Updated publication reference for PubMed record(s): 34088668. Two models have been put forward for cyclin-dependent kinase (Cdk) control of the cell cycle. In the qualitative model, cell cycle events are ordered by distinct substrate specificities associated with successive waves of G1, S and mitotic cyclins. Alternatively, the gradual quantitative rise of Cdk activity from G1 phase to mitosis could lead to ordered substrate phosphorylation at sequential thresholds. Here, we study the relative contributions of qualitative and quantitative Cdk control in the budding yeast S. cerevisiae. S-phase cyclins can be replaced by a single mitotic cyclin, albeit at the cost of reduced fitness. The single cyclin can in addition replace G1 cyclins to support ordered cell cycle progression, fulfilling key predictions of the quantitative model. However, single-cyclin cells fail to polarize or grow buds and thus cannot sustain proliferation. Our results suggest that budding yeast has become dependent on G1 cyclin specificity to couple cell cycle progression to essential morphogenetic events.