7.4: Unraveling Nucleic Acid Structure

Knowing that DNA is the genetic material was a tremendous break through, but it left a mystery - how was genetic information stored and replicated. Nucleic acids were initially thought of as boring 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 five-carbon sugar (ribose in RNA or deoxyribose in DNA), 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 functionally important group is a hydroxyl group attached to the 3’ carbon of the ribose/deoxyribose moiety.^[283] RNA differs from DNA in that there is a hydroxyl group attached to the 2’ carbon of the ribose, the absence of this hydroxyl makes DNA a “deoxy” ribonucleic acid! We take particular note of the 5’ phosphate and 3’ hydroxyl groups of the ribose/deoxyribose because they are directly involved in the linkage of nucleotide monomers together to form nucleic acid polymers.

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