Ans; A (G0) "The cell cycle consists of G, (presynthetic), S (DMA synthesis), G, (premitotic), and SI (mitotic) phases.Quiescent cells are in a physiologic state called Ga. Tissues may be composed primarily of quiescent ceils in Gv but most mature tissues contain some combination of continuously dividing cells, terminally differentiated cells, stem ceils, and quiescent ceils that occasionally enter into the cell cycle"- Abeloff's Clinical Oncology/ 4^/428;Phases of Cell CycleThe cell cycle consists of four distinct phases: G, phase. S phase (synthesis91, G phase (collectively known as interphase) and M phase (mitosis)Q,M phase is itself composed oftwo tightly coupled processes: mitosis, in which the cell's chromosomes are divided between the two daughter cells, and cytokinesis, in which the cell's cytoplasm divides in half forming distinct cells.Activation of each phase is dependent on the proper progression and completion of the previous one. Ceils that have temporarily or reversibly stopped dividing are said to have entered a state of quiescence called G phaseS Phase# Th e ensuing S phase starts when DNA synthesis commenceswhen it is complete, all of the chromosomes have been replicated, i.e., each chromosome has two (sister) chromatids.# During this phase, the amount of DNA in the cell has effectively doubled, though the ploidv of the cell remains the same.StatePhaseAbbreviationDescriptionQuiescent/ senescentGap 0G0A resting phase where the cell has left the cyde and has stopped' dividing. Gap 1G1Cells increase in size in Gap 1. The checkpoint control mechanism ensures that everything is ready' for DNA synthesis.SynthesisSDNA replication occurs during this phase.Interphase Gap 2G2During the gap between DNA synthesis and mitosis, the cell will continue to grow. The G, checkpoint control mechanism ensures that everything is ready to enter the M (mitosis) phase and divide.Cell growth stops at this stage and cellular energy' is focused on the orderly' division into two daughter cells. A checkpoint in the middle of mitosis (Metaphase Checkpoint) ensures that the cell is ready to complete cell division.Cell divisionMitosisM Fig: Sequence of events during the cell cycle