Which molecule is important for signaling checkpoints during the cell cycle?

Cyclin-dependent kinase (CDK) is crucial for signaling checkpoints during the cell cycle because it acts as an enzymatic regulator that helps control the progression of cells through various phases of the cycle. CDKs are activated when they bind to specific proteins called cyclins, which are produced and degraded at specific intervals during the cycle. This activation leads to the phosphorylation of target proteins that drive cellular processes, such as DNA replication and mitosis.

The role of CDKs at checkpoints is to ensure that the cell only proceeds to the next phase of the cell cycle when conditions are suitable; for instance, checking for DNA damage or complete DNA replication before moving from the G1 phase to S phase or from G2 to mitosis. These checkpoints are critical to maintaining genomic integrity and preventing uncontrolled cell division, which can lead to cancer.

In contrast, the other options do not function in this regulatory capacity. ATP is primarily an energy currency and fuels various cellular processes; RNA polymerase is involved in transcription, synthesizing RNA from a DNA template; and amino acids serve as building blocks for proteins but are not directly involved in cell cycle regulation.