Deoxyribonucleotide synthesis and the emergence of DNA in molecular evolution.

DNA replication requires monomeric deoxyribonucleotides, which cannot be regarded as primary products of organic syntheses on a primitive earth. However, the present biosynthetic pathway--reductive elimination of the 2'-OH group from ribonucleotides, catalyzed by ribonucleotide reductases and thioredoxins--suggests an early, polyphyletic combination of protein-nucleotide interactions and metal catalysis. That key process had to precede the upcome of RNA-DNA dualism on the way from RNA-protein protocells to true organisms.