Cell Cycle Phases and Regulation (2C) Practice Test

A laboratory compared two populations of cells after transient nutrient deprivation. Population 1 entered a quiescent state and showed low cyclin levels; upon nutrient repletion, cells re-entered the cycle. Population 2 carried a mutation that constitutively activates a G1 cyclin-CDK complex and did not enter quiescence under the same deprivation. Which phase is most likely bypassed in Population 2 under nutrient-poor conditions?

In an experiment on replication stress, researchers treated cells with hydroxyurea to reduce nucleotide availability and stall replication forks. They then pharmacologically inhibited ATR signaling. Compared with hydroxyurea alone, ATR inhibition results in fewer cells pausing before mitosis and increased chromosomal breaks observed in daughter cells. Which phase transition is most directly compromised by ATR inhibition under these conditions?