Answer:
DNA is first checked during the G1/S. This is when the integrity of DNA is checked to make sure it's correct and ready to be replicated in the S phase. If the DNA fails the check, it can leave the cell cycle or enter a resting phase called G0, where it stays until the DNA is corrected. Any damage is fixed by proteins, the key one being the p53 protein, a tumor suppressor. It works by halting the cell cycle and activating repair proteins. If the damage is irreparable, it initiates apoptosis, programmed cell death, to prevent the damaged DNA from replicating and dividing into cells. If this gene is missing or mutated, damaged cells continue to replicated rapidly and lead to the rise of tumors and thus cancer. If nothing is wrong or the errors are fixed, the G1/S checkpoint gives the green light, to allow DNA to replicate. The now replicated chromosomes are once again checked in the G2 checkpoint. Here, they're checked again for integrity and whether or not replication occurred correctly. If the DNA fails these checks, the cell once again attempts to fix the problems. If these problems aren't corrected due to mutations in the repair proteins, the cells containing the damaged chromosomes will continue to replicate rapidly and cause cancer. Should no problems occur, cyclin dependent kinases (CDKs) are activated and allow the cell to continue into mitosis. DNA damage can basically lead to the rapid division of mutated cells and cancer.
