During the G1 phase of the cell cycle (blue), the promoter for a given gene opens and closes to produce mRNA molecules (black waves) in bursts. However, during the S phase and G2 phase of the cell cycle (green), the cell contains twice as many copies of each gene as a result of replication. Cells must therefore compensate for these extra copies in order to maintain constant mRNA levels throughout the cell cycle. Skinner et al. found that this compensation is achieved by reducing the number of times per hour that the promoter is open. The challenge of maintaining a constant level of mRNA is similar to the challenge of maintaining a constant speed in a car in which a second driver periodically appears.