W Ford Doolittle

Date:
[1980]
Reference:
PP/CRI/H/6/13/5
Catalogue details

Licence: In copyright

Credit: W Ford Doolittle. Source: Wellcome Collection.

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    cell evolution seems unlikely. These features, together with the evidence that eukaryotic cytoplasmic ribosomes contain not only distinct classes of rRNA but function differently in protein synthesis [24,25] can be most easily accommodated by the hypothesis that the nuclear-cytoplasmic components of eukaryotic cells diverged from prokaryotes (or at least from eubacteria) before final solutions to the problems of effective genetic organization and expression had been found. Eukaryotic cell s th emse lves re sult from the repeated fusi ons of r epresentatives of two of the three major cellular lineages There are two general ways to explain the functional compartmentalization of nuclear, mitochondrial and (in eukaryotic algae and plants) plastid DNA in eukaryotes (Figs.3,4). Neither is new; what is new is the fact that molecular sequence data now at last allow us to choose between them. In autogenous origin hypotheses (Fig. 3 ), mitochondrial and plastid genomes derive in some fashion from nuclear (or protonuclear) genomes [17,18]. In endosymbiotic origin hypotheses, mitochondrial genomes descend from the genomes of once free-living aerobic bacteria trapped in permanent endo- symbioses within the cytoplasms of protoeukaryotic cells, while plastid genomes similarly descend from the genomes of once free-living oxygen- evolving photosynthetic prokaryotes similar to modern blue-green algae. The second hypotheses has gained recent renewed prominence through the efforts of Lynn Margulis and others [19]. Not everyone regards it as proven, but molecular sequence data leave little room for autogenous alternatives. T1 oligonucleotide catalogs show strong affinities between plastid and blue-green algal 16S rRNAs and no affinity between either of these and the nuclear-gene-coded cytoplasmic 18S rRNA present