Spindle microtubules

A signal transduction pathway required for proper chromosome alignment during mitosis. The spindle assembly checkpoint is activated during mitosis in response to the presence of chromosomes that are not attached to spindle microtubules or that are not properly aligned at the metaphase plate. The spindle checkpoint... 

During telophase, a midbody forms between the separating daughter cells (see Figure 4), and disassembly of the remaining spindle microtubules and kinetochores takes place. 

In terms of evolutionary biology, the complex mitotic process of higher animals and plants has evolved through a progression of steps from simple prokaryotic fission sequences. In prokaryotic cells, the two copies of replicated chromosomes become attached to specialized regions of the cell membrane and are separated by the slow intrusion of the membrane between them. In many primitive eukaryotes, the nuclear membrane participates in a similar process and remains intact the spindle microtubules are extranuclear but may indent the nuclear membrane to form parallel channels. In yeasts and diatoms, the nuclear membrane also remains intact, an intranuclear polar spindle forms and attaches at each pole to the nuclear envelope, and a single kinetochore microtubule moves each chromosome to a pole. In the cells of higher animals and plants, the mitotic spindle starts to form outside of the nucleus, the nuclear envelope breaks down, and the spindle microtubules are captured by chromosomes (Kubai, 1975 Heath, 1980 Alberts et al., 1989). 

Fig. 2.6 The moqjhological events of sporulation in Saccharomyces cerevisiae. (a) starved cell V, vacuole LG, lipid granule ER, endoplasmic reticulum CW, cell wall M, mitochondrion S, spindle pole SM, spindle microtubules N, nucleus NO, nucleolus, (b) Synaptonemal complex (SX) and development of polycomplex body (PB) along with division of spindle pole body in (c). (d) First meiotic division which is completed in (e). (f) Prepararation for meiosis II. (g) Enlargement of prospore wall, culminating in enclosure of separate haploid nuclei (h). (i) Spore coat (SC) materials produced and deposited, giving rise to the distinct outer spore coat (OSC) seen in the completed spores of the mature ascus (j). Reproduced from the review by Dickinson (1988) with permission from Blackwell Science Ltd.

At the cellular level, rotenone inhibits cellular respiration by blocking electron transport between flavoprotein and ubiquinone. It also inhibits spindle microtubule assembly. ... 

Mechanism of Action Acytotoxicagent that directly affects epithelial cell metabolism by arresting mitosis through binding to a protein subunit of spindle microtubules. Therapeutic Effect Removes soft genital warts. 

Destruction of cohesin allows the spindle microtubules to pull the separated chromatids to opposite poles of the cell. Failure of spindle attachment to a single kinetochore activates the SAC (spindle assembly checkpoint), which arrests cells at metaphase until corrections are effected and equal distribution of chromosomes has been ensured. A sensory mechanism initiates the wait anaphase signal from an imattached kinetochore and triggers the accu mulation of the checkpoint components that comprise the Bub (budding uninhibited by benomyl)-Mad (mitotic arrest deficient) families of proteins. 

Other chemicals, which affect tubulin polymerization or spindle microtubule stability, are podophyllotoxin and the drugs, paclitaxel, benomyl, griseofulvin, nocodazole, and colecimid. 

Cyclic uptake and release of Ca2+ from the extracellular medium occur during mitosis in Physarum pofycephalum, and correlate with specific structural and kinetic events in the mitotic nuclei.442 The membrane system in the mitotic apparatus in Haemantkus endosperm cells functions in the localized release of Ca2+, so regulating the events of mitosis.443 It is known that calcium exerts effects on the stability of spindle microtubules. An alternative view is that free magnesium concentration acts as the fundamental regulator of the cell cycle.444 Tubulin polymerization depends on the presence of magnesium and the absence of calcium, and control of the Ca2+/Mg2+ ratio is relevant to spindle assembly. 

It is clear from observing chromosome movements that cell division occurs in an ordered sequence of events (Fig. la). First chromosomes attach to spindle microtubule fibers and move to the spindle equator. Only after completion of this step do sister chromosomes separate at anaphase and move to opposite sides of the cell, followed by their division into two daughter cells. Events must occur in this order for successful chromosome segregation. If the cell enters anaphase prematurely, before chromosomes have attached properly to the spindle, the sister chromosomes will not segregate equally, which leads to aneu-ploid daughter cells. Therefore, mechanisms that determine the timing of anaphase onset are critical for the success of mitosis. 

Whereas the benomyl screens established the existence of the spindle checkpoint and identified some of the key components in checkpoint signaling, a fundamental question that remained unanswered was what exactly is monitored by the checkpoint. Two general models have been proposed. One is that the checkpoint monitors the attachment of spindle microtubules at the kinetochore, a stmcture that forms on each chromosome to mediate microtubule binding. Unattached kinetochores keep the checkpoint active and delay anaphase (27). A second model is that the checkpoint monitors force across the centromere, the region of the chromosome where kinetochores assemble... 

Figure 3 Manipulation of chromosome-microtubule attachments with small molecules, (a) In the absence of microtubule poisons, the attachment of both kinetochores to spindle microtubules creates tension across the centromere, (b) Taxol reduces tension across the centromere by inhibiting microtubule dynamics, (c) Nocodazole creates unattached kinetochores by depolymerizing microtubules.

Feedback control of anaphase onset, or mitotic checkpoint signaling, is one mechanism that contributes to ensuring accurate chromosome segregation. Delaying anaphase in response to unattached kinetochores, however, is not sufficient. Chromosomes must attach to spindle microtubules in a particular... 

All four vinca alkaloids block mitosis with metaphase arrest. Their antitumor activity is based on their high binding affinity to intracellular tubulin, which is the protein subunit of the spindle microtubules. The binding constants of vincristine, vinblastine, and vindesine for tubulin are 8, 6, and 3.3 nmol/1 respectively (9,10). The formation of complexes between the vinca alkaloids and tubulin prevents the polymerization of the tubulin subunits to microtubules, which results in depolymerization of microtubules and inhibition of microtubule assembly. Based on the fact that microtubule assemblies also play a pivotal role in the movement of neurotransmitter substances along neuronal axons, vinca alkaloids can cause neurotoxicity, particularly at higher concentrations (9,10). 

Most satellite DNA is composed of repeats of 14-500 base pairs in tandem repeats of 20-100 kb. In situ hybridization studies with metaphase chromosomes have localized these satellite DNAs to specific chromosomal regions. In most mammals, much of this satellite DNA lies near centromeres, the discrete chromosomal regions that attach to spindle microtubules during mitosis and meiosis. Satellite DNA is also located at telomeres, the ends of chromosomes, and at specific locations within chromosome arms in some organisms. These latter sequences can be useful for identifying particular chromosomes by fluorescence in situ hybridization (FISH), as Illustrated in Figure 10-5. 

Simple-sequence DNA located at centromeres may assist in attaching chromosomes to spindle microtubules during mitosis. As yet, however, there is little clear-cut experimental evidence demonstrating any function for most simple-sequence DNA, with the exception of the short repeats at the very ends of chromosomes discussed in a later section. 

Cheeseman, I. M., D. G. Drubin, and G. Barnes. 2002. Simple centromere, complex kinetochore linking spindle microtubules and centromeric DNA in budding yeast. J. Cell Biol. 157 199-203. 

Microtubule dynamics change drastically at the onset of mitosis, when long interphase microtubules disappear and are replaced by astral and spindle microtubules. These mitotic microtubules, which are nucleated from the newly duplicated centrosomes, are more numerous, shorter, and less stable than interphase microtubules. The average lifetime of a microtubule decreases from 10 minutes In Interphase cells to 60-90 seconds in the mitotic apparatus. This Increase In dynamic instability enables microtubules to assemble and disassemble quickly in mitosis. 

When the duplicated centrosomes have become aligned, formation of the spindle proceeds, driven by simultaneous events at centrosomes and chromosomes. As just discussed, the centrosome facilitates spindle formation by nucleating the assembly of the spindle microtubules. In addition, the (—) ends of microtubules are gathered and stabilized at the pole by dynein-dynactin working with the nuclear/mitotic apparatus protein. The role of dynein in spindle pole formation has been demonstrated by reconstitution studies in which bipolar spindles form in Xenopus egg extracts in the presence of centrosomes, microtubules, and sperm nuclei. The addition of antibodies against cytosolic dynein to this in vitro system releases and splays the spindle microtubules but leaves the cen-trosomal astral microtubules in position (Figure 20-35). 

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