Levels of Structure in Nucleic
identified four levels of structure-primary, secondary, tertiary, and
quaternary-in proteins. Nucleic acids can be viewed in the same way. The primary structure of nucleic acids is
the order of bases in the polynucleotidesequence, and the secondary structure is the three-dimensional conformation of the
backbone. The tertiary structure is
specifically the supercoiling of the molecule.
are two principal types of nucleic acids, DNA (deoxyribonucleic acid) and RNA
differences between DNA and RNA appear in their secondary and tertiary
structures, and so we shall describe these structural features separately for
DNA and for RNA. Even though nothing in nucleic acid structure is directly
analogous to the quaternary structure of proteins, the interaction of nucleic
acids with other classes of macromolecules (for example, proteins) to form
complexes is similar to the interactions of the subunits in an oligomeric ribosomes (the polypeptide-generating
machinery of the cell); another is the self-assembly of tobacco mosaic virus,
in which the nucleic acid strand winds through a cylinder of coat-protein
The two principal kinds of nucleic acids are
DNA and RNA.
The primary structure of nucleic acids is the order of bases. The second-ary structure is the three-dimensional conformation of the backbone. The tertiary structure is the supercoiling of the molecule.