Here's why:

* S phase stands for "synthesis phase". This is the phase where the cell replicates its entire DNA genome, ensuring that each daughter cell receives a complete set of genetic information.

* G1 phase (before S phase) is for cell growth and preparation for DNA replication.

* G2 phase (after S phase) is for further growth and preparation for cell division (mitosis or meiosis).

* M phase (mitosis or meiosis) is when the cell divides into two daughter cells.

So, DNA synthesis is a crucial step in the cell cycle, and it takes place specifically during the S phase.