By Dr. CHIU Chi Ming Instructor, Department of Biology, The Chinese University of Hong Kong Cell cycle refers to the series of events that a eukaryotic cell has to proceed orderly before it can divide into two daughter cells. The cell cycle is made up of different phases, namely G1, S, G2 and M. During each of these phases, the cell has specific duties to complete; for example, it has to duplicate its DNA during the S phase so that the genetic material can be divided equally into its daughter cells in the M phase, when mitosis and cytokinesis occur. There are a number of checkpoints in the cell cycle, so that the cell has to complete all its duties in a particular phase before it can move on to another. It would be a disaster if the cell has not completely duplicated its DNA before it starts mitosis. Progression of cell cycle and passage through different checkpoints are genetically regulated. The mitosis at M phase is further subdivided into prophase, metaphase, anaphase and telophase. The duplicated DNA, now in the form of sister chromatids, have to be aligned properly at the equatorial plate and are attached to the opposite poles of the mitotic spindle apparatus via microtubules. It is now know that there is a protein complex, called kinetochore, at the centromere of the chromatids. One of its proteins, Mad2, will inhibit degradation of the “protein glues” that stick the arms of the sister chromatids and prevents the onset of anaphase. However, when all the sister chromatids are attached properly to the poles, Mad2 will disappear at the kinetochore so that the “protein glues” will be degraded, and anaphase begins. Therefore, the disappearance of Mad2 is a distinct signal, just likes traffic light turning from yellow to green, indicating that all the chromatids have been properly attached to the poles so that anaphase can begin. This molecular mechanism ensures that each of the daughter cells will receive a complete set of chromosomes after the cell division.
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