A cell in 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 cell's mass increases. 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.
During the following S phase the double helix of the DNA strand separates (like a zipper) into two parent strands (which are still bonded, but open) Each single strand is now joined by the appropriate matching opposite bases (A to T, G to C) to create a complimentary strand. The result is two new double strands, each made of an original parent strand and a complimentary strand.