Biology Reference
In-Depth Information
used to describe a type of organelle found only in plants; the best-studied type of
plastid is the chloroplast, the green organelle that carries out photosynthesis.
Mitochondria originated from a group of bacteria called the
-proteobacteria,
while plastids originated from cyanobacteria, so-called because of their photosyn-
thetic pigments. Thus the appearance of the vital processes of aerobic respiration
and photosynthesis in eukaryotes is due to endosymbiosis and massive lateral gene
transfer from the ingested bacteria to the nucleus of the host cell. But the phe-
nomenon of endosymbiosis also includes many other cases of bacteria that have
taken up residence inside eukaryotic cells, where they both enjoy and confer some
metabolic benefit, but have not evolved into either mitochondria or plastids. For
instance many insects that live on plants, such as aphids, contain bacteria in their
cells. These bacteria provide the insects with certain compounds that are lacking in
the sap that these insects extract from the plants. There are even cases of endosym-
bioses where eukaryotic cells have taken up residence inside other eukaryotic
cells.
The evidence for endosymbiosis is summarized in Fig. 4.10. A German botanist
called Schimper suggested in a footnote of a paper published in 1883 that maybe
chloroplasts originated from cyanobacteria because they looked similar, and more-
over he could see chloroplasts dividing inside plant cells, as though they were cells
themselves. This idea was developed much further in 1905 by the Russian biologist
Mereschkowsky, who stressed not only that chloroplasts were seen to divide, but
that they continuing dividing and carrying out photosynthesis in parts of plant cells
from which the nucleus had been removed by dissection. This latter observation
implied that chloroplasts were independent of the nucleus to some extent, as might
be expected if they originated from free-living cyanobacteria. Today we know that
chloroplasts have only a very limited independence from the nucleus because most
of the genes for the three thousand or so proteins that make up chloroplasts are
encoded in the nucleus. The chloroplast does contain about one hundred genes, the
exact number depending on species, and it does make some of its own proteins using
its own ribosomes, but the vast majority of chloroplast proteins are made by cytoso-
lic ribosomes and then transported across the chloroplast envelope. So chloroplasts
do not divide and photosynthesize when isolated from plant cells for more than a few
hours. It follows that during evolution there has been a massive lateral transfer of
genes for chloroplasts and mitochondrial proteins from the original endosymbionts
to the nucleus. An American scientist called Ivan Wallin extended the endosymbiont
idea to the origin of mitochondria in 1923.
Like many scientific ideas that later turn out to be correct, this suggestion by
Schimper, Mereschkowsky and Wallin was not accepted for a long time. Most biolo-
gists found it hard to swallow the idea the chloroplasts and mitochondria could have
descended from free-living cells that had somehow ended up inside larger cells. It
was research in the 1960s showing that both mitochondria and chloroplasts contain
their own complete genetic systems, separate from that in the nucleus and cytosol,
that revived the idea. A genetic system is defined as one that contains both DNA
with protein-encoding genes, the enzymes to transcribe these genes into messenger
RNAs, and the ribosomal translation apparatus that uses these messenger RNAs to
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