Bit by Bit: The Darwinian Basis of Life
(A) The replication cycle involves paired RNA enzymes (E in blue, E′ in orange) that catalyze each other's synthesis by joining two corresponding oligonucleotide substrates (A′ and B′ to form E′, A and B to form E). Each substrate contains six nucleotides of variable sequence that are recognized by Watson-Crick pairing to the enzyme. The tertiary structure is based on homology modeling to the crystal structure of the L1 ligase RNA enzyme . (B) Sequence and secondary structure of the E·A′·B′ complex, with the two genetic regions (boxed) and two corresponding regions within the functional domain of the enzyme shown in color, and with the immutable nucleotides that are essential for replication shown in black. Curved arrow indicates the site of ligation of A′ and B′ to form E′. The E′·A·B complex has reciprocal composition.