DNA controls all functions of cells and determines characteristics of organisms. It is a long molecule composed of nucleotides, each containing a phosphate group, deoxyribose sugar, and nitrogenous base. The DNA double helix structure contains two nucleotide chains bonded together via base pairing between adenine and thymine, and cytosine and guanine. DNA replicates before cell division by unwinding and using each strand as a template to make a new complementary strand, ensuring each new cell contains an identical copy. Genes, which are sequences of DNA triplets coding for proteins, determine the structure and function of cells.