| Abstract: | It is suggested that DNA in first organisms on the Earth was enriched by GC-pairs due to intensive solar UV-irradiation. In the course of progressive evolution, nucleotide composition of DNA shifted to the favour of AT-pairs. This shift to higher photosensitivity became possible because of the development of reparation systems and screening of DNA in cells. The latter phenomenon is better revealed in higher organisms, accounting their stable TA-type of DNA. In lower forms, peculiarities of their habitats provided some conditions for variability of nucleotide composition. High content of GC-pairs remained in those of them which are subjected to the influence of solar irradiation. Parasitic microorganisms and those safely protected from solar radiation developed towards the AT-type of DNA. Both in higher and lower organisms, AT-pairs are richer in those zoms of DNA which are presented by several copies and in which mutations are less significant for the organism therefore being permissible. Structural genes exhibit heterogeneous nucleotide composition which provides for higher variability of the synthesized proteins. |
