The reparation trough excision is more complex. Here a complex of
enzymes participates and there is no necessity of daylight as a source of
energy. It is realized in dark. Its essence is an enzymatic recognition and
removal of the damaged part in one of DNA-chains, resynthesizing the
same under the action of DNA-polymerase and sewing up the rupture by
means of a specific group of enzymes called DNA-ligases. This mode of
raparation refers not only to pyrimidine dimers, but also to other kinds of
damages causes by UV-light, X-rays, γ-rays, alkylating agents, etc.
Postreplication restoring (reparation) is referred to the processes and
changes occurring during and after the replication of the damaged DNA.
That determines the importance of the connection of these processes
with S-period, i.e. the synthesis of DNA. It is supposed that some of the
enzymes, participating in the reparation processes through excision and
resynthesis of damaged DNA-segments, may play an important role in the
normal realization of genetic recombinations and in maintaining the
integrity of the two-chain molecule of DNA. Besides, it is accepted that
pyrimidine dimers do not block the replication of DNA but cause the
formation of gaps in the locuses of newly synthesized chains,
corresponding to the dimers in initial chains. Fujiwara (1972) has
established that the number of gaps corresponds to the number of
pyrimidine dimers. But the mechanism of their filling remains unclear.
All stated in this Section leads to the conclusion that the knowledge
about the mutation changes is insufficient. Most processes and
mechanisms of their realization are given schematically. That gives
grounds to consider this problem one of the “white spots” in biology.
Undoubtedly, the mutations are connected with the hereditary material,
the chromosomes and genome as a whole, respectively. However, the
chromosomes are built not only by DNA. As it was already mentioned,
also RNA and a large set of proteins participate in chromatin composition.
Before undoing this Gordian knot of biology, i.e. to elucidate the
connection and interactions among them objectively, the problem about
the mutation changes will be open.