Sprecher
Beschreibung
The earliest form of RNA replication may have been non-enzymatic, without requiring polymerase ribozymes. Template-directed synthesis of complementary strands forms double strands that are unlikely to separate unless temperature cycling drives melting. If there are multiple copies of identical sequences, re-annealing of existing strands prevents subsequent cycles of copying. However, if there is a diverse mixture of sequences, partially matching sequences can reanneal in configurations that allow continued strand growth. Here we present simulations that incorporate melting, reannealing, primer extension, and ligation. Strand growth occurs over multiple heating/cooling cycles, producing strands over 200 nucleotides in length. However, there is no exact copying of sequences, even if single base additions are fully accurate (no mutational errors). It has been proposed that RNA systems may contain a virtual circular genome consisting of partially overlapping sequences that can be assembled into a circle. We show that this situation is unlikely to arise naturally and cannot maintain itself in the presence of mutational errors or inflow of random oligomers. We show that even a short functional sequence like a tRNA cannot be encoded on a virtual circle because it contains repeated tetramers; hence sequence information on a longer length scale is not maintained. In contrast, we argue that the most likely way for replication to begin in the RNA world involves real circular strands that use the rolling circle mechanism. Multiple copies are produced from a single circle via strand displacement without requiring temperature cycling.
