1.9.17 Cell Cycle Entry Definition

Cell Cycle Entry Definition is the description of the transition by which a cell residing within the resting, quiescent state departs from that state and reenters the actively cycling sequence of phases leading toward DNA replication and division, a transition triggered by the reception of sufficient growth-promoting signaling and marked by resumption of the cellular activities characteristic of the first growth phase of the cycle. Cell cycle entry represents the initiating event by which a previously non-dividing cell begins the process that will ultimately culminate in its division into two daughter cells, distinguishing this transition from the subsequent internal checkpoints and phase transitions that govern progression once the cycle has already been entered.

Conceptual Basis of Cell Cycle Entry

The Initiating Transition From Quiescence to Active Cycling

Cell cycle entry marks the specific point at which a cell shifts from the resting, quiescent condition into the actively cycling sequence of phases, representing a qualitatively distinct transition from the internal transitions that occur between phases once a cell is already actively cycling.

Dependence on External Growth-Promoting Signaling

Cell cycle entry is normally dependent on the reception of adequate external growth-promoting signals from the cell's surrounding environment, reflecting the role of this transition as the point at which external conditions are translated into a cellular decision to resume progression toward division.

Molecular Events Underlying Cell Cycle Entry

Reception and Transmission of Growth Signals

Cell cycle entry begins with the binding of external growth-promoting signals to receptors present on the cell's surface, triggering an intracellular signaling cascade that ultimately activates the regulatory proteins responsible for driving progression through the first growth phase of the cycle.

Activation of Early Cell Cycle Driving Proteins

Following reception of adequate growth signaling, specific driving regulatory proteins associated with the earliest stage of the first growth phase become activated, initiating the sequence of cellular activities that will eventually lead the cell toward the restriction point and, beyond it, toward DNA replication.

Regulation of Cell Cycle Entry

A Point Subject to Restraining Influence

Cell cycle entry remains subject to restraining influence from tumor suppressor proteins capable of counteracting the effect of growth-promoting signals, providing a mechanism by which a cell can resist inappropriate entry into the cycle even in the presence of some degree of growth signaling.

Reversibility Prior to Full Commitment

Unlike passage through the restriction point later in the first growth phase, cell cycle entry itself remains a reversible transition, meaning that a cell that has begun to reenter the cycle can still withdraw back into the quiescent state if growth signaling is subsequently withdrawn before the restriction point has been reached.

Significance of Cell Cycle Entry Within Cancer Cell Biology

A Frequent Target of Abnormal Activation in Cancer Cells

Cancer cells frequently display abnormal, inappropriate cell cycle entry, whether through oncogenic activation of the signaling pathways that normally require external growth factor stimulation, or through loss of the tumor suppressor proteins that would otherwise restrain this transition, allowing cancer cells to reenter the cycle under conditions that would not support entry in a corresponding normal cell.

A Foundational Step Preceding Subsequent Cell Cycle Deregulation

Because cell cycle entry represents the initiating event from which all subsequent progression through the cycle proceeds, abnormal or inappropriate entry establishes the foundation upon which the further deregulation of downstream checkpoints and phase transitions characteristic of cancer cells is subsequently built.