Interphase begins as a cell from a previous division (see the "fried egg with blue yolk" cell, left) enters the G1 stage of interphase, a period of growth in which protiens and other cellular molecules are synthesized, but DNA is not replicated (the "G" refers to the "gap" or break in DNA synthesis during this stage).  At some point, if the cell is going to divide, DNA replication begins.  The initiation of DNA replication ends G1 and begins the S period of interphase (from S= DNA synthesis).  During S phase, the entire nuclear complement of DNA is duplicated.  As repliction is completed, S ends and the cell enters the final stage of interphase, G2.  Most cell types remain in G2 only briefly; and at the end of G2, which marks the end of interphase, mitosis begins.   After mitotic division is complete, the two cell products each enter G1 of the next cell cycle.
        Each stage of interphase is characterized by a distinct pattern of cellular synthesis.  During G1, most of the proteins, carbohydrates, and lipids typical of a given cell type are synthsized, but no DNA is replication takes place.  Although assembly of  the major classes of cellular molecules usually continues during S and G2, S is uniquely marked by DNA replication.  The histone and nonhistone chromosomal proteins associated with DNA are also duplicated during S.  G2 is characterized by synthesis of a group of proteins necessary for progress through mitosis.  G0 is a state of cell cycle arrest.  The dark orange wedge in the diagram represents cytokinesis (cytoplasmic cell division).