G1 is considered to be the most important checkpoint in the cell cycle. G1 precedes S and G2, so it decides if the cell is ready to commit the resources to go through the S phase, G2 phase, M phase, and then finally division. S is crucial for the replication of cellular materials and the duplication of DNA, but if the cell cannot pass G1, then it will never reach S phase.

The G2 checkpoint of cell division prevents the cell from entering the mitotic or dividing phase until all chromosomes have accurately been replicated. Other answers: "Prevents sister chromatids from separating before each chromatid is secured to a spindle microtubule" refers to the spindle or M checkpoint. "Initiates the cell division process if conditions are favorable" and "Ensures that the cell has matured to a sufficient state before beginning synthesis" refer to functions of the restrictive or G1 checkpoint. "Ensures that each daughter cell has received the correct number of chromatids after cell division" is incorrect since there is no checkpoint in cell division to error check the results of mitosis.

During interphase, you have G1, or gap 1, in which the cell grows. Then you have S phase, in which the cell synthesizes its DNA. Finally, you have G2, or gap 2, in which the cell continues to grow again.