Cell cycles

Cell division and DNA synthesis are coordinated in most cells.

In synchronously dividing cell populations, the levels per cell of RNA and protein rise steadily during culture after division. DNA synthesis, however, occurs only during a limited time period some time after cell division is completed. Mitosis and cell division, also, occur discontinuously.
The time during which DNA synthesis happens is called S phase. The time during which chromosomes condense, segregate and the cell divides is labeled M phase. These two non-consecutive distinct phases of the life of a cell define two other phases, one after M and before S, called G1 and one after S and before M, called G2.

In the traditional, unicycle, model of the stages in the life of a cell, each phase follows logically after the other. The model is often represented as a circle and called the cell cycle. It implies that events in the previous phase need to be completed before the next phase can start.
In a competing, bicycle, model, there are two distinguishable cycles. One cycle is the DNA replication (S) cycle. The other is the cycle of mitosis and cell division (M). The model postulates that the two cycles are interconnected by regulatory signals from one cycle that either promote or inhibit events in the other cycle. In the diagram, replicating DNA prevents entry into mitosis and mitosis prevents initiation of the events that lead to DNA synthesis.

Lack of coordination of replication and division leads to endoreduplication.
Details of the replication cycles differ in fission and budding yeasts.
Errors in regulation of the cell cycle can lead to uncontrolled growth and cancer.
Uncoupling cell division from mitosis can occur in some fungi and in some mutants deficient in cytokinesis. Multinucleate cells result. These observations suggest a tricycle model: DNA synthesis, mitosis and cytokinesis cycles.

E-mail inquiries to U. Melcher------------Last Updated: 3 September, 2003