-period, were in accord with the results obtained from the DNA distributions. A parallel analysis of DNA and histones showed a correspondence in the timing and direction of shifts in rate for both during the middle part of the S-period.