12.3 Concepts

Purpose of the cell cycle control system

To regulate the timing and accuracy of cell division.

Checkpoint in the cell cycle

A control point where the cycle can pause to ensure everything is ready before continuing.

Most important checkpoint

The G₁ checkpoint—it determines whether the cell will divide or enter the G₀ phase.

G₀ phase

A non-dividing, resting state entered by cells that do not receive a go-ahead signal at G₁.

Cyclins

Regulatory proteins whose concentrations fluctuate during the cell cycle.

Cdks

Cyclin-dependent kinases; enzymes that become active only when bound to cyclins.

MPF

A Cyclin-Cdk complex that triggers the cell's passage past the G₂ checkpoint into M phase.

Cyclin levels during the cell cycle

They rise during interphase and fall during mitosis.

M checkpoint

Whether all chromosomes are properly attached to spindle fibers.

Density-dependent inhibition

Cells stop dividing when they sense that they are crowded.

Anchorage dependence

Cells must be attached to a surface to divide.

Transformation in cancer biology

The process where normal cells become cancerous.

Cancer cells evading the cell cycle control system

They may produce their own growth signals or disable checkpoint mechanisms.

Benign tumor

A mass of abnormal cells that remains at its original site.

Metastasis

The spread of cancer cells to distant parts of the body.