The four key phases of the eukaryotic cell cycle are: G1 phase, where cells grow in size & produces organelles, cytoskeletons, some DNA & proteins for DNA synthesis; S phase, DNA synthesis occurs where chromosomes are duplicated, forming sister chromatids held together by centromeres; G2 phase, where cells grow in size & produce required proteins, for checkpoints to confirm that cells are ready for M phase; and M phase, involving mitosis (i.e. prophase, prometaphase, metaphase, anaphase & telophase), the division of nucleus, and cytokinesis, the division of cell plasma for two daughter cells.