The cell cycle is made of the four stages G1, S (synthesis), G2, and M (mitosis). The cell grows in G1 phase, the DNA and chromosomes are duplicated in S phase, the cell grows and prepares to divide in G2 phase, and the cell with its nucleus divide in M phase. However, if the cell cycle does not proceed from G1 to S phase, the cell may differentiate and enter G0, the cell will stop grow in G0. If the cell does not differentiate, a signal is received from cyclin D and E, and the cell cycle goes into S phase and replicate its DNA. After several hours, another signal from cyclin A activate the cell to enter G2. The cell will enter mitosis and divide when the cyclin B become active. Eukaryotic cells use cyclins as checkpoint to control the progression through the stages of cell division. The cell will reach a checkpoint at each stage, which ensure all the proteins and DNA are in the correct position before moving to the next stage .