50 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”be the first spark in the engine of life, promoting changes in other mol-
ecules, including synthesis of mono- and di-saccharides, and probably
nucleosides, nucleotides, and di- and tri-nucleotides. Some of the amino
acids might spontaneously link together through the amide bond,^57 making
available di- and tri-peptides, further improving their catalytic function.
Over time, oligomers of amino acids and of nucleotides could
appear. These snippets of peptides and RNAs did not have to be more
than ten units long, and they might not contain all the subunits observed
in today’s biochemistry. For example, they might have only four simple
types of amino acids (e.g., glycine, alanine, serine, aspartic acid), and
two or three kinds of nucleobases (e.g., adenine, guanine and uracil). At
this early stage, base pairing of RNA was probably unimportant, but it
could be present. These oligomers of amino acids and nucleotides could
undergo mutual or reciprocal catalysis (with broad substrate specific-
ity) or, more significantly, form complexes (metal ions included) with
improved catalytic function — a prototype of the modern day “protein/
organic-cofactor/metal-cofactor” tripartite metabolic enzyme system. As
metabolism became increasingly elaborate, more varieties of building
blocks were available in the repertoire for incorporation into the poly-
mers. At the same time proteins and RNAs could increase in length
and begin to fold into 3-dimensional structures, greatly enhancing their
respective enzyme functions. The proto-proteins and proto-RNA were
initially confined to metabolic activities, but somewhere along the line
complementary strands of RNA could appear, starting rudimentary
molecular replication. Initially, the sequence of amino acids and nucleo-
tides were largely random, and any RNA replication, if present, would be
imprecise. How the useful sequences of the biopolymers were selected
is a mystery, but I believe it could not have taken place without involving
the organizational power of the proto-cell, which by this time should
have been in the formative stage.
A plasma membrane was essential for the proto-cell. It was specu-
lated that the cell membrane started as lipid vesicles or liposomes, to be
replaced later by a phospholipid bilayer (as we have it today), and much