G0 involves a cell that is quiescent and senescent. Some cells are always in the G0 phase after being formed, such as neurons, which do not divide. Fully differentiated cells also often are in the G0 phase because they no longer divide. Certain cells do not ever get into the G0 stage, such as epithelial cells, which always divide and replace themselves throughout the organism’s lifetime. Interphase is when the cell prepares for cell division. It is also referred to as the preparatory stage or intermitosis. More than 90 percent of the cell cycle happens in this phase. The three phases in this stage (G1, S, and G2) are involved in the preparation for mitosis. Growth and biosynthesis happen during this phase; it slows down considerably in the M (mitotic) phase. The S phase starts with DNA synthesis to make two sister chromatids. There is little protein synthesis and RNA transcription happening while this division of chromosomes happen. The G2 phase involves the beginning of microtubule organization in order to from a mitotic spindle.
MITOSIS Mitosis is relatively brief and consists of nuclear division or karyokinesis. It is broken down into five tightly regulated steps. In this phase, the five steps include prophase, prometaphase, metaphase, anaphase, and telophase. This is followed by cytokinesis, which separates the cells. The sister chromatids are placed together in the nucleus and lines up along a mitotic plate. The chromosomes separate and are pulled to opposite sides of the cell. The nuclear envelope breaks down in the cell so that the chromosomes are actually pulled to opposite poles of the cell. Figure 18 shows what mitosis looks like:
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