Final answer:
Regulatory proteins typically function as either activators or repressors to control gene expression and are crucial in cell cycle regulation. Proteins such as retinoblastoma protein (Rb), p53, and p21, which act as negative regulators, were often found to be malfunctioning in cancerous cells.
Step-by-step explanation:
Regulatory proteins often exist as either activators or repressors. These proteins are crucial in managing the regulation of transcription, where they bind to DNA at regions known as regulatory elements to control gene expression. Activators enhance the interaction of RNA polymerase with the promoter to promote transcription, whereas repressors impede the progress of RNA polymerase, preventing transcription.
In the context of the cell cycle, negative regulatory molecules like retinoblastoma protein (Rb), p53, and p21 play a critical role. These proteins were discovered to be non-functional or damaged in cancerous cells, leading to unregulated cell proliferation. For instance, Rb monitors cell size and, in its active state, can bind to transcription factors like E2F to prevent the transcription of genes necessary for the G1/S transition, effectively blocking cell cycle progression until certain conditions are met.