If the cell cycle fails to progress, which of the following is a possible explanation?
(A) There are adequate phosphate groups available for the cyclin-dependent kinase.
(B) An oncogene inhibits the signal for apoptosis.
(C) A cyclin is unable to release from its cyclin dependent kinase.
(D) An activator of a cyclin gene has been highly expressed.

The cell cycle is a series of events in the life of a dividing eukaryotic cell. It consists of four stages: G₁, S, G₂, and M. The duration of the cell cycle varies from one species to another and from one cell type to another. The G₁ phase varies the most. For example, embryonic cells can pass through the G₁ phase so quickly that it hardly exists, whereas neurons are arrested in the cell cycle and do not divide.

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M mitosis) G₁₂ 4 hours 9 hours S- -G₁ 10 hours