Which of the following is NOT a phase of the cell cycle?

In the context of the cell cycle, the phases include well-established stages such as G1, S, G2, and mitosis (which encompasses phases like prophase and telophase). The G1 phase is the first gap phase where the cell grows and prepares for DNA replication, followed by the S phase where DNA synthesis occurs, and then the G2 phase where the cell prepares for mitosis. Prophase and telophase are indeed phases of mitosis, which is part of the overall cell cycle.

The G1/S checkpoint is critical in regulating the transition from the G1 phase to the S phase, ensuring that the cell is ready for DNA synthesis. This checkpoint assesses various factors such as DNA integrity and cell size before allowing the cell to proceed.

In contrast, "G3 phase" is not a recognized phase in the standard cell cycle terminology. The phases are generally limited to G1, S, G2, and the stages of mitosis. Therefore, identifying "G3 Phase" as not being a correct phase of the cell cycle is accurate, as it does not exist within the established framework of cell cycle phases.