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.