G1 phase

The g1 phase, or Gap 1 phase, is the first of four phases of the cell cycle that takes place in eukaryotic cell division. In this part of interphase, the cell synthesizes mRNA and proteins in preparation for subsequent steps leading to mitosis. G1 phase ends when the cell moves into the S phase of interphase. The g1 phase, or Gap 1 phase, is the first of four phases of the cell cycle that takes place in eukaryotic cell division. In this part of interphase, the cell synthesizes mRNA and proteins in preparation for subsequent steps leading to mitosis. G1 phase ends when the cell moves into the S phase of interphase. G1 phase together with the S phase and G2 phase comprise the long growth period of the cell cycle called interphase that takes place before cell division in mitosis (M phase). During G1 phase, the cell grows in size and synthesizes mRNA and proteins (known as histones) that are required for DNA synthesis. Once the required proteins and growth are complete, the cell enters the next phase of the cell cycle, S phase. The duration of each phase, including the G1 phase, is different in many different types of cells. In human somatic cells, the cell cycle lasts about 18 hours, and the G1 phase takes up about 1/3 of that time. However, in Xenopus embryos, sea urchin embryos, and Drosophila embryos, the G1 phase is barely existent and is defined as the gap, if one exists, between the end of mitosis and the S phase. G1 phase and the other subphases of the cell cycle may be affected by limiting growth factors such as nutrient supply, temperature, and room for growth. Sufficient nucleotides and amino acids must be present in order to synthesize mRNA and proteins. Physiological temperatures are optimal for cell growth. In humans, the normal physiological temperature is around 37 °C (98.6 °F). G1 phase is particularly important in the cell cycle because it determines whether a cell commits to division or to leaving the cell cycle. If a cell is signaled to remain undivided, instead of moving onto the S phase, it will leave the G1 phase and move into a state of dormancy called the G0 phase. Most nonproliferating vertebrate cells will enter the G0 phase. 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. Biochemical triggers known as cyclin-dependent kinases (Cdks) switch on cell cycles events at the corrected time and in the correct order to prevent any mistakes. There are three checkpoints in the cell cycle: the G1/S Checkpoint or the Start checkpoint in yeast; the G2/M checkpoint; and the spindle checkpoint. During G1 phase, the G1/S cyclin activity rises significantly near the end of the G1 phase. 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.