Final answer:
Retinoblastoma protein (Rb) is the primary regulator of cyclin transcription during the G1/S checkpoint, inhibiting gene expression until the cell reaches a sufficient size. Cdk and Myc are also involved in regulating the cell cycle, but are not directly responsible for cyclin activation.
Step-by-step explanation:
The activation of cyclin transcription during the G1/S checkpoint is primarily controlled by Retinoblastoma protein (Rb).
Rb is a negative regulator that monitors cell size. In its active state, Rb binds to transcription factors, such as E2F, inhibiting the transcription of genes necessary for the G1/S transition. As the cell grows, Rb becomes phosphorylated and releases E2F, allowing the production of the transition protein and progression through the checkpoint.
Cdk (Cyclin-dependent kinase) and Myc also play important roles in regulating the cell cycle, but they are not directly responsible for the activation of cyclin transcription during the G1/S checkpoint.