a, Influence of temperature upon RNA synthesis by ribozymes Y and R18: quantification of average extension (primer A / template I, 7 days) in aqueous (left panel) and frozen (right panel) reactions (error bars represent s.d. of 3 repeats, except for −7°C frozen (6 repeats)). b, Primer extension by Y in ices at different temperatures (primer A / template I; 40 days; * = −25°C, no NTPs).