|Cell cycle. Image: Richard Wheeler|
It's the time (indicated by "I" in the figure at right) when the cell isn't undergoing division. So it isn't part of mitosis ("M" in the figure), even though it ends in "-phase" like the names of the various steps of mitosis (the Latin prefix inter-, meaning "between," 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 (typically, more than 90%) 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. The process is called replication because each of the two sister chromatids that make up a replicated chromosome is a exact copy, or replica, of the unreplicated, or parent, chromosome. This replication is accomplished by splitting the parent chromosome's double-stranded DNA into two single-stranded DNA chains and then assembling matching nucleotides all along the length of each of the two single strands to produce two identical chromatids (watch a video showing how replication occurs). However, though they have been entirely separated from each other elsewhere, the two chromatids are attached to each other (at the centromere), and remain attached as prophase begins. Besides connecting the two chromatids of a single replicated chromosome together, the centromere also functions as an attachment point where the cellular machinery attaches to each chromosome during the process of segregating the chromosomes of the parent cell into separate daughter cells, which occurs during mitosis and meiosis. Replication occurs during the synthesis, or S phase ("S" in the diagram).
S phase is preceded by G₁ phase, which in many cells is a time when cell growth occurs (thus, the G). 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.