was obtained thus disclosing the first triplet (UUU). After that the codons for a
number of amino acids were established: AAA — for lysine, CCC — for
proline, etc. In this way the path leading to the in vitro synthesis of amino
acids, polypeptide chains (proteins) and genes (Khorana, 1979) has been
widely opened. Since these studies are strictly in the domain of molecular
biology they will not be discussed in detail.
As to the “age” of the genetic code it was determined by Eigen et al.
(1989) to not more than 3.8 (±0.6) billions of years, i.e. approximately the age
of the Earth, supposing the self-generation of life, if not transferred from the
outside world. The thesis supporting the extraterrestrial origin of the genetic
code is backed by F. Crick as shown in Section 1. 5.
The genetic code is assumed universal though it was basically
worked out on the well-known bacterium Escherichia coli. It must be
noted that it has attracted researchers attention mainly by posing three
great questions: 1) when has it arisen?; 2) is it universal?; 3) has it
undergone evolutionary changes or not?
T. Jukes (1973, 1990) considered the possibilities for an evolution of the
genetic code from a preceding form and has reported some exceptions to the
universal genetic code. They were initially established in mitochondria, then in
the nuclear codons of bacteria, yeasts, ciliated protozoa and algae. These