Interphase - G1, S and G2
For most cells, the majority of their life is spent in interphase. It is during this time that the cell carries out most of its normal functions, such as growth and protein synthesis. Interphase has three stages called G1, S and G2. In many tissues, such as in the brain, most mature cells will remain in interphase throughout their lives.
G1 and G2 stand for "first Gap" and "second Gap." Interphase begins with G1. This is the long period of time following mitosis but before DNA synthesis (S phase). During this time, the cell grows and carries out protein systhesis and other cellular functions (except DNA replication). The chromosomes are all unreplicated, that is, each contains only one molecule of DNA. The chromatin is very diffuse within the nucleus and so the individual chromosomes are not visible. The nucleus is complete, bound by a membrane and the nucleoli are clearly visible with proper staining.
Following G1 is the S phase. This is the period when the cell replicates its DNA. At the completion of this stage, all of the chromosomes have two chromatids (two molecules of DNA per chromosome). The chromosomes are still decondensed and not distinctly visible.
After the S phase, the cell enters G2. The cell continues to carry out all normal functions and may continue growth. Near the end of G2, the cytoplasmic organelles replicate in preparation for the cell to divide during mitosis.