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The Cell Cycle In a population of dividing (or cycling) cells, each cell passes through four consecutive stages: Gt. S, G2, and M (gap 1, DNA synthesis, gap 2, and mitosis). During Gh preparations are made for DNA synthesis; this is the stage that usually occupies the longest portion of the cell cycle. Also, differences in the duration of cell cycles between different kinds of cells can usually be attributed to differences in the duration of G1. DNA undergoes a process called semiconservative replication during the S stage. The two complementary strands of the mother cell's DNA separate, and each one acts as a template (a guide; see details in Chapter 4) for a new complementary strand. Barring mutations, two complete identical copies of the original DNA are formed (Fig. 3-2).