Abstract
The eukaryotic cell cycle is one of the most fundamental biological processes, ensuring the accurate duplication and segregation of the genome during mitosis. Decades of research across model systems have shown that this process is orchestrated by a family of protein kinases known as cyclin-dependent kinases (Cdks). Together with their cyclin partners, Cdks act as master regulators of cell division, coordinating DNA replication, chromosome segregation, and cytokinesis with remarkable precision. The discovery of Cdks and cyclins in yeast and sea urchins, celebrated with the Nobel Prize of Hartwell, Hunt, and Nurse (awarded in 2001), established the conceptual framework for understanding how oscillations in kinase activities drive cell cycle progression in a unidirectional and irreversible manner. Over the past thirty years, a central question has been whether cell cycle control relies primarily on the quantitative level of Cdk1 activity or whether distinct qualitative functions of cyclin-Cdk1 complexes ensure the correct ordering of events. Addressing this question required new genetic and biochemical tools capable of controlling Cdk1 activity with high temporal resolution and specificity. A turning point came in 2000 with the development of the analogue-sensitive Cdk1 allele by the Shokat laboratory. This approach replaced classical temperature-sensitive alleles with a version of Cdk1 that can be selectively inhibited by bulky ATP analogues. Beyond specific inhibition, the system was soon adapted to directly label and identify Cdk1 substrates, coupling chemical genetics with the emerging power of mass spectrometry. This review outlines the conceptual frameworks of quantitative and qualitative models of Cdk1 control. It also highlights how these ideas have been experimentally dissected, tracing the development of the Cdk1 Shokat system and advances from synthetic biology and phosphoproteomics in decoding phosphorylation logic, and how these concepts apply to meiosis. These studies draw primarily on budding yeast and fission yeast which have a single Cdk, making them convenient models for studying core principles of cell cycle regulation. Key insights from vertebrates are also integrated to illustrate principles that extend to other eukaryotes.