The Cell Cycle
All the cells in living organisms must replicate their genetic material and divide,
this process is known as the cell cycle. There are primarily three phases in the cell cycle:
interphase, mitosis, and cytokinesis. The time that it takes to complete a cell cycle varies
greatly among organisms. Typical animal embryos can complete their cell cycle in less
than twenty minutes. Most dividing mammalian cells complete their cell cycle in
approximately twenty-four hours. Some cells found in the human liver have cell cycles
that last over a year. Figure 1 illustrates the different phases of the cell cycle and the
length of the phases by the length of the arrows.
Figure 1: The Cell Cycle
Source: Peter H. Raven, George B. Johnson, Jonathan B, Losos, Susan R. Singer,
Biology Seventh Edition, McGraw Hill 2005
INTERPHASE
Interphase is a portion of the cell cycle between cell divisions. During this phase,
the cell grows and prepares for division. The longest duration of the cell cycle is spent in
interphase. Interphase can be divided into three sub-phases: G1, S, and G2.
G1
The primary growth phase of the cell, commonly referred to as G1, is the first step
of interphase. For many organisms, G1 encompasses the major portion of the cells life
