What characterizes the secondary structure of DNA?

The secondary structure of DNA is characterized by its stable three-dimensional helical shape, which is formed by the specific base pairing between nucleotides (adenine with thymine, and cytosine with guanine) and the twisting of the two strands around each other. This helical structure is essential for DNA’s stability and functionality, allowing it to efficiently store and transmit genetic information. The formation of the double helix is a critical feature that separates DNA's secondary structure from its primary structure, which refers to the linear sequence of nucleotides.

Other aspects, such as the arrangement of nucleotides in linear sequence or the presence of free nucleotides in solution, pertain more to the primary structure or the environment of DNA rather than its secondary structure. Therefore, the helical arrangement is the defining characteristic that is key to understanding the secondary structure of DNA.