The portion of the cell cycle during which replication occurs is called S phase.

Here's a breakdown of the cell cycle phases and what happens in each:

* G1 phase (First Gap): The cell grows and carries out normal metabolic functions.

* S phase (Synthesis): DNA replication occurs, creating two identical copies of the genome.

* G2 phase (Second Gap): The cell prepares for mitosis, synthesizing proteins and organelles.

* M phase (Mitosis): The cell divides into two daughter cells, each with a complete copy of the genome.

So, S phase is the key stage where DNA replication takes place, ensuring that each daughter cell receives a full set of genetic instructions.