(A) As cells grow and prepare to divide they pass through four different phases of the cell cycle: G1, S, G2, M. During the course of this cycle, the activity of cyclin-dependent kinases (CDKs) changes. These fluctuations in activity can be monitored using a fluorescent reporter protein that contains a portion of human DNA helicase B (DHB), which moves from the nucleus to the cytoplasm when phosphorylated by active CDKs. Thus, changes in the levels of DHB in the nucleus and cytoplasm (depicted in shades of orange/red) can be used to determine a cell’s CDK activity. At the start of G1, CDK activity is low and DHB remains in the nucleus (shown in red). Some G1 cells maintain this low level of activity (CDKlow) and exit the cell cycle to become quiescent, while cells with increasing levels of CDK activity (CDKinc) commit to another cell cycle and enter S-phase. (B) Adikes et al. showed that the DHB reporter could monitor cell cycle progression of individual cells in live animals, such as the embryo of a zebrafish. It can also identify which cells have exited the cell cycle and which are preparing for division.
Image credit: Joy H Meserve.