The S or synthesis phase marks the period during which DNA synthesis or replication takes place. During this time, the amount of DNA per cell doubles. For example, if the initial amount of DNA is denoted as 2C, it increases to 4C. However, there is no increase in the chromosome number; if the cell had a diploid or 2n number of chromosomes at G1, even after the S phase the number of chromosomes remains the same, i.e., 2n .