Identifying the correct function of cyclins, which statement is true?

Cyclins are crucial regulatory proteins that play a significant role in controlling the progression of the cell cycle. They function by activating cyclin-dependent kinases (CDKs), which are essential for the transition between different phases of the cell cycle, such as moving from the G1 phase to the S phase for DNA replication, and from the G2 phase to the M phase for mitosis. The levels of cyclins fluctuate throughout the cell cycle, which allows for tight regulation of cell division and ensures that each phase is completed before moving on to the next. This regulation is vital for maintaining proper cell function and preventing uncontrolled cell growth, which can lead to cancer.

While cyclins are involved in various cellular processes, their primary and defining function is related specifically to cell cycle progression, making that statement true. Other options pertain to functions that are performed by different proteins or processes unrelated to cyclins directly. For instance, DNA repair mechanisms are managed by repair enzymes and pathways, apoptosis is regulated by a different set of proteins, and cyclins are not limited to activity during mitosis, as they have roles in several phases of the cell cycle.