Figure 17.22 The Mitotic Phase of a Cell's Life Cycle Animation Video
Our bodies make new cells by dividing, a process that occurs during the mitotic phase of a cell's life cycle. Mitosis is the division of a cell's nucleus and chromosomes into two nuclei, each with its own set of identical chromosomes. During prophase, the first step of mitosis, the chromatin, the blue and turquoise areas you see here, condenses to form chromosomes, which resemble narrow x's. At the same time that the chromosomes are forming, the cell membrane is breaking down, and the centrioles are moving to opposite sides of the cell. Also during this phase microtubules begin to grow away from the centrioles, forming spindle fibers. During metaphase, some spindle fibers "lock on" to the centromeres of the chromosomes and pull on the chromosomes, causing them to be positioned along the midline of the cell. During anaphase, the centromeres are cut in half and the sister chromatids pull themselves along a spindle toward one of the centrioles. We only see eight chromatids here, four going each way, but in a human cell 46 chromatids go to one side and 46 to another. During telophase, the chromosomes gather together at each centriole, where they "decondense". The spindles disappear and a new nuclear membrane forms. At this point one nucleus has become two and the cell's DNA has been equally divided. Cytokinesis is the division of the cytoplasm into two separate parts—the final step in cell division. A ring of acting microfilaments forms a circle around the cell and myosin proteins pull on the filaments, making them shorten. As a result of this pulling, a cleavage furrow forms around the middle of the cell. The furrow gets tighter and tighter, until the cell finally splits apart, into two daughter cells. Cell division is now complete.
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