Self And The Phenomenon Of Life: A Biologist Examines Life From Molecules To Humanity

Self and the Beginning of Life 51

“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity

later to be embellished with ion pumps and receptors, both of protein
nature. The first membrane would be imperfect, permitting ions, amino
acids and nucleotides/nucleosides to pass through but trapping biopoly-
mers inside. The permeability of the crude membrane to ions suggests
that the early cells might have their internal environment similar to the
outside, implying that life started in terrestrial ponds adjacent to volca-
nic geothermal outlets, where the salt composition is closer to modern
intracellular fluid than is seawater.^58
Getting back to the RNA-protein relationship, a key molecule is
peptidyl transferase (a ribozyme), which sits at the interface of RNA
and protein worlds by making peptide bond connections between amino
acids. The primitive transferase could well be much simpler than the
modern counterpart, and might even function in the absence of a ribo-
some. Though grossly inefficient, this rickety machine should be able
to “crank out” a few strands of long peptides (or short proteins), thus
starting the protein “ball” rolling. Therefore, as soon as the transfer-
ase appeared, proteins could be made with relative ease, allowing them
to gradually take over the metabolic processes. At the start, the pro-
teins could be, for illustrative purposes, from 10 to 20 amino acids long,
gradually increasing in length over millions of years. With improved
efficiency of peptidyl transferase and the help of a proto-ribosome, the
protein product could reach a length sufficient to fold on itself, forming
3-dimensional structures that enhanced their catalytic function. At the
same time, proteins can help produce and stabilize long chains of RNA.

3.16 The Origins of the Genetic Code and the Translation
Machinery

Despite much speculation and intensive experimentation, the origins of
the genetic code and its assignment, along with the translation mecha-
nism, are shrouded in mystery (Table 3.3; Figs. 3.3 and 3.4). Based on
our knowledge that the code is made of three letters, each taken from an
alphabet of four nucleobases, there are 4 × 4 × 4 = 64 possible codons.