G₁ - G₁ stands for "Gap 1." During this phase cells carry out basic cell functions. Most cell growth occurs during G₁. The cells begin to prepare for DNA replication. Proteins needed for DNA replication are made during this phase. Some cells exit the cell cycle and enter the G₀ phase while other cells continue on to the S phase.

S - During the S phase, or synthesis, DNA is replicated. At the end of the S phase, one cell contains twice as much DNA as it did when it began the S phase. Cell growth continues during the S phase. It is estimated that it takes about 10 hours to replicate the information contained in the nucleus of a single human cell.

G₂ - Once a cell's DNA has been replicated it will enter the G₂ or "Gap 2" phase. This phase is the cell's final chance to grow. The organelles found in the cytoplasm are replicated, and more proteins are made.

G₀ - G₀ is referred to as the resting phase. When a cell stops dividing it enters the G₀ stage. During the G₀ phase, cells continue to perform all of the normal process but do not grow or divide. Some cells may never leave G₀ while others skip this stage altogether. Examples of cells that enter and never leave the G₀ phase are neurons (nerve cells) and red blood cells.