Knowing that the genetic material was DNA was a tremendous break through, but it left a mystery - how was genetic information stored and replicated. Nucleic acids were thought to be aperiodic polymers, that is, molecules built from a defined set of subunits, known as monomers, but without a simple overall repeating pattern. The basic monomeric units of nucleic acids are known as nucleotides. A nucleotide consists of three distinct types of molecules joined together, a 5-carbon sugar (ribose or deoxyribose), a nitrogen-rich “base” that is either a purine (guanine (G) or adenine (A)) or a pyrimidine (cytosine (C), or thymine (T)) in DNA or uracil (U) instead of T in RNA, and a phosphate group. The carbon atoms of the sugar are numbered 1’ to 5’. The nitrogenous base is attached to the 1' carbon and the phosphate is attached to the 5’ carbon. The other important group attached to the sugar is a hydroxyl group attached to the 3’ carbon. RNA differs from DNA in that there is a hydroxyl group attached to the 2’ carbon of the ribose, this hydroxyl is absent in DNA, which is why it is “deoxy” ribonucleic acid! We take particular note of the 5’ phosphate and 3’ hydroxyl groups because they are directly involved in the linkage of nucleotides together to form nucleic acid polymers.