The cell cycle refers to the sequence of events throughout a cell's typical life, including growth, DNA replication, sometimes more growth, and division. For sexually reproducing organisms, life begins as a zygote, a fertilized egg. Over time, that original single cell grows and divides in a controlled manner to produce a multicellular complex individual. Cells continue this process, especially to maintain and repair tissues during the progression of aging. In contrast, single-celled organisms like bacteria rely on cell division to simply reproduce via binary fission. Unfortunately, when cell division is left unchecked, cancer results, with a loss of control over genomic sequences that regulate the normal process.