Short answers, pulled from the story.
The cell cycle is the sequence of events by which a cell grows, duplicates its DNA, and divides into two daughter cells. In multicellular organisms it drives development from a fertilized egg to a mature body, and also regenerates tissues such as hair, skin, and blood cells.
The four phases are G1 (first growth), S (DNA synthesis), G2 (second growth and mitosis preparation), and M (mitosis and cytokinesis). G1, S, and G2 together form interphase, which typically accounts for at least 91% of the total cycle time.
Leland H. Hartwell, R. Timothy Hunt, and Paul M. Nurse shared the 2001 Nobel Prize in Physiology or Medicine for discovering cyclins and cyclin-dependent kinases, the central regulatory molecules of the cell cycle.
Palbociclib, ribociclib, and abemaciclib are FDA-approved CDK4/6 inhibitors that block the phosphorylation of the retinoblastoma protein, preventing cancer cells from entering the cell cycle. They are approved for advanced or metastatic hormone-receptor-positive, HER2-negative breast cancer, and only work in tumors that still express Rb.
G0, or quiescence, is a resting state in which a cell has exited the active cycle and stopped dividing. Some cells, such as neurons, may remain in G0 indefinitely, while others, such as epithelial cells, never enter G0 and continue dividing throughout an organism's life.
Bacteria divide by binary fission and organize their cycle into B, C, and D periods covering birth, DNA replication, and final cell splitting. Unlike eukaryotes, bacteria can begin a new round of DNA replication before the previous cell division is complete, and they lack the checkpoint-based regulatory systems that govern eukaryotic progression.