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, cell growth, and gene expression. 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, making them vital for maintaining cellular functions and overall homeostasis in organisms.