Evolutionary prokaryotes

In terms of evolutionary biology, the complex mitotic process of higher animals and plants has evolved through a progression of steps from simple prokaryotic fission sequences. In prokaryotic cells, the two copies of replicated chromosomes become attached to specialized regions of the cell membrane and are separated by the slow intrusion of the membrane between them. In many primitive eukaryotes, the nuclear membrane participates in a similar process and remains intact the spindle microtubules are extranuclear but may indent the nuclear membrane to form parallel channels. In yeasts and diatoms, the nuclear membrane also remains intact, an intranuclear polar spindle forms and attaches at each pole to the nuclear envelope, and a single kinetochore microtubule moves each chromosome to a pole. In the cells of higher animals and plants, the mitotic spindle starts to form outside of the nucleus, the nuclear envelope breaks down, and the spindle microtubules are captured by chromosomes (Kubai, 1975 Heath, 1980 Alberts et al., 1989). 

Figure 5.3 The deduced evolutionary tree for selected members of the transferrin superfamily, based on comparisons of structures and sequences. The tree combines the transferrins with a number of prokaryotic periplasmic transport proteins. From Bruns et al., 1997. Reproduced by permission of Nature Publishing Group.

Martin, W. and Russell, M.J. (2003). On the origin of cells a hypothesis for the evolutionary transition from abiotic chemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells. Philos. Trans. R. Soc. London, B 358, 27-85... 

In modern anaerobic prokaryotes and in fact in all the prokaryotes we know about, the metabolic paths have changed little from those described above but details have. The essential feature is the flow of C, H, N, O, P and S compounds into and out of cells in an incomplete cycle. The first big evolutionary advance must have been made by introducing coenzymes - both freely mobile, where mobile includes swinging attached arms, and at fixed sites, to aid flow. The known coenzymes are so-called because they all also assist catalysis. This cannot be an... 

Details of the evolutionary path from prokaryotes to eukaryotes cannot be deduced from the fossil record alone, but morphological and biochemical comparisons of modern organisms have suggested a sequence of events consistent with the fossil evidence. 

A multiauthor book on the citric acid cycle, including molecular genetics, regulatory mechanisms, variations on the cycle in microorganisms from unusual ecological niches, and evolution of the pathway. Especially relevant are the chapters by H. Gest (Evolutionary Roots of the Citric Acid Cycle in Prokaryotes),... 

The striking differences between eukaryotic and prokaryotic cells have led to many speculations about the evolutionary relationship of these two great classes of living organisms. A popular theory is that mitochondria, which are characteristic of most eukaryotes, arose from aerobic bacteria. After cyanobacteria had developed and oxygen... 

Fig. 2. An evolution diagram illustrating a suggestion of common ancestry of some present-day organisms. The essential features of present-day photosynthesis may have originated in the prebiotic era and is preserved in its most primitive form in (at least some) present-day phototrophs. The heterotrophs may have developed parallel with the aerobic nonphotosynthetic bacteria, some l to 1.5 x 109 years after the emergence of the cyanobacteria. The eukaryotic photosynthetic organisms developed much later, perhaps some 1.5 to 0.5 x 109 years ago. The archaebacteria are primitive organisms that seem to have no evolutionary relation with the present prokaryotes.21 Little is known about their energy metabolism. Tentatively, they are considered as a very early form of cellular life.

As we will see, the evolutionary tree is bisected into a lower prokaryotic domain and an upper eukaryotic domain. The terms prokaryote and eukaryote refer to the most basic division between cell types. The fundamental difference is that eukaryotic cells contain a membrane-bounded nucleus, whereas prokaryotes do not. The cells of prokaryotes usually lack most of the other membrane-bounded organelles as well. Plants, fungi, and animals are eukaryotes, and bacteria are prokaryotes. The biochemical functions associated with organelles are frequently present in bacteria, but they are usually located on the inner plasma membrane. 

Classical evolutionary tree. All living forms have a common origin, believed to be the ancestral prokaryote. Through a process of evolution some of these prokaryotes changed into other organisms with different characteristics. The evolutionary tree indicates the main pathways of evolution. 

Gest, H., Evolutionary roots of the citric acid cycle in prokaryotes. Biochem. Soc. Symp. 54 3-16, 1987. A fascinating recount of how the evolution of the cycle is traced by studying the way in which enzymes of the cycle are used in present day microorganisms. 

The control of mating type in yeast illustrates a specialized role of mobile or transposable genetic elements in which mobile elements hop from one site to another. Mobile genetic elements are commonly found in both prokaryotes and eukaryotes, but most of them show little site specificity. Indeed, their almost random insertion behavior has led to the proposal that their major function in most cases is to promote evolutionary change. 

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