Mitogens

Mitotic poisons, also called mitotic inhibitors, are chemical substances that act as inhibitors of mitotic cell division, doing the opposite job of mitogens. A well-known example, referenced elsewhere in this chapter's practice questions, is colchicine, a plant-derived alkaloid that binds to tubulin and prevents it from polymerising properly into functional spindle microtubules; because the spindle cannot form correctly, cells treated with colchicine become arrested specifically at metaphase, since the spindle assembly checkpoint will not permit the cell to proceed to anaphase without a properly attached spindle. This metaphase-arresting property is exploited in two very different contexts: in genetics and cytology laboratories, colchicine treatment is used deliberately to accumulate large numbers of metaphase cells (whose chromosomes are maximally condensed and easiest to study) for karyotyping, and, at a higher dose, to induce chromosome doubling (polyploidy) by preventing the cell from completing division at all. In medicin …

Endomitosis is the replication of chromosomes in the complete absence of nuclear division and cytoplasmic division, so that numerous replicated copies of the genome accumulate within a single, undivided cell rather than being distributed into separate daughter cells. In endomitosis, the individual chromatin threads (chromonemata) that would normally separate to form distinct daughter chromosomes fail to separate and instead remain closely associated side by side with each other; because the nuclear membrane never ruptures during this process, no spindle ever forms, unlike in ordinary mitosis. Endomitosis is best known from the salivary gland cells of Drosophila and other flies, where repeated rounds of endomitotic replication without division produce giant chromosomes displaying a condition called polyteny: each of these polytene chromosomes consists of many thousands of intimately associated, tightly synapsed chromatids bundled together as one visibly enormous structure, easily seen under a light microscope. Polytene chromosomes are of particular value to gen …

Two contrasting patterns of spindle organisation are recognised based on whether asters (radiating star-shaped arrays of microtubules at the spindle poles) form during division. Anastral spindles are found only in plant cells: no asters and no centrioles are formed, and only the spindle fibres themselves are present, running between the two poles without any surrounding aster of microtubules radiating outward - consistent with the fact, noted earlier in this chapter, that plant cells generally lack centrioles altogether. Amphiastral spindles, by contrast, are the pattern found in animal cells: here, a full aster of radiating microtubules, together with a pair of centrioles, is present at each of the two spindle poles, giving the spindle a distinctive dumbbell-like appearance with a star at either end. This anastral-versus-amphiastral distinction is really just a more formal restatement, using the aster/centriole terminology, of the plant-versus-animal mitosis …