${\mathrm{G}}_1$ phase corresponds to the interval between mitosis and initiation of DNA replication. During ${\mathrm{G}}_1$ phase, the cell is metabolically active and continuously grows but does not replicate its DNA. 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. If the initial amount of DNA is denoted as 2C, then it increases to 4C. However, there is no increase in the chromosome number; if the cell had diploid or 2n number of chromosomes at ${\mathrm{G}}_1$, even after S phase the number of chromosomes remains the same, i.e., 2n. In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm. During the ${\mathrm{G}}_2$ phase, proteins are synthesised in preparation for mitosis, while cell growth continues.