1.9.3 Cell Cycle Phase Definition

Cell Cycle Phase Definition is the description of one of the discrete, sequentially ordered intervals that together compose the complete cell division cycle, each phase characterized by a distinct set of cellular activities and each required to be completed before the cell advances into the subsequent phase. A cell cycle phase represents a defined stage of progress toward division, distinguished from adjacent phases both by the specific biological processes occurring during that interval and by the particular regulatory proteins that become active to drive or restrain progression through it.

Conceptual Basis of the Cell Cycle Phase

A Defined Interval With Characteristic Activities

Each cell cycle phase is defined by the particular cellular activities that predominate during that interval, such as growth and synthesis of cellular components, duplication of genetic material, or physical separation of chromosomes, providing a basis for distinguishing one phase from another according to what the cell is actively doing at that point in its progression toward division.

Sequential Dependency Between Phases

The phases of the cell cycle occur in a fixed, sequential order, with completion of each phase's characteristic activities generally required before the cell can appropriately advance into the next phase, reflecting the logical dependency between successive stages of preparation for and execution of division.

Identifying the Major Cell Cycle Phases

Phases Dedicated to Growth and Preparation

Certain cell cycle phases are dedicated to cellular growth and the synthesis of components required for subsequent stages, occurring both before and after the phase dedicated to DNA replication, and providing the cell with the opportunity to accumulate necessary resources and to verify readiness before proceeding further.

The Phase Dedicated to DNA Replication

A specific, dedicated phase of the cell cycle is set aside for the accurate duplication of the cell's entire genetic material, producing two complete copies of each chromosome in preparation for their eventual distribution to two separate daughter cells.

The Phase Dedicated to Physical Division

A specific, dedicated phase of the cell cycle is set aside for the physical separation of the duplicated chromosomes and the division of the cell itself into two complete daughter cells, representing the culminating event toward which all preceding phases have been directed.

A Distinct Phase Representing Withdrawal From Active Cycling

Beyond the sequentially ordered phases directly involved in progression toward division, cells can enter a distinct phase representing withdrawal from the actively cycling sequence, remaining metabolically active without progressing toward further division until appropriate signals prompt reentry into the cycle.

Transitions Between Cell Cycle Phases

Checkpoint Control at Phase Boundaries

The boundary between each pair of successive cell cycle phases is subject to checkpoint control, at which the cell's readiness to proceed is assessed before permission to advance into the next phase is granted, providing an opportunity to halt progression if conditions within the preceding phase have not been satisfactorily completed.

Phase-Specific Regulatory Proteins

Distinct combinations of regulatory proteins become active during each specific cell cycle phase, driving the particular activities characteristic of that phase and contributing to the eventual transition into the subsequent phase once those activities have been completed.

Significance of Cell Cycle Phase Definitions Within Cancer Cell Biology

A Framework for Localizing Deregulation to Specific Points in the Cycle

Defining the cell cycle in terms of its constituent phases provides a framework for identifying precisely where within the overall cycle a given cancer-associated alteration exerts its effect, since many oncogenic and tumor suppressor alterations act specifically upon the regulatory proteins governing transition through one particular phase or phase boundary rather than the cycle as an undifferentiated whole.