Formation of Nucleotides
We hypothesize a process for multiplication of nucleotides on prebiotic Earth [11]. It is posited that the subunits of the nucleotides of terrestrial or exterrestrial origin existed in prebiotic oceans. The multiplication of nucleotides took place at tideland or estuary at which wet & dry cycle or pH fluctuation occurred due to tidal cycle. The subunits dissolved in the oceans were selectively adsorbed on clay minerals at the bottom of tideland or estuary. Since fresh subunits were supplied in every tidal cycle, the concentration of the subunits on the mineral surfaces were gradually increased. Once the concentration reached a certain level, it was extremely rare, but a rection that joined three subunits, say, base (U), D-ribose and phosphate occurred, and an original nucleotide formed. The rate of the reaction is proportional to the concentration of reactants but independent of the concentration of the nucleotides. The nucleotides might degrade before the next reaction occurred; thus, a significant increase in the concentration of the nucleotides was unlikely if the formation was solely relied on the synthesis reaction unless the concentration of the reactants were unnaturally high. In order to increase the concentration, the nucleotides must be formed faster than degradation. As a plausible process, we propose a cross complementary self-replication of the nucleotides. Although various proto-nucleotides including extant RNA nucleotides were produced in the prebiotic Earth, the extant nucleotides eventually dominated. The sequential steps of the formation of the extant RNA nucleotides are described in the following:
Step 1: For example, a free complementary base (A) resided close to the original nucleotide is joined to the base (U) by hydrogen bond. This reaction is spontaneous because the bonding does not require to overcome an activation energy for establishing the bonding.
Step 2: A free D-ribose is joined to the base (A) by covalent bond. A nucleoside is formed. This reaction is possible because the original nucleotide is preferentially oriented and in a state of lower activation energy for the bonding.
Step 3: Next, a free phosphate is joined to the D-ribose by covalent bond to form the second nucleotide. Again, the activation energy is lowered due to the preferential orientation.
Step 4: The hydrogen bond between the original and the second nucleotides is broken by cyclic variation of the environment due to tidal cycle, resulting in two separate nucleotides.
Steps 1-4 repeat and the original nucleotide with (U) and the second nucleotide with (A) produce the third nucleotide with (A) and the fourth nucleotide with (U), respectively. Two different nucleotides are self-replicated. The process continues as long as the subunits are supplied.
The same process also occurs to the base (G) and (C) pair. If the rate of complementary self-replication exceeds the rate of degradation of existing nucleotides, the concentration of the nucleotides is expected to increase exponentially.