G1 is the period between the last mitotic division and the beginning of DNA synthesis. It can vary in duration and substantially determines the overall length of the cell cycle. In the G1 phase, the cell regains its original size after its previous division. This also involves an increase in the amount ofand the replication of organelles. The cell increases in size, monitors its environment, and arrives at checkpoint G1, where it decides if it should enter the S phase, in which the DNA is replicated. If the necessary size has been attained and the environmental conditions are favorable, signal proteins are activated to prepare for cell division and the cell enters the S phase.
Some of the activating proteins are called cyclins and cyclin-dependent kinases (CDKs). These activate several genes that drive the cell through all of the steps of the cell cycle. Some oncogenes are a special class of mutated genes for activating proteins that constantly drive continuation of the cell cycle, initiating constant cell divisions, which leads to cancer. One well known example of this is the Ras protein. The cytoplasmic S phase activator induces DNA replication and preparation for the subsequent S phase and mitosis through preparation of replication enzymes and synthesis of DNA components. If the situation is found to be unfavorable for cell division, preparation for the S phase is halted and the cell enters the resting phase, G0.
At the G1 checkpoint, another effector protein, gene product p53, prevents cells with damaged DNA from entering the S phase. If the cell can repair the DNA defect, it is allowed to enter the S phase; however if reparation fails, cell death (apoptosis) is initiated to prevent transmission of the defective DNA.