Control of the Cycle

During the G: and G2 phases, cells grow and make sure that conditions are proper for DNA replication and cell division. During the G: phase, cells monitor their environment and determine if conditions, including the availability of nutrients, growth factors and hormones, justify DNA replication. The decision to initiate replication is made at a specific "checkpoint" in G: called the "restriction point."

replication duplication The processes of DNA replication and mitosis, and intervening events during the cell cycle, occur in a highly ordered and specific manner. A complex network of proteins ensures that these events occur at the proper times. Intracellular and extracellular signals block cell-cycle progression at checkpoints if certain events have not yet been completed. After the restriction point, the cell is committed to replicating its genome and dividing, completing one round of the cell cycle. If, prior to the restriction point, cells sense inadequate growth conditions or receive inhibitory signals from other cells, they enter G0 (G-zero) phase, also called quiescence. In the G0 phase, they are maintained for prolonged periods in a nondividing state. If cells sense such conditions after the restriction point, they complete the current of DNA

A TYPICAL ANIMAL CELL

Smooth endoplasmic reticulum

Golgi apparatus

Ribosomes

Stalk Basal body Rootlet

A TYPICAL ANIMAL CELL

Smooth endoplasmic reticulum

Golgi apparatus

Ribosomes

Stalk Basal body Rootlet

Peroxisome

Mitochondrion

Centrioles

Vacuole

Vacuole

Peroxisome

Mitochondrion

Centrioles

Chromosome

Nucleolus Nuclear membrane

Plasma membrane Lysosome

OOOc

Animal cell parts. Adapted from Robinson, 2001.

round of the cell cycle and exit to G0 during the subsequent G: phase. The G2 phase is shorter than Gx, but it, too, consists of important mechanisms that control the completion and fidelity of DNA replication and that prepare the cell for entry into mitosis. Whereas some conditions cause cells to enter the G0 phase, others trigger apoptosis. One such signal that may trigger apoptosis is if a cell's DNA has undergone significant damage.

After the restriction point, at the transition from the G2 to the M phase, another checkpoint occurs. Mitosis is prevented if DNA damage has occurred or if genomic replication is not complete. The final key checkpoint occurs at the end of mitosis, when the cycle stops if chromosomes are not properly attached to the mitotic spindle.

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