Figure 1 The Cell Cycle. During the cell cycle, tRNA, mRNA, ribosomes, and enzymes are produced in the G1 stage. DNA replication occurs in the S stage. Proteins required for the spindles are synthesized in the G2 stage. Once some organs, such as the brain, have completely developed, certain types of cells, such as never cells, remain in the G2 stage. The time periods indicated are relative and vary depending on the type of cell and the age of the organism.