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GENETIC SYSTEMS IN THE CYTOPLASM
Dr. Hans Ris
Univo of Wisconsin
Madison, Wisc.
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GENETIC SYSTEMS IN THE CYTOPLASM
It has been known for many years that in addition to the chromosomal
genes many organisms possess genetic factors which are located in the cytoplasm
and do not show Mendelian inheritance (chloroplasts, mitochondria, kappa end
related particles of Paramecium, etc.).
Recent work on the ultrastructure and
chemistry of cell orgauelles has provided new insight into this problem and
suggests an interesting hypothesis on the possible origin of such cytoplasmic
genetic systems.
On the basis of complexity of nuclear organization, one can
distinguish two major cell types:
the prokaryote as in bacteria and blue-green
algae and the eukaryote as in plants and animals.
These two cell types also
show characteristic differences in the organization of the cytoplasm.
In the
eukaryote cell, the cytoplasm contains specialized organelles such as
mitochondria and plastids which carry out specific functions for the cell,
These orgsmelles have a certain autonomy since their presence is apparen$1y
required for the formation of new organelles.
It has been shown that a cell
which has lost its plastids cannot make them de novo and remains plastid-free.
Lederberg has called such cell components "self-dependent".
The recent
developments in chemical genetics stimulated the analysis of these cell organelles
for the presence of nucleic acids, especially DNA.
In a variety of species, it
has now been demonstrated that both pastids and mitochondria contain DNA.
This
cytoplasmic DNA usually has a base composition different from that of nuclear DNA,
and therefore the two types of DNA can be separated on a cesium chloride density
gradient.
j_
What is the role of this DNA?
properties of plastids and mitochondria?
Is it connected with the genetic
These questions cannot be answered
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Autoradiographic studies have shown that _A
synthesized in cell organelles.
is
Both plastids and mitochondria contain
ribosomes which synthesize proteins.
In plastids, DNA-dependent RNA synthesis
has been demonstrated and such RNA stimulates protein synthesis when added to
plastid ribosomes in vitro.
Such studies which are now vigorously pursued in
many laboratories indicate that these cytoplasmic organelles of eukaryote cells
have all the basic properties of living systems, namely DNA, _NA-dependent RNA
synthesis and a ribosome system for protein synthesis.
Electron microscope
studies of the ultrasturcture of plastids and mitochondria have demonstrated
amazing structural similarity be tween chloroplasts and cells of free-living
blue-green algae, and between mitochondria and certain bacteria.
In other
words, cytoplasmic organelles of eukaryote cells resemble certain free-living
prokaryote cells.
In plastids and mitochondria, the DNA is present as thin
strands about 25A thick and even after fixation all structure is removed with
deoxyribonuclease.
_he DNA does not seem to be associated with protein but
forms a nucleoplasm as in prokaryotes.
The photosynthetic pigments and enzyme
systems in blue-green algae and in chloroplasts are localized in similar membranes
and lamellae.
Enzymes and carrier proteins for electron transport are located
in infoldings of the inner of the two mitochondrial membranes (cristae)and
certain bacteria are associated with infoldings of the plasma membrane.
in
The
inner surface of the membrane which forms c_istae in mitochondria is characterized
by the presence of particles which have been claimed to contain part of the
electron transport system. - Similar particles have now been found to be attached
to the plasma membrane of bacteria.
>
What is the meaning of the similarities
between certain free-living prokaryotes and the organelles of eukaryote cells?
I
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have suggested that these recent findings lend support to the long-abandoned
hypothesis of Fereschkowski and of Famintzin which suggests that during the
evolution of the eukaryote cell plastids and mitochondria originated as
endosymbiotic prokaryote cells resembling present-day blue-green algae and
bacteria.
This suggestion is supported by the fact that endosymbiotic blue-green
algae and bacteria are found in some organisms.
The kappa-like particles of
Paramecium which are now considered to be bacteria-like endosymbiotes are
particularly interesting in this regard.
This hypothesis explains most elegantly
and with the smallest number of assumptions why cells should contain accessory
genetic systems, why these genetic systems resemble those of prokaryotes and
why in the eukaryote cells cellular respiration and photosynthesis are carried
out by highly-organized, self-dependent organelles which have a striking similarity
to certain primitive free-living forms.
Selected References
Abram, D.
1965.
Electron Microscope Observations on Intact Cells, Protoplasts
and the Cytoplasmic Membrane of Bacillus stearothermophilus.
J. Bact.
89:
855-873.
Brawerman, G., and Eisenstadt, J. M.
196_.
Template and Ribosomal Ribonucleic
Acids Associated with the Chloroplasts and the Cytoplasm of Euglena gracilis.
J. Mol. Biol.
10:
403-_11°
Edelman, M., Cowan, C. A., Epstein, H. T., and Schiff, J.A.
Chloroplast Development in Euglen_.
Nat. Acad. Sci. 52:
VIII.
Studies of
Chloroplast-Associated DNA.
Proc.
121_-1219.
Eisenstadt, J. M., and Brawerman, O.
1964.
The Protein-Synthesizing Systems from
the Cytoplasm and the Chloroplasts of Euglena gracilis.
392-_02.
1964.
J. Mol. Biol.
iO:
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Gibor A., and Granick, S.
Systems.
Science
Kellenberger, E.
196_.
lh5:
1962.
Plastids and Mitochondria:
890-897.
The Study of Natural and Artificial DNA-Plasma by Thin
Sections, In Interpretation of Ultra-structure.
Academic Press, New York.
and Mitochondria in Swiss Chard.
1952.
R. J. C. Harris, ed.,
pp. 233-2h9.
Kislev, N., Swift, H., and Bogorad, L.
Lederberg, J.
Inheritable
1965.
Nucleic Acids of Chloroplasts
J. Cell. Biol. 25:
327-3h4.
Cell Genetics and Hereditary Symbiosis.
Physiol. Rev°
32:
h03-430.
Luck, D. J. L.
Biol.
2h:
1965.
Formation of Mitochondria in Neurospora crassa.
h61-hTO.
Ray, D. S. and Hanawalt, P. C.
1964.
Properties of the Salellite _NA Associated
with the Chloroplasts of Euglena gracilis.
Ris, H.
1961.
1962.
J. Mol. Biol.
9:
812-824.
Ultrastructure and Molecular Organization of Genetic Systems.
Canad. J. Genst. Cytol.
Ris, H.
J. Cell
3:
95-120.
m
Ultrasturcture of Certain Self-Dependent Cytoplasmic Organelles.
Proc. Fifth Int. Congress Electron Microscopy
2:
XX-1.
Academic _ess,
New York.
Ris, H. and Plaut, W.
1962.
Ultrastructure of DNA-Containing Areas in the
Chloroplast of Chlamydomonas.
Ris, H. and Singh, R. N.
1961.
J. Biophys. Biochem. Cytol.
5
J. Cell Biol.
13:
383-391.
Electron Microscope Studies on Blue-Green Algae.
9:
w
63-80.
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_,
DR. HANS RIS - "GENETIC SYSTEMS IN THE CYTOPLASM"
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._- .
AMP HICKS:
Lush was a boy) is it possible that what are now chloroplasts were free living
organisms
_! i
RIS:
chloroplast
__:___
Going far back into evolutionary time (say about the time Dr.
and their
present
condition
arose
through
some symbiotic
This is exactly what I would conclude from the recent studies on
ultrastructu_,
the
presence
of
DNA, DNA-dependent
and protein synthesis on ribosomes inside of chloroplasts.
formed
through
division
of pre-existing
ones.
They resemble
!:il
,!'/"i_"ii_:
of their organization free-living blue-green algae.
v
RNA synthesis
Plastids are only
in every
The occurrence of
/
•
function?
endosymbiotic blue-green algae further supports this idea.
aspect