24 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”in the mRNA. Nature resolves this discrepancy by assigning more than
one codon to some amino acids. This redundancy of the codes is called
degeneracy. All tRNAs possess a specific codon-recognition site called
anticodon, and there are over sixty tRNAs corresponding to the number
of codons in the mRNA. The job of matching the twenty amino acids
to so many tRNAs is the work of a set of remarkable enzymes called
aminoacyl-tRNA synthetases (AARS). There is one AARS assigned to
each amino acid. Each AARS, therefore, recognizes its specific amino
acid on one part of its molecule and at the same time the correspond-
ing tRNA on another part. What is remarkable is that, in the event
of degeneracy, one AARS is able to match one amino acid to several
tRNAs. This versatile matching ability has strong implication in the
origin of life and will be discussed later.
In summary, the direction of information flow for the biopolymers
is DNA → mRNA → protein, a phenomenon known as the Central
Dogma (Fig. 3.5). The other function of DNA, which is to replicate itself
by complementary base pairing, is depicted in Fig. 3.2.
Proteins are polymers of amino acids linked together by peptide
bonds. Out of a great variety of amino acids found in nature, only twenty
kinds are assembled into a protein. Just as the 26 letters of the English
alphabet can be arranged in limitless combinations and permutations
to make words, the living system uses the 20 amino acid letters to con-
struct thousands of different proteins. A typical protein chain contains
from about 100 to several thousand amino acids, the sequence of which
decides what 3-dimensional appearance (conformation) the protein
molecule assumes. Conformation is extremely important for protein
Fig. 3.4. (Continued) to its cognate tRNA. The enzyme catalyzes the formation of a
covalent bond between an amino acid and the tRNA, now called aminoacyl tRNA (AA-
tRNA). (B) Detail of an aminoacyl-tRNA showing an amino acid linked to the adenylic
acid nucleotide at the 3′ end (on top) of the tRNA. The location of the anticodon, which
is to match the complementary sequence (codon) on the mRNA, is shown at the bottom.
(C) Drawing showing the assemblage of a protein chain of amino acids on a ribosome
by matching the codons (on messenger RNA) and anticodons (on tRNA). [B and C: US
Natl. Lib. Med.]