Deoxyribose nucleic acid. DNA is a long molecule constructed of building blocks called deoxynucleotides. The different building blocks of DNA are often referred to by the letters A, G, T and C. DNA usually contains two separate strands of building blocks, and is thus “double-stranded” in structure. The pairing of strands in double-stranded DNA is determined by the nature of the building blocks in the respective strands. A in one strand pairs with T in the other strand, and G pairs with C, to hold the two DNA strands together by many easily broken A-T and G-T linkages. The overall structure is something like a spiral ladder, with the A-T and G-C linkages being the steps of the ladder. These paired ladder-like double strands are coiled, forming a coiled ladder which is often called the double-helix. (See also RNA).