The normal function of one tumor-suppressor gene acting at the G1 checkpoint is to prevent cells with damaged chromosomes and DNA from “progressing from G1 to S.” Another tumor-suppressor gene, acting...

The normal function of one tumor-suppressor gene acting at the G1 checkpoint is to prevent cells with damaged chromosomes and DNA from “progressing from G1 to S.” Another tumor-suppressor gene, acting at the G2 >M checkpoint, prevents “passage from G2 to M.” When these tumor-suppressor genes fail to work, cancer can result. Explain what the phrases in quotations mean.