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The cell cycle consists of four main phases: G1 (gap 1), S (synthesis), G2 (gap 2), and M (mitosis). The cell cycle is essential for cell growth, replication, and division. During G1, the cell prepares for DNA replication that occurs during S phase. In the G2 phase, the cell prepares for division, and finally, cell division occurs during the M phase.

In normal cells, the cell cycle is tightly regulated by specific proteins, including cyclins and cyclin-dependent kinases (CDKs). In addition to these regulators, several checkpoints exist to ensure the correct progression of the cell cycle. These checkpoints (G1 checkpoint, G2 checkpoint, and M checkpoint) help ensure that damaged or incomplete DNA does not get passed on to daughter cells.

In cancer cells, cell-cycle regulation becomes disrupted, which results in uncontrolled cell growth and division. This deregulation can occur due to the overexpression of positive cell-cycle regulators (such as cyclins and CDKs) or the inactivation of negative regulators (such as tumor suppressor genes). The cell-cycle checkpoints may also become dysfunctional, allowing cancer cells to continue dividing even with damaged DNA or incomplete replication.

Examples of specific regulators that differ between normal and cancer cells include: - p53: A tumor suppressor gene that helps regulate the cell cycle and maintain genomic stability. In many types of cancer, the p53 gene is mutated or inactivated, leading to a loss of cell-cycle regulation. - Rb: Another important tumor suppressor gene which binds to the E2F transcription factor and prevents cell cycle progression. In cancer cells, Rb may be mutated or inactivated, allowing cells to bypass the G1 checkpoint and enter S phase. - Cyclins and CDKs: In cancer cells, cyclins, and CDKs can be overexpressed, leading to uncontrolled cell division and subsequent growth of a tumor. By understanding these key differences between normal cells and cancer cells in terms of cell-cycle regulation, we can better comprehend the uncontrolled growth and division characteristic of cancer.