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The somatic cell cycle, as described briefly in the previous topic, consists of a set of events responsible for duplication of the cell, so that a cell undergoing division produces two daughter cells genetically identical, not only to each other, but also to the mother cell. The success of this cell division depends on faithful alternation of chromosome replication (S phase) and chromosome segregation (mitosis), which is genetically controlled in all eukaryotic cells (see details later). The speed of the cell cycle varies enormously, but in an orderly fashion, particularly in early embryo development in eukaryotic systems. For example in Drosophila, cells occupying different positions exhibit G2 phases that differ in length from 30 minutes to 150 minutes. The cell divisions though usually symmetric, may also be asymmetric as in the production of three polar bodies and one egg from an egg mother cell through a meiotic division. In view of this variation (and even otherwise), one may ask questions such as the following : |
what are the factors which order a particular cell to undergo cell division, mitosis or meiosis ? Does the mechanism of cell division involve a mere difference between interphase and mitosis or is it much more complicated ? Are there specific genes and gene products which regulate and control orderly sequence of events in a cell division ? Significant progress has been made in recent years to answer these questions, utilizing the following two different approaches : (i) Geneticists analysed mutations that arrested the cell cycle of somatic cells (excluding embryonic cells) at specific points (particularly in yeast). This approach may suggest that the cell cycle follows a pathway, like any metabolic pathway, where each step in the pathway is dependent on the completion of the preceding step. Thus the cell cycle is described as a set of dependent reactions having checkpoints and the theory is described as domino theory. (ii) Physiologists, biochemists and embryologists, on the other hand, analysed the points of arrest of the cell cycle and used agents that restored the normal cell cycle. For this approach, embryonic cell divisions in sea urchin and frog (Xenopus) eggs were often used, since embryonic cells undergo rapid cell divisions and are, therefore, the simplest systems for a study of cell cycle. In this approach, cell cycle is described as a biochemical machine (having two states) that oscillates between interphase and mitosis. This theory is described as the clock theory. Initially these two approaches and theories seemed incompatible, but recently these could be used to present a unified picture to explain the mechanism of cell division.
A study of the mechanism of cell division is important not only for satisfying one's curiosity, but also for finding solutions to uncontrolled cell division in cancerous tissues or for a study of differentiation and pattern formation in metazoan tissues. For instance, valuable information has become available in Drosophila and Xenopus, where early embryo development has been shown to be under the control of maternal genome and only later the zygotic (or biparental) control is exercised resulting in pattern formation and differentiation. These different aspects involving a study of genetics and biochemistry of cell division will be discussed in this section.
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