Mitosis Interphase

The influences of herbicides on cell division fall into two classes, ie, dismption of the mitotic sequence and inhibition of mitotic entry from interphase (G, S, G2). If ceU-cycle analyses indicate increases in abnormal mitotic figures, combined with decreases in one or more of the normal mitotic stages, the effect is upon mitosis. Mitotic effects usually involve the microtubules of the spindle apparatus in the form of spindle depolymerization, blocked tubulin synthesis, or inhibited microtubule polymerization (163). Alkaloids such as colchicine [64-86-8J,viahla.stiae [865-21-4] and vincristine [57-22-7] dismpt microtubule function (164). Colchicine prevents microtubule formation and promotes disassembly of those already present. Vinblastine and vincristine also bind to free tubulin molecules, precipitating crystalline tubulin in the cytoplasm. The capacities of these dmgs to interfere with mitotic spindles, blocking cell division, makes them useful in cancer treatment. 

Protoanemonin, which has been isolated from Anemone pulsatilla and Ranunculus spp., was reported to inhibit root growth by slowing down metabolism and blocking mitosis 35). Erickson and Rosen 35) observed cytological effects in corn root tips at concentrations of 10M and lower. Cells undergoing division appeared to accumulate in the interphase or prophase stages. Metaphase, anaphase, and telophase stages were not observed. Cytoplasmic and vacuolar structures were disturbed and the presence of mitochondria could not be demonstrated in treated tissue. Thimann and Bonner 141) reported that protoanemonin was 10 to 30 times more inhibitory than coumarin in coleoptile and split pea stem tests, and that BAL prevented the inhibitory action. 

When cells enter mitosis, the interphase array of MTs is dismantled. The centrosome duplicates, and the daughters move to opposite poles of the nucleus. After disassembly of the nuclear envelope, MTs emanat-... 

Hunt I don t think she would claim what Jacek Kubiak is claiming, that the first mitosis is a particularly long one. It is a prolongation of the interphase, not the... 

FIG. 1. Localization of key proteins involved in the neuroblast asymmetric cell division. During late interphase a complex of proteins including Insc, Baz (and Pins) are localized to the apical cell cortex. This complex acts to mediate the basal cortical localization of the cell fate determinants Numb (and its partner Pon), Pros (and its partner Miranda) and pros RNA (and its partner Staufen) during mitosis. During interphase, Numb is cytoplasmic and Pros is localized to the apical cortex. 

FIG. 2. The formation of the apical protein complex involves two distinct steps. Bazooka is localized apically in the epithelium from which neuroblasts are derived. In the interphase (G2), delaminating neuroblast formation of the apical complex is initiated. It is thought that Baz acts to allow neuroblasts to retain the apical/basal polarity inherent in the epithelium. Baz recruits Insc to the neuroblast apical stalk during delamination before Pins becomes part of the complex. During this initiation step Baz, Insc and Pins are part of a linear hierarchy. However following delamination and during mitosis, the maintenance of the apical localization of each of these proteins requires all three proteins. 

Epithelial cells express but do not apically localize Pins, and do not express Insc. We have previously shown that ectopically expressed Insc localizes to the apical cortex in wild-type epithelial cells (Kraut et al 1996). Interestingly, ectopic Insc expression causes Pins, which is normally localized to the lateral cortex, to localize to the apical cortex. Conversely, apical localization of ectopically expressed Insc is dependent on pins. Insc ectopically expressed in Pins- epithelial cells does not localize as an apical crescent it adopts a cytoplasmic distribution which is enriched towards the apical side of the cell during interphase and is undetectable during mitosis, presumably due to rapid degradation. This instability of ectopically expressed Insc may be why the 90° rotation in the mitotic spindles which occurs as a consequence of Insc ectopic expression in the wild-type epithelial cells no longer occurs when Insc is expressed in Pins-embryos. These results indicate that Insc is necessary and sufficient for the recruitment of Pins to the apical cortex of wild-type epithelial cells. 

Cdc2 kinase activity is required during mitosis but not during interphase for apical Insc localisation... 

Newport-. The way that spindles and microtubules are normally reoriented is by the stabilization of dynamic instability at the plus-end of the microtubule. So if microtubules were to embed in this apically localized complex, they would effectively be capped and this would reorient the spindle. One would expect that this would happen to the centriole prior to mitosis, so that the interphase microtubules would be stabilized at that location as well. Are you saying this doesn t happen If it doesn t happen, perhaps Cdc2 is necessary to activate this apical region for stabilizing plus ends, and this would explain why it rocks about. Are any of these molecules potential candidates for capping microtubules at the plus-end, for instance ... 

From this stage onward, interphase and mitosis (or division) alternate until the final organization of the agent is reached. Again, interphase is the phase in which each and every cell (1) computes its neighborhood vector, (2) allows its RBN to settle into a fixed state, and (3) decides whether to divide or not based on the state of bit 4 of its (settled) state vector. One more cycle of development will be described in detail. All following cycles, which are too many to be described here in detail, follow the same pattern. 

Figure 1. Hierarchical model of chromosome structure, (a) In interphase cells, DNA is packed in a nucleus as forming nucleosome and chromatin, (b) DNA forms nucleosome structure together with core histone octamer, which is then folded up into 30nm fiber with a help of linker histone HI. This 30 nm fiber is further folded into 80 nm fiber and 300 nm loop structures in a nucleus. In mitosis, chromosome is highly condensed. Proteins which are involved in each folding step are indicated above and non-protein factors are indicated below, (c) The amino acid sequences of histone tails (H2A, H2B, H3 and H4) are shown to indicate acetylation, methylation and phosphorylation sites. (See Colour Plate 1.)...

During the cell cycle, chromosome structures shuttle between de-condensed interphase and condensed mitosis states. Dynamic changes also occur at the lower levels of architectures, i.e., at the chromatin and nucleosome levels. Upon gene activation and inactivation, folding and unfolding of the nucleosome structure and the chromatin fibers occur at limited loci of the genome. Namely, the structures of the chromosome are dynamic and mobile. Nevertheless, there are basic structural units that remain stable and constitute the fundamental chromosome architecture. 

The concept of the cell cycle (Figure 1-1-2) can be used to describe the timing of some of these events in a eukaryotic cell. The M phase (mitosis) is the time in which the cell divides to form two daughter cells. Interphase is the term used to describe the time between two cell divisions or mitoses. Gene expression occurs throughout all stages of interphase. Interphase is subdivided as follows ... 

During subsequent interphase, the DNA replicates to create identical chromatids tor each chromosome in preparation fora subsequent mitosis in the cell cycle... 

Figure 20.28 Diagrammatic representation of mitosis in a cell with a single pair of homologous chromosomes. In prophase, the chromatin condenses into chromosomes, each of which consists of a pair of chromatids that have been formed by replication during interphase, and the nuclear envelope disappears. In metaphase, each chromatid attaches to the spindle fibres (microtubules) at a centre point, the centromere. In anaphase, the two chromatids of each chromosome become detached from each other and move to opposite poles of the cell along the microtubules. In telophase, the chromatids have reached the poles. Two nuclear envelopes then form and enclose each new set of chromatids, now once again called chromosomes. The microtubules disappear and the chromosomes uncoil and re-form into the long chromatin threads. Finally the cell membrane is drawn inward by a band of microfilaments to form a complete constriction between the newly formed nuclei, and two new cells are formed. The process is called cytokinesis.

When mitosis has been completed, cyclin B is marked with ubiquitin and broken down proteolytically by proteasomes (see p. 176). Protein phosphatases then regain control and dephosphorylate the proteins involved in mitosis. This returns the cell to the interphase. 

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