EUGENE M. MCCARTHY, PHD Google+ Profile
The interphase, or growth, period of the eukaryotic cell cycle (indicated by "I" in the figure at left below) alternates with mitosis ("M" in the figure). It's the time when the cell isn't undergoing division. So it isn't part of mitosis, even though it ends in "-phase" like the names of the various steps of mitosis (the Latin prefix inter- indicates that this stage occurs between rounds of mitosis). Beginning students are often confused about this distinction. In reality, prophase is the first stage of mitosis, and telophase is the last. Cytokinesis occurs only during mitosis and meiosis.
Interphase is the stage of the cell cycle in which cells spend most of their time and perform their customary functions, including preparation for cell division. When this phase begins, the chromosomes have not yet replicated, but by the beginning of prophase replication is complete, so that each chromosome is composed of two sister chromatids. Replication occurs during the synthesis, or S phase ("S" in the diagram at right).
S phase is preceded by G₁ phase, which in many cells is a time when cell growth occurs. From G₁, a cell may exit the cell cycle and go into a long-term stable state known as G₀ where the cell functions but does not divide.
At the beginning of the third and last of these substages, G₂ phase, replication is complete. During G₂ the cell prepares for mitosis as it undergoes rapid growth.
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