When the first DNA polymerase (DNA-polymerase I) was discovered, the
transcription mechanism of the DNA chain appeared to be comparative simple.
Afterwards it proved to be much more complex. In the small bacterial cell E. coli
there have been identified not one but three DNA-polymerases — I, II and III.
For some time it was thought that DNA-polymerase I is the main polymerizing
enzyme binding the deoxynucleotides. It is now accepted that this role is played
by DNA-polymerase III, i.e. the last one in the order of its discovery. The function
of DNA-polymerase II remains unclear.
In eukaryotic cells three forms of RNA-polymerase with different functions
have been identified as well. One of them is responsible for the rRNA
synthesis, the other one — for the mRNA and the third one — for tRNA.
It is accepted in principle that any DNA-segment can be transcribed
with the formation of two different mRNA molecules (one for each of the
two DNA-chains). In reality, it is more likely for this to take place on only
one of them, which is determined by the promotor (start-signal) in DNA.
Figure 2–35. A scheme clarifying the transition of mRNA from the nucleus into the cytoplasm where by the help of the ribosomes the translation and synthesis of polypeptide chains (proteins) take place in eukaryotic cells
The mRNA synthesized in the nucleus of eukaryotic cells passes
into the cytoplasm and binds to the ribosomes. There the process of
translation of mRNA genetic information into polypeptide chains is
performed (Fig. 2–35). This process is very complex and multistage, since
translation is not directly connected with the α-amino acids which are to
be polymerized. The preparation of their incorporation into polypeptide
chains is started with the amino acid COOH-group activation by ATP and
its transfer on a specific transport RNA (tRNA). For each of the 20
amino acids participating in protein synthesis there are one or more
tRNAs. An important role for the successful decoding of the information
in mRNA is played by the precise binding of the bases between the
mRNA codon and the tRNA anticodon (Fig. 2–36). Each amino acid is
attached to the corresponding tRNA molecule specific for it. The
“recognition” of the amino acid by tRNA and its adaptation to it is