What is DNA replication and how does the cell cycle regulate cell division?

DNA replication is the production of two identical DNA strands from one original DNA molecule and is a nearly error-free process that occurs in all living organisms. This is because, in order for a cell to divide, it must first replicate its DNA. It is therefore a vital biological process and the basis for inheritence. 

Cell cycle regulator genes strictly regulate when DNA replication occurs and therefore when cell division can take place.

G0 is the resting phase of the cell cycle which occurs after mitosis, but when a cell is NOT preparing to divide again. At any one point, most cells are in the quiescent (G0) phase. G1 is the phase after mitosis and before the next round of DNA synthesis. During G1, the cell enlarges and makes new proteins. Towards the end of G1, there is a restriction point. This is for the cell to assess whether it is prepared for cell division. Once this point is passed, the cell is committed to DNA replication. S is the phase of DNA synthesis, when the new DNA is actually formed. This process takes 6-8 hours in eukaryotes and only 40min in prokaryotes. In the G2 phase, DNA is condensed into chromosomes in preparation for mitosis. Just before mitosis, there is a checkpoint which assesses if the DNA has replicated properly and whether there are any mutations requiring repair. In the M (mitotic) phase, the chromosomes actually pull apart and there is subsequent cell division.