Cell Division: Mitosis
Threadlike structures in dividing cells were first observed by Walter Hemming in 1879. He intrQduced the term mitosis for cell division. Hemming also observed the longitudinal division of chromosomes during mitosis. In 1884, Strasburger coined the terms prophase, metaphase, and anaphase for the different stages of cell division. A mitosis results in two genetically identical daughter cells.
A. Mitosis
During the transition from interphase to mitosis, the chromosomes become visible as elongated threads (prophase). In early prophase. each chromosome is attached to a specific site on the nuclear membrane and appears as a double structure tsister chromatids), the result
•of the foregoing DNA synthesis. The chromosomes contract during late prophase to become thicker and shorter (chromosomal condensation). In late prophase, the nuclear membrane disappears and metaphase begins. At this point, the mitotic spindle becomes visible as thin threads. It begins at two polelike structures (centrioles). The chromosomes become arranged on the equatorial plate, but homologous chromosomes do not pair. In late metaphase during the transition into anaphase, the chromosomes divide also at the centromere region. The two chromatids of each chromosome migrate to opposite poles, and telophase begins with the formation of a nuclear membraile. Finally the cytoplasm also divides (cytokinesis). In early interphase the individual chromosomal structures become invisible. Interphase chromosomes are called chromatin (Flemming 1879), i.e., nuclear structures stainable by basic dyes.
B. Metaphase chromosomes Waldeyer (1888) coined the term chromosome for the stainable threadlike structures visible during mitosis. A metaphase chromosome consists of two chromatids (sister chromatids) and the centromere, which holds them together. The centromere may divide each of the chroFnatids into two chromosome arms. The regions at both ends of the chromosome are the telomez res. The point of attachment to the mitotic spindle fibers is the kinetochore. During metaphase and prometaphase, chromosomes can be visualized under the light microscope as discrete elongated structures, 3—7 itm long (see p. 182).
C. Cell cycle (mitosis) Eukaryotic cells go through cell division cycles (cell cycle). In eukaryotic cells, each cell division begins with a phase of DNA synthesis, which lasts about 8 hours (S phase). This is followed by a phase of about 4 hours (G2). During the C2 phase (gap 2), the whole genome is double. Mitosis (M) in eukaryotic cells lasts about an hour (see A). This is followed by a phase, C1 (inter- phase), of extremely varied duration. It corresponds to the normal functional phase. Cells that no longer divide are in the C phase. Every cell must have a “memory” to tell it whether it is in C1 or in C2. since division of the chromosomes before they have been doubled would be lethal. All phases of the cell cycle are regulated by specific proteins encoded by numerous cell division cycle (cdc) genes. In particular, the transition from G1 to Sand from C2 to Mis regulated by specific cell-cycle proteins (see preceding plate).
References
HoIm, C.: Coming undone. How to untangle a chromosome. Cell 77:956—957, 1994.
Koshland, D.: Mitosis. Back to the basics. Cell 77:951 —954,1994.
North, C.: Regulating the cell cycle. Nature 339:97—98,1989.
Nurse, R: The incredible life and times of biological cells. Science 289:1711 —1716, 2000.
Sharp, D.J., Rogers, CC., Scholey, j.H.: Micro- tubule motors in mitosis. Nature 407:41 — 47, 2000.
Whitehouse, L.H.lC: Towards the Understanding of the Mechanism of Heredity. 3rd ed. Edward Arnold, London, 1973. Passarge, Color Atlas of Genetics © 2001 Thieme All rights reserved, Usage subject to terms and conditions of license.
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