Mitotic divisions

Kubiak In mouse, there is a weak checkpoint in meiosis for spindle formation. There are some responses where if the spindle is destroyed, the passage from metaphase I to metaphase II is blocked. There is other evidence that there is no checkpoint. I think there is a special checkpoint in meiosis. Starting from the first mitotic division there is no spindle checkpoint. 

The neural tube is a pseudostratified epithelium, with cells extending between the apical and basal surfaces of the epithelial wall. The neuroepithelium contains undifferentiated populations of stem cells and radial glia (see Ch. 29). With time, both of these cell types give rise to the three main lineages for neurons, astrocytes and oligodendrocytes. After the final mitotic division, neurons migrate away from the ventricular surface of the neural... 

The nervous system is subject to a unique set of constraints during development. Many more neurons are generated during development than are required in the mature nervous system. Importantly, after their last mitotic division neurons are unable to re-enter the cell cycle and can no longer proliferate. As development proceeds there is a period during which as many as half of all neurons die, a process known as normal or programmed cell death that occurs by apoptosis of the cell (see Ch. 35). 

Conventional cytogenic analysis is performed on a small sample of heparinized blood. Phytohemagglutinin is added to induce mitotic division. After 65 to 72 hours in culture, colchicine is added, which by preventing spindle... 

Chromosomal aberrations were not induced by di(2-ethylhexyl) phthalate in any of eight studies in various types of cultured cells in the absence of metabolic activation. Only three of these studies for chromosomal aberrations included an exogenous metabolic activation system. Of these, one, using Syrian hamster embryo cells, found an increase in aberration frequency. Weak effects were detected for the induction of aneuploidy and mitotic division aberrations in Chinese hamster lung cells. 

Aberrations in chromosomes or chromatids, which are sometimes microscopically visible, may arise during mitotic division when newly divided chromosomes fail to separate or do so incorrectly. The absence of a chromosome is usually lethal, and an excess is often poorly tolerated, giving rise to serious defects. Aberrations of the sex chromosomes are more readily tolerated, however. Chromosome aberrations may be caused by foreign compounds as indicated in the section on mutagenesis (see chap. 6). However, those cells with aberrations seem to be rapidly eliminated and so may contribute to cell death rather than a heritable mutation. 

The embryo develops from a zygote formed by fusion of a sperm nucleus originating from the pollen and an egg cell. The fertilized egg is surrounded in the gynmosperms by a nutritive layer or endosperm which is haploid and is derived from the same game-tophyte tissue that produced the egg. In angiosperms two sperm nuclei form one of these fertilizes the egg, while the other fuses with two haploid polar nuclei derived from the female gametophyte. (The polar nuclei are formed by the same mitotic divisions that formed the egg.) From this develops a 3n triploid endosperm. 

Soil-borne bacteria of the family Rhizobiaceae and leguminous plants form a symbiotic relationship during which a new organ, the root nodule, is developed. Within these root nodules the bacteria fix atmospheric dinitrogen and the product of nitrogen fixation, ammonia, is exported to the plant [69,70]. Root nodules develop from primordia which are established at specific sites in the root cortex shortly after Rhizobium infection. The peptide enod40 is believed to play a critical role in inducing the de-differentiation and the mitotic division of root cortical cells, i.e. the initial steps in nodule development. This however, is not entirely undisputed [3,4,69-72]. 

The overall life cycle of a particular yeast Saccharomyces (S.) cerevisiae, is summarised in Fig. S.6 which shows how it is possible for the cells to fuse to form various cell and spore types. The figure shows the possible types of reproduction in yeast. Generally, industrial strains of S. cerevisiae, brewers yeast, reproduce by budding/ fission processes and only sporulate under specialised conditions. However, many strains of yeast are capable of cell fusion to form spores or cells with increased genetic complements. Such strains have many sets of chromosomes and are termed polyploid. Active fermentation of industrial strains involves growth by mitotic division and nutrient depletion which results in stationary cells with little or no spore formation. 

The existence of different morphotypes, two ploidy levels related to phase changes, and the ability of both haploid and diploid stages to divide mitotically (Kornmann 1955 Rousseau et al. 1994 Vaulot et al. 1994), support the existence of a haploid-diploid life cycle in P. globosa. In such life cycles, both haploid and diploid stages are related by sexual processes, meiosis and syngamy, and both are capable of mitotic division (Fig. 3 ... 

DNA synthesis commences just 10 to 12 hours after partial liver resection, and after another 10 to 20 hours, mitotic division reaches maximum activity. Replication in the endothelial cells and bile duct epithelia follows within 1 to 3 days. Consequently, 3 or 4 weeks subse-... 

The sequence of events that occurs in a cell from one mitotic division to the... 

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