Publisher Summary Mitosis is defined as all those types of nuclear division that produce two, or rarely more, daughter nuclei, each containing a chromosome complement approximately similar to that of the original nucleus. The greatest range of variations by which mitosis is accomplished, occurs in the protistan and fungal kingdoms, some members of which are probably most similar to the ancestors of higher plants and animals. The variations in the higher organisms are secondarily derived from the division patterns of typical plants and animals. This chapter discusses the characteristics and evolution of mitosis. The efficiency of mitosis consists of two basic components; the frequency with which each daughter nucleus receives the necessary complete genome complement (genetic efficiency) and the amount of energy and materials expended in the synthesis and operation of the mitotic apparatus. The chapter also discusses the use of mitosis as a phylogenetic marker. It is applicable to all eukaryotic cells and thus is valuable across boundaries where other structures are absent on one side and present in various forms on the other side.

Variant mitoses in lower eukaryotes: indicators of the evolution of mitosis.

Cytoplasmic microtubules and their functions

I. I n t r o d u c t o r y Review of Some Recen t Comparat ive Cytological Studies in the Green Algae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 II. A Classification of the Green Algae Based on Cytological Characterist ics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 III. Problems and Impl ica t ions of a Cytological Classification . . . . . . . . . 119 IV. The Origin of Land Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 V. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 VI. Zusammenfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 VII. References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

Comparative cytology, evolution and classification of the green algae with some consideration of the origin of other organisms with chlorophylls A and B

Publisher Summary This chapter discusses the evolution of the mitotic spindle. The recounting of the evolutionary past of the mitotic processes of chromosome movement would entail tracing a gradual shift from membranes to micros as the primary producer of forces for chromosome movement. In addition, it was suggested, microtubules themselves might in the course of evolution have acquired the capacity for force production as an adjunct to preexisting static properties. That at least remnants of some of the intermediate stages in such transitions probably survive in unusual forms of nuclear division should be apparent from the foregoing account. Thus it is find among these not only organisms such as C. cohnii in which membrane phenomena may exert the sole direct force for chromosome movement, but also forms such as hypermastigote flagellates in which membranes and microtubules may both take a direct role in chromosome movement. There are certainly indications that microtubules can assume roles in nuclear division quite unrelated to force production, as in C . cohnii in which a cytoskeletal role for the exclusively extranuclear microtubules seems most likely, and in Syndinium sp . in which chromosomal microtubules effect a permanent connection of kinetochores and poles but seem not to function actively in chromosome movement. Such signs that possible intermediates in the evolution of the mitotic apparatus still exist among living organisms, rather than having been casualties of the selective process, gives idea that some of the unusual patterns of nuclear division of other lower organisms such as euglenoids, trypanosomes, ciliates, fungi, and sporozoa are of evolutionary significance.

The evolution of the mitotic spindle.

Publisher Summary Membranes are universal components of the mitotic (or meiotic) apparatus (MA). The nuclear envelope (NE) is present in cells of all eukaryotic organisms from the most primitive to the most advanced and it becomes closely associated with the spindle fibers during their formation and/or function. In addition to the NE, there are other membranes—the endoplasmic reticulum (ER) and the Golgi apparatus that occur in the MA of many different cells. During the process of mitosis, these membranes undergo transformations and changes in disposition that correlate with the formation and function of the microtubular spindle fibers. During prophase, the nucleus, although surrounded by an envelope, is devoid of membranes. However, upon breakage and dispersal of the NE, in cells of higher eukaryotes, membranes enter the nuclear region, and some elements accumulate at the spindle perimeter and create a boundary between the MA and the rest of the cytoplasm. This chapter highlights the structural contribution of membranes to the MA and discusses the variability in this morphology that exists in different cell types.

Membranes in the Mitotic Apparatus: Their Structure and Function

Nuclear division in O. cardiacum is described. Before division, the nucleus enlarges considerably. At prophase, the nucleolus starts dispersing and kinetochores appear on the condensing chromatin, situated and oriented apparently at random in the nucleus. By prometaphase, the kinetochore pairs become aligned along the spindle axis before moving into the metaphase-plate configuration; this supports an earlier theory explaining metakinesis. During prophase and metaphase particularly, the nuclear envelope at the poles forms channels that extend for some distance into the cytoplasm; these may also bifurcate. The nucleolus disperses but remains in the intranuclear spindle throughout division as a loosely knit skein of granular material. The kinetochores have a complex structure, up to seven distinct layers being detectable; the kinetochore pairs split, and then migrate polewards at anaphase with the rest of the chromosome trailing behind. Large numbers of microtubules run from the kinetochore into evaginations of the nuclear envelope which increase in size during anaphase. The spindle grows in length considerably during anaphase, this coinciding with a proliferation of interzonal microtubules, first seen amongst the trailing chromosome arms. The nuclear envelope enclosing the spindle becomes severely stretched at this stage; it contracts closely around each of the daughter nuclei, isolating them from the rest of the spindle (including microtubules and the remains of the nucleolus). The spindle then collapses; the nuclei come together and then flatten against one another; between them, vesicles and other components of the septum collect amongst a large number of transversely oriented micro tubules.

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JD Pickett-Heaps

Papers

Cell Division in Oedogonium II. Nuclear Division in O. Cardiacum