Figure 3

Evolution of eukaryotes from a posibacterium, emphasizing changes in DNA segregation caused by internalization of DNA-membrane attachments. (a). An FtsZ ring between daughter DNA termini (T) divides bacteria; cortical skeletal MreB (blue) and rigid murein wall (brown) control cell shape. (b). Disruptive effects of phagotrophy. Left: flexible glycoproteins (yellow) replaced murein, allowing MreB to become actin (blue) and power phagocytosis, which internalised DNA-membrane attachments (centre); evolution of COP-coated vesicle budding, and fusion with plasma membrane after uncoating, made permanent endomembranes (EM: precursor of ER, NE, Golgi, lysosomes; peroxisomes (P) separated earlier) and disrupted bacterial DNA segregation. (c). Hypothetical origin of simple mitosis in a prekaryote where FtsZ gene duplications evolved stable microtubules and γ-tubulin-containing centromeres still attached to the surface membrane. (d). Accidents in centrosome duplication and phagotrophic membrane internalisation generated a more complex prekaryote II in which stable endomembranes differentiated into peroxisomes (P) and protoendomembranes (EM, i.e. the ancestors of ER and Golgi; see [27]), some associated with the internalised centrosome and DNA; another centrosome remained at the cell surface stabilising it; the actin ring controlled the site of cytokinesis. Ultimately the surface centrosome generated pellicular microtubules and centriolar and ciliary microtubules of the cenancestral eukaryote; the ER-associated MNC nucleated its intranuclear spindle. (e). The first eukaryote. (f). Adding NPCs, mitochondria, and cilium, and nuclear chromosome linearization and kinetochore evolution, made the cenancestral eukaryote, shown in G1 of the cell cycle; bacterial ingestion was via a specialised microtubule-supported pocket-like cytostome (CY) at the apical ciliary end, making the cell asymmetric.