(A) In the central dogma of molecular biology, information flows from DNA (red oval) to RNA (green oval) to protein (blue box). DNA is formed of building blocks called deoxynucleoside triphosphates (dNTPs) and can be replicated (solid looping red arrow); RNA is formed of nucleoside triphosphates (NTPs). Enzymes called reverse transcriptases (RT) enable complementary DNA to be made from the building blocks of RNA (dashed arrow). Blue rectangles represent processes catalyzed by proteins; green rectangles show processes catalyzed by RNA; translation is mediated by an RNA catalyst (green inner rectangle) that has proteins that modulate its activity (blue outline). (B) In the RNA world, ribozymes (RdRp) replicate RNA genomes (solid looping red arrow). Based on the work of Joyce and Samanta, if dNTPs were present in the RNA world, reverse transcriptase ribozymes could have constructed DNA genomes using RNA genomes as a template (straight red arrow). Ribozymes could also have potentially replicated DNA genomes (dashed red arrow).