such a case the admissible genetic autonomy of flagella is 20%, and the
dependence on the cell nucleus is 80%.
The presence in the cells of organelles having its own DNA, specific
RNA and ribosomes, and their capability to divide independently posed a
difficult dilemma before the investigators: eukaryotic cell — is it
elementary structure or endosymbiotic system?
This dilemma supposes two possible ways of evolutionary developing
of the eukaryotic cells: 1) through intracellular differentiation, i.e.
compartmentalization of the above-mentioned autonomous organelles; 2)
through endosymbiosis of protists existed independently earlier. The first
concept is supported by the successive theory, and the second — by the
endosymbiotic. Also, there exists so-called combination theory. According
to it eucists have as endosymbiotic, as well as successive origin. This
theory cannot answer the above dilemma, since accepting endosymbiosis
though to a certain extent, in reality it supports the endosymbiotic theory.
What is important in this case is whether the extranuclear DNA is a product
of the cell itself, or it is included in it by ancient ancestor.
Vast theoretical and experimental work was carried out to confirm one
or the other of the two basic theories. There were used different methods of
investigation — analyses of microfossils and DNA-containing organeles,
revealing similarities between their nucleotide sequences. making
analogies with contemporary independently existing protocists, studying the
genetic autonomy of individual organelles, etc. Most results obtained up to
now evidence in favour of endosymbiotic theory, but they still cannot prove
it completely.
Because of the importance of the problem, in 1980 and 1983 in
Tübingen (Germany) there were held 1st and 2nd International Symposia
on “Endosymbiosis and Cell Research”. As a result of these two forums a
new branch of biology appeared — Endocytobiology.
Having in mind contemporary rapid development of science and the
possibilities and methods of investigation in molecular biology and gene
engineering, one can expect creation of organelle cultures like the cellular
ones. Thus, there will be an answer to many questions related with the
extranuclear heredity. If the endosymbiotic theory is proven indisputably,
then according to W. Schwemmler (1977) “in the future it will play the same
role, as at its time the atomic theory for the chemistry”. Studying of the cell
will enter upon a new stage of its development. This on its part will impose
reconsidering the contemporary idea on the genome of eukaryotic cells in
the light of the obtained new data on their hereditary endowments.