Describe the cell cycle phases and checkpoints.

• A. G1—growth; S—DNA replication; G2—preps for mitosis; M—mitosis/cytokinesis; checkpoints ensure proper replication and chromosome alignment.
• B. G1—cell division; S—protein synthesis; G2—growth; M—DNA replication.
• C. G1—mitosis; S—DNA repair; G2—preps for cytokinesis; M—cytokinesis.
• D. G1—growth; S—DNA replication; G2—preps for mitosis; M—mitosis/cytokinesis; checkpoints ensure proper replication and chromosome alignment.

Answer: (D)

Understanding the cell cycle means knowing the order and purpose of each phase and how the cell makes sure everything is ready before moving on. The cycle begins with G1, a growth and metabolic phase where the cell increases in size and carries out regular functions. Next is S, where DNA is replicated so each new cell will have a complete genome. Then G2, another growth phase where the cell grows more and checks that DNA replication happened correctly and is ready for division. Finally M spans mitosis, where chromosomes are separated, and cytokinesis, where the cell splits into two. Checkpoints act like quality control, pausing the cycle if there are problems or if key steps aren’t finished properly. They monitor DNA integrity during G1/S, ensure replication is complete during S, verify readiness for mitosis during G2/M, and confirm proper chromosome attachment and alignment during mitosis. This combination—G1 growth, S DNA replication, G2 preparation for mitosis, M mitosis/cytokinesis, with checkpoints guarding replication and chromosome alignment—best matches how the cell cycle actually works. Descriptions that swap these roles, such as labeling G1 as cell division or S as protein synthesis, don’t align with how the phases and checkpoints function.