mitochondria it was established that organelle DNA-molecules are circular,
specific and differ from the average nucleotide composition of nuclear DNA.
That is used as a basis for their identification. It was proven that the
replication of DNA in the nucleus and that of chloroplasts are realized in
different phases of the cell cycle (Chiang, Sueoka, 1967). Also there
appeared reports about a presence of DNA in basal small bodies of the
flagella in some unicellular algae and Protozoa.
Extranuclear heredity which in many cases determines a non-
Mendelian type of inheritance, the presence of specific DNA, RNA and
ribosomes, as well as the differences during the replication of nuclear and
chloroplast DNA-molecules gave serious grounds to accept the existence
of genetic autonomy of some organelles participating in the composition of
eukaryotic cells. By means of modern methods numerous studies were
carried out in this field. As a result of them an opinion was confirmed, that
these organelles are endosymbionts originating from common protist
ancestors — bacteria and unicellular blue-green algae.
At present there exist the following ideas on the genetic autonomy of
some cellular organelles.
Chloroplasts. Circular DNA-molecule of the chloroplasts has a
molecular weight of about 1×10⁸ daltons, which corresponds to the
genome of lower bacteria like mycoplasms and rickettsiae. By means of
the available 250 000—300 000 base pairs the chloroplast genome may
code the synthesis of about 200 proteins with average molecular weight
of 40 000. In reality, according to the measurements, only 20% of the
genetic information is active. That means a synthesis of maximum 40—
50 proteins. If juxtapose the amount of their own proteins — about 50 to
100 — the corresponding minimum in protists determined by Kaplan
(1972), the chloroplasts code only 50% of their essential proteins. In this
case the proportion between chloroplast genetic autonomy and the
dependence on the nucleus is 50:50.
Mitochondria. These organelles also have circular DNA-molecules
with a molecular weight of about 1×10⁷ daltons, with dimensions less than
the average of now-existing protists. The mitochondrial genome is active
and with its 15 000—20 000 base pairs is capable to synthesize about 30—
35 proteins with average molecular weight of 20 000. If juxtapose the
mitochondria-own proteins with the total 100 proteins in protists, then
mitochondria code only 35% of their essential proteins, i.e. their genetic
autonomy is up to 35%, and the dependence on the cell nucleus is up to
65%.
Flagella. This is the third organelle in eukaryotic cells considered to
have its own DNA. It is accepted that DNA-molecule in the bases of
flagella (i.e. in the basal small bodies) having a molecular weight of about
1×10⁶ daltons, approximately equal to 10 000 base pairs, is capable to
code 20 proteins with average molecular weight of 20 000 daltons. In