Cyclic nucleotides are important signaling molecules in cells, primarily including cyclic AMP (cAMP) and cyclic GMP (cGMP). They are formed from ATP and GTP, respectively, and play crucial roles in various biological processes, such as regulating metabolism, gene expression, and cell growth. These molecules act as secondary messengers, transmitting signals from hormones and neurotransmitters to target proteins within the cell. By activating specific enzymes, cyclic nucleotides help modulate physiological responses, including smooth muscle relaxation and neuronal signaling, making them vital for maintaining cellular functions and overall health.