File:Mitosis variations protists eukaryotes.jpg

English: Various forms of mitosis in protists. Figures show post-replication paired chromosomes being separated. A. Open orthomitosis: dissociation of the nuclear envelope follows kinetosome duplication that are the MTOC, separation and formation of a microtubular spindle. The nuclear envelope membranes scatter as cisternae in the cytoplasm and chromosomes attach to the spindle at their kinetochore. A metaphase with equatorial plate usually forms, although it may not be obvious when there are too many chromosomes. The mitotic spindle is bipolar with axial symmetry. Examples are found in animals, Cryptista, Haptista, Chrysophyceae, Amoebozoa, Sagenista (the MTOC forms de novo), some gregarines, and Embryophyta. B. Semi-open orthomitosis: the nuclear envelope opens (fenestrae) at two poles to allow passage of the mitotic spindle which forms after. The spindle is bipolar with axial symmetry and MTOC are in the cytoplasm outside the openings. Chromosomes attach to the spindle at their kinetochore but an equatorial plate at metaphase does not always form. Examples are found in most gregarines, Chlorophyta, Rhodophyceae, Phaeophyceae, Chytridiomycota (Fungi), and Myxogastria (Amoebozoa). C. Closed extranuclear pleuromitosis: The microtubular spindle form entirely outside of the nucleus, the MTOC is amorphous but has nearby a kinetosome derived from the mastigont. Chromosomes attach to the nuclear envelope at their kinetochore and link to microtubules from lateral half-spindles. The nuclear envelope remains throughout, and the spindle is bipolar with axial symmetry. At anaphase-telophase the spindle lowers into the centre of the nucleus producing a U-shaped nucleus. Examples are found in dinoflagellates, and the hypermastigotes and trichomonads (Metamonada). D. Variant of C found in Dinoflagellata, where the mitotic spindle penetrates through tunnels in the nucleus, with chromosomes attached to the tunnel nuclear envelope at their kinetochore. The MTOC is extranuclear and amorphous forming a bipolar spindle with axial symmetry. E. Semi-open pleuromitosis: Two independent half-spindles form that penetrate the nucleus through a pair of openings. The extra-nuclear MTOC forms the initial microtubules that attach to the chromosomes at the kinetochores. Duplication and radial separation of the MTOC along the nuclear envelope produces the two half-spindles that pull the chromosomes apart. The nuclear envelope remains intact with only two openings where the spindles penetrate the nucleus. An equatorial plate does not form at metaphase, as the MTOC pull chromosomes closer to the nuclear envelope at anaphase, and kinetochores approach and eventually adhere to the nuclear envelope for telophase. Chromosome condensation is thought to be atypical. It is the typical mitosis in Coccidia and Aconoidasida, and the Eugregarina Grebnickiella gracilis. F. Closed intranuclear mitosis: Two independent half-spindles form inside the nuclear envelope, and the amorphous MTOC are intranuclear. As in E an equatorial plate does not form at metaphase, and chromosome condensation is limited. In some variants the MTOC are on the cytoplasmic face but integrated with the nuclear envelope, and without fenestrae. These are common in Microsporidia (Fungi), Kinetoplastea (Discoba), Preaxostyla (Metamonada), Foraminifera and Radiolaria (Rhizaria), and prasinomonads (Chlorophyta). G. Closed intranuclear orthomitosis: the microtubular spindle begins as a bipolar structure with intranuclear MTOC already at opposite poles and axial symmetry. A metaphase equatorial plate is common but not always obvious. Although rare, this form is found in some gregarines, euglyphid amoebae, Trichosphaera (Ascomycota), green algae like Cladophora (Ulvophyceae). A variation of this type of mitosis is found across Ciliophora in the micronucleus (H, I). The MTOC is usually not visible until anaphase, and metaphase is often obscure and there is no obvious metaphase or equatorial plate; this is due to the small volume and the number and size of chromosomes. At telophase, a new nuclear envelope forms inside the old micronucleus around the clustered chromosomes. Citation: S. Adl (2024). Cell cycle regulation. In: Protistology (Elsevier), Chapter 17.