Spindle fibers

The mode of action has not yet been elucidated but the manufacturer states that it probably behaves like the herbicide triflurolin and its congeners. These materials inhibit cell division by binding to tubuHn thereby internipting micro-tubule development. This, in turn, stops spindle fiber formation essential to mitosis and cell division. Experiments with C-labeled Prime+ show that it is acutely toxic to fish with estimated LC q (96 h) of less than 100 ppb for rainbow trout and bluegiU. sunfish. However, channel catfish did not exhibit any toxic response at the maximum attainable water concentration (10). 

In addition to effects on biochemical reactions, the inhibitors may influence the permeability of the various cellular membranes and through physical and chemical effects may alter the structure of other subcellular structures such as proteins, nucleic acid, and spindle fibers. Unfortunately, few definite examples can be listed. The action of colchicine and podophyllin in interfering with cell division is well known. The effect of various lactones (coumarin, parasorbic acid, and protoanemonin) on mitotic activity was discussed above. Disturbances to cytoplasmic and vacuolar structure, and the morphology of mitochondria imposed by protoanemonin, were also mentioned. Interference with protein configuration and loss of biological activity was attributed to incorporation of azetidine-2-carboxylic acid into mung bean protein in place of proline. 

Colchicine is a poisonous tricyclic tropane alkaloid from the autumn crocus (Colchicum autumnale) and gloriosa lily (Gloriosa superba). This alkaloid is a potent spindle fiber poison, preventing tubulin polymerization.25 Colchicine has been used as an effective anti-inflammatory drug in the treatment of gout and chronic myelocytic leukemia, but therapeutic effects are attainable at toxic or near toxic dosages. For this reason, colchicine and its analogs are primarily used as biochemical tools in the mechanistic study of new mitotic inhibitors. 

When microtubules were visualized by electron microscopy (EM), after the improvement of methods of fixation, it was realized that they formed the structural basis of flagellar axonemes and of so-called spindle fibers, as well as occurring as individual filaments in the cytoplasm. Their designation as part of the cytoskeleton suggested that they acted mainly as fixed structural supports. Subsequent research has focused more and more on their dynamic behavior and on their role as tracks for motor proteins, which may, for example, transport chromosomes during cell division. Microtubules are found in all eukaryotic cells and are essential for many cellular functions, such as motility, morphogenesis, intracellular transport, and cell division. It is that dynamic behavior that allows microtubules to fulfill all of these functions in specific places and at appropriate times in the cell cycle. 

Micronucleus Test The micronucleus test is an in vivo test usually carried out in mice. The animals are treated in vivo, and the erythrocyte stem cells from the bone marrow are stained and examined for micronuclei. Micronuclei represent chromosome fragments or chromosomes left behind at anaphase. It is basically a test for compounds that cause chromosome breaks (clastogenic agents) and compounds that interfere with normal mitotic cell division, including compounds that affect spindle fiber function. 

In the fine structure of cells, microtubules make up fibers such as the spindle fibers that attach to centromeres of chromosomes to pull chromatids apart during mitosis and meiosis. Microtubules function in a number of cellular processes, including motility of cells and subcellular components. Microtubules assemble into tubulin, a substance that can change the shape of cells. 

CENTROMERE A specialized part of a chromosome that attaches to a spindle fiber in mitosis or meiosis. 

G2 ends as the first of the five stages of cell division, prophase, begins. Prophase is defined as the first appearance of condensed chromosomes. This is followed by metaphase, in which spindle fibers appear leading from centromeres to opposite sides, or poles, of the cells. The sister chromosomes (i.e., the pairs formed by replication of single chromosomes in S) become aligned in one plane... 

Figure 1 Overview of mitosis, (a) (i) Chromosomes are replicated before mitosis, and sister chromosomes are held together, (ii) The spindle forms and chromosomes attach to spindle fibers, (iii) Chromosomes move to the center of the spindle at metaphase, (iv) Sister chromosomes separate at anaphase and move in opposite directions, (v) The cell divides as the cleavage furrow forms between the separated chromosomes, (vi) Two daughter cells form, each with exactly one copy of each chromosome, (b) Stmcture of colchicine, a small molecule that targets microtubules.

Inone S. Polarization optical stndies of the mitotic spindle. I. The demonstration of spindle fibers in living cells. Chromosoma 1953 5 487-500. 

Inone S, Sato H. CeU motility by labile association of molecnles. The natnre of mitotic spindle fibers and their role in chromosome movement. J. Gen. Physiol. 1967 50 259-292. 

Satellite DNA is thought to play a structural role in chromosomes. Certain satellite DNA sequences are concentrated near the centromeres of chromosomes, the site where spindle fibers attach when sister chromatids are separated. Other satellite DNA sequences are located in... 

Howell BJ, Hoffman DB, Fang G, Murray AW, Salmon ED (2000) Visualization of Mad2 dynamics at kinetochores, along spindle fibers, and at spindle poles in living cells. J Cell Biol 150 1233-1250... 

In higher eukaryotes, a complex protein structure called the kinetochore assembles at centromeres and associates with multiple mitotic spindle fibers during mitosis. Homologs of most of the centromeric proteins found in the yeasts occur in humans and other higher eukaryotes and are thought to be components of kinetochores. The role of the centromere and proteins that bind to it in the segregation of sister chromatids during mitosis is described in Chapters 20 and 21. 

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