The S-phase is called the synthesis stage because it is the phase of the cell cycle where DNA replication occurs.

Here's why:

* DNA is the blueprint of life: It contains the genetic instructions for building and maintaining an organism.

* Cell division requires two copies of DNA: Before a cell can divide, it needs to duplicate all its DNA so that each daughter cell receives a complete set of genetic information.

* Replication is the process of making a copy: This process is called DNA replication, and it happens during the S-phase.

Therefore, the S-phase is aptly named "synthesis" because it's the stage where the cell synthesizes a new copy of its entire DNA genome.