accomplished by another specific group of enzymes — aminoacyl-tRNA-
synthetase leading to the formation of an aminoacyl-tRNA-molecule. For
each amino acid there is a separate aminoacyl-tRNA-synthetase: (one of them
joins glycine to — tRNAᵍˡʸ another — alanine to — tRNAᵃˡᵃ, etc. The extension of
the polypeptide chain is continued until one of the three stop-codons is
reached, which is a signal for stopping the synthesis.
Figure 2–36. Scheme (A) and spatial model (B) of codon and anticodon in the molecule of the phenylalanine tRNA of yeast (Courtesy of Sung-Hou Kim; From Alberts et al., 1986)
It is supposed that in prokaryotic cells transcription and protein
synthesis take place almost simultaneously. Here the main unsolved
problem is how the despiralization of the circular DNA-molecule is
effectuated at the time of the replication. Most probably this occurs by its
severing at a given segment under the action of one or more enzymes.
These processes are not clearly elucidated.
One of the most important functions attributed to proteins for now is
the specific catalysis of biochemical reactions, i.e. their action as
enzymes. This differs them a lot from nucleic acids. It is considered that
oxygen transfer is a characteristic property of hemoglobin and not of the
gene encoding for that protein. It is quite possible from such a standpoint
to draw the conclusion that although DNA contains the whole genetic
information necessary for the structure and development of the cells, it is
kept in an “academic” state and does not immediately participate in the