Interphase

Interphase is a phase of the cell cycle, defined only by the absence of cell division. During interphase, the cell obtains nutrients, and duplicates its chromosomes. Most eukaryotic cells spend most of their time in interphase. For example, human skin cells, which divide about once a day, spend roughly 22 hours in interphase. Cells during interphase may or may not be growing. At any given time, even in an area of rapid cell division such as the tip of a plant root, 90 percent of cells are in interphase. Some cells, such as nerve cells, can stay in interphase for decades. The cell grows and replicates its DNA and centrioles. There are 3 parts of interphase: G₁ (growth 1 in which the cell creates organelles and begins metabolism), S phase (DNA synthesis in which the chromosomes of the cell are copied) and G₂ (growth 2 in which the cell grows in preparation for cell division). Sometimes the cells exit the cell cycles(usually from G1 phase) and enter the G0 phase. In the G0 phase, cells are alive and metabolically active, but do not divide. In this phase cells do not copy their DNA and do not prepare for cell division. Many cells in the human body, including those in heart muscle, eyes, and brain are in the G0 Phase. If these cells are damaged they cannot be replaced. Interphase is the step before the mitotic phase, though it is frequently erroneously thought to be part of mitosis. The mitotic phase follows interphase, and begins with prophase.

During interphase, the chromosomes are found arranged in the nucleus and appear as a network of long, thin threads, called the chromatin. At some point before prophase begins, the chromosomes begin to replicate themselves to form pairs of identical chromosomes, called chromatids; the deoxyribose nucleic acid (DNA) of the chromosomes is in use only during interphase, when the cell is in a stable and self reliant phase. In prophase the two chromatids are still connected by something called the centromere. The sister chromosomes contract tightly. Meanwhile, the nucleolus and the nuclear envelope break down and disappear, since their components have been sufficiently altered. Outside the nucleus are two centrosomes which sprout microtubules by polymerizing free-floating proteins. The centrosomes push themselves to opposite ends of the cell. The network of microtubules forms the beginning of the mitotic spindle. These spindle fibers become visible. The centrioles separate starting to radiate bundles of fibers, called asters. The spindle fibers run from one centriole to the other, at both poles of the cell. At the onset of prophase, chromatin condenses together into a highly ordered structure called a chromosome.