G1 phase

In this part of interphase, the cell synthesizes mRNA and proteins in preparation for subsequent steps leading to mitosis.

[1] During G1 phase, the cell grows in size and synthesizes mRNA and protein that are required for DNA synthesis.

Sufficient nucleotides and amino acids must be present in order to synthesize mRNA and proteins.

[1] Within the cell cycle, there is a stringent set of regulations known as the cell cycle control system that controls the timing and coordination of the phases to ensure a correct order of events.

Complexes of cyclin that are active during other phases of the cell cycle are kept inactivated to prevent any cell-cycle events from occurring out of order.

Three methods of preventing Cdk activity are found in G1 phase: pRB binding to E2F family transcription factors downregulate expression of S phase cyclin genes; anaphase-promoting complex (APC) is activated, which targets and degrades S and M cyclins (but not G1/S cyclins); and a high concentration of Cdk inhibitors is found during G1 phase.

At the G1/S checkpoint, formation of the G1/S cyclin with Cdk to form a complex commits the cell to a new division cycle.

[1] Many sources have linked irregularities in the G1 phase or the G1/S checkpoint to uncontrolled growth of tumors.