In order for living things to produce cells for the purpose of growth, repair or to reproduce, cell division is necessary. The cell cycle describes the stages a cell goes through before it eventually divides into two daughter cells. The cell cycle includes interphase which includes three stages-- G1 or first gap, S phase, G2 which is then followed by mitosis. A cell grows during interphase and produces necessary organelles. It is during the S phase that the chromosomes are replicated.
Of utmost importance is the replication of the DNA which contains the heredity information of the cell which must be copied into two identical sets for each daughter cell. This complete set is known as the genome and the cell's DNA is packaged into chromosomes. In eukaryotic cells, this stage of the cell cycle occurs inside a cell's nucleus where its DNA is located.
During the cell cycle, when the cell is not actively undergoing mitosis, the chromosomes do not appear condensed but rather, are thin, long fibers known as chromatin. Once DNA has replicated during the S-phase of interphase, the chromatin condenses, folds and coils making it more compact and visible under a microscope as chromosomes.
The cell is ready for mitosis (M phase) of the cell cycle after the DNA has been replicated. At the end of mitosis, cytokinesis occurs resulting in the division of the cytoplasm to form two smaller, but genetically identical daughter cells.