Primordial genetics: phenotype of the ribocyte.

The idea that the ancestors of modern cells were RNA cells (ribocytes) can be investigated by asking whether all essential cellular functions might be performed by RNAs. This requires isolating suitable molecules by selection-amplification when the predicted molecules are presently extinct. In fact, RNAs with many properties required during a period in which RNA was the major macromolecular agent in cells (an RNA world) have been selected in modern experiments. There is, accordingly, reason to inquire how such a ribocyte might appear, based on the properties of the RNAs that composed it. Combining the intrinsic qualities of RNA with the fundamental characteristics of selection from randomized sequence pools, one predicts ribocytes with a cell cycle measured (roughly) in weeks. Such cells likely had a rapidly varying genome, composed of many small genetic and catalytic elements made of tens of ribonucleotides. There are substantial arguments that, at the mid-RNA era, a subset of these nucleotides are reproducibly available and resemble the modern four. Such cells are predicted to evolve rapidly. Instead of modifying preexisting genes, ribocytes frequently draw new functions from an internal pool containing zeptomoles (<1 attomole) of predominantly inactive random sequences.