Which is not a phase of the cell cycle?

The correct answer is G5, as it is not recognized as a phase of the cell cycle. The cell cycle consists of a series of stages through which a cell goes to grow and divide. These stages include G1 (the first gap phase), S (the synthesis phase where DNA is replicated), G2 (the second gap phase where the cell prepares for mitosis), and M (mitosis, the phase where the cell divides).

G0 is known as a resting or quiescent phase, where cells may exit the active cycle and remain metabolically active but not proliferate. G1 is the initial growth phase after a cell has divided, where it increases in size and synthesizes the necessary components for DNA synthesis. Mitosis is the actual process of cell division. In contrast, G5 is not a recognized phase in this context, as the primary cell cycle phases are limited to the defined stages mentioned above.