29. • DNA also has primary, secondary, tertiary, and quaternary structures.
• (1) The primary structure of DNA is the primitive linear deoxyribonucleotide chain composed of
deoxyribonucleotides in order. The deoxyribonucleotides contain different nitrogenous base (A/T/C/G),
deoxyribose and one or more phosphate groups, and are linked by phosphodiester bonds.
• (2) DNA's secondary structure is a double helix formed on the basis of two complementary polynucleotide
strands by hydrogen bonds in base pairs. Actually, the stronger forces holding the two strands together
are stacking interactions between the bases. These stacking interactions are stabilized by Van der Waals
forces and hydrophobic interactions, and show a large amount of local structural variability. The two
polynucleotide chains form the right hand double helix with the same rotation around the same common axis.
The diameter of the spiral is 2.0nm. The sugar phosphate backbones of two polynucleotide chains are
located at the outer side of the double helix, while bases are located at the inner side. There are two
grooves in the double helix, which are called major groove and minor groove based on their relative size.
The axial distance between adjacent base pairs is 0.34nm, and the wheelbase of each helix is 3.4nm.
• (3) On the basis of the double helix, DNA molecules twist the coil further in order to form a super helix
to compress the volume. The resultant is called tertiary structure of DNA. The tertiary arrangement has
large-scale folding taking into consideration into geometrical and steric constraints. Usually, it
includes A-DNA, B-DNA and Z-DNA in space.
• (4) DNA’s quaternary structure is similar to that of the protein to some extent. The quaternary structure
of nucleic acids is the interaction between nucleic acids and other molecules. For example, the chromatin
is formed by interaction between DNA and small histones.