9 - Chapter 9. THE CELL CYCLE AND MITOSIS A. Introduction...

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Chapter 9. THE CELL CYCLE AND MITOSIS A. Introduction Histologists have long been aware of the presence of nuclei in cells and the appearance of dense arrays of chromosomes in a small proportion (1-5%) of cells in sections cut from fixed tissues (Figure 9-1a). These mitotic figures are the hallmark of mitosis, the common process of cell division, and their detection enabled the first classification (with regard to cellular proliferation): cells were either mitotic or, in the absence of mitotic figures, were described as being interphase . In the simplest sense, this alternation between mitosis and interphase was the first description of the cell cycle . Cells in culture also displayed mitotic figures (Figure 9-1b) and developments in microscopy provided the means of detecting an intact nucleus in many cell types in culture without the need for killing the cells. Mitosis can be inferred from the absence of an intact nucleus, the appearance in sparse cultures of pairs of smaller daughter cells and from the fact that most cells loosen their attachment to the substrate and round-up for division, then the daughter cells settle back down on the substrate, attach tenaciously and take on the characteristic, flattened morphology. Any representation of the fraction of cells in a population (tissue section, culture dish) displaying mitosis is called a mitotic index ( MI ). Figure 9-1a. Mitotic figures. Densely stained chromatin in two cells in this highly magnified light micrograph reveal nuclei undergoing karyokinesis (nuclear division). The upper cell is likely to be at the late anaphase stage as it has well separated chromosomes. Figure 9-1b. Division in cultured cells. The pair of cells near the upper left are in telophase, just completing cell division, and the large cell at lower right is at the metaphase stage. The green fluorescence is detecting a molecule normally confined to the nucleus (all other cells) except when the nuclear envelope is broken down. The 2007 version – page 56
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B. The sequential events of mitosis The individual stages of mitosis are readily classified as prophase, metaphase, anaphase, and telophase (see Figure 9-2) and are frequently subclassified into early, intermediate, and late portions thereof. This finest level of description is not critical for present purposes. The interphase cell has a distinct nucleus bounded by a nuclear membrane (or envelope), described earlier as being a specialized portion of the endoplasmic reticulum. The nuclear contents include a nucleolus (plural, nucleoli) and indistinct chromosomes dispersed in the nucleoplasm. Peripheral to the nuclear membrane, one particular organelle of significance in mitosis is the centrosome , consisting of two centrioles . The remainder of the cytoplasm was described briefly in Chapter 2 and some further detail will be mentioned in Chapter 12. Prophase is characterized by
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This note was uploaded on 03/23/2009 for the course ANSCI 1110 taught by Professor Brucecurrie during the Fall '08 term at Cornell University (Engineering School).

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9 - Chapter 9. THE CELL CYCLE AND MITOSIS A. Introduction...

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