You inject cells with a growth factor to take them into G1. After 10 hours, you inject one group of cells with an antibody against cyclin D (inhibiting its function). After 14 hours, you inject a second group of cells with this antibody. After 16 hours, you inject a third group. The fourth group of cells, a control group, is not injected with antibodies. You label these cells in order to see whether DNA synthesis (i.e. growth) has occurred in them. You find that the cells injected with anti-cyclin D after 10 hours show no DNA synthesis; those injected after 14 hours show about half as much synthesis as the control group; those injected after 16 hours show the same amount of synthesis as the control group. From these results, what can you conclude about the presence or absence of a restriction point related to the G1 phase?

MCQs on the cell and its structures, including molecular compounds, structural features, and organelles including the nucleus, cytoskeleton, and extracellular matrix. Particular emphasis on transport mechanisms and membrane trafficking, cellular signaling, the cell cycle including mitosis and meiosis, and gene expression.